A novel population of composite mounds: their initiation, growth and demise. San Emiliano Formation, Cantabrian Mountains, Spain

A novel population of composite mounds: their initiation, growth and demise. San Emiliano... Purpose Mounds from the Pennsylvanian aged San Emiliano Formation (Cantabrian Mountains, Spain) are commonly well exposed. These mounds range from 2 to 50 m in height and are observed to be primary geological features. The mounds are described and classified and the factors and controls of mound nucleation, growth and demise have been established. Methods Microfacies analysis of 177 thin sections has revealed the composition of the mounds and surrounding carbonates. Results Composite mounds, exhibiting characteristic components of both Cluster mounds and Agglutinated Microbial mounds are described. The mounds are skeletal-microbial/pack-wackestones. Peloidal, homogenous and clotted micrites are the main sedimentological constituents of the mounds. Microfossils are dominant with Donezella, Claracrusta, Rothpletzella and Girvanella being common. Small foraminifera, bryozoans, corals and algae are all present within the mounds, but are more common within off-mound carbonates. Conclusions The formation of the mounds was controlled by a dynamic relationship between Donezellacean algae, and microscopic encrusters, a bio-mechanism not observed in mud mounds elsewhere. Fluctuating environmental conditions lead to the alternate dominance between the two groups, resulting in accretion and stabilisation of carbonate muds. These mounds are compositionally different to their Pennsylvanian counterparts. Keywords Composite mound · Agglutinated microbial mound · Cluster mound · Donezella · Encrusting biota · Microfacies analysis · Pennsylvanian Resumen Propósito Los montículos de la Formación San Emiliano de edad Pensilvaniense (Cantabrian Mountains, España) están comúnmente bien bexpuestos. Estos montículos tienen una altura de 2 a 50 m y se observa que son rasgos geológicos prima- rios. El los montículos se describen y clasifican y los factores y controles de la nucleación, el crecimiento y la fallecimiento tienen que ver con los siguientes factores se ha establecido. Métodos El análisis de microfacias de 177 sección delgada ha revelado la composición de los montículos y carbonatos circundantes. Resultados Montículos compuestos, que exhiben los componentes característicos de ambos montículos Cluster y Microbial aglutinado. Se describen los montículos. Los montículos son esqueléticos-microbianos lodogranular/lodosa. Peloide, homo- géneo y coagulado micrites son los principales componentes sedimentológicos de los montículos. Los microfósiles dominan con Donezella, Claracrusta, Rothpletzella y Girvanella son comunes. Pequeños foraminíferos, briozoarios, corales y algas son todos ellos presente dentro de los montículos, pero son más comunes dentro de los carbonatos fuera del montículos. Conclusiones La formación de los montículos estaba controlada por una relación dinámica entre las algas donezellaceanas, y incrustaciones microscópicas, un mecanismo biológico no observado en montículos de lodo en otras partes. Medio ambiente fluctuante condiciones conducen a la dominación alternada entre los dos grupos, resultando en la creación y estabilización de lodos de carbonato. Estos montículos son compositivamente diferentes a sus contrapartes de Pensilvaniense. Palabras claves Montículo compuesto · Montículo microbiano aglutinado · Montículo de cúmulos · Donezella · Biota incrustante · Análisis de microfacias · Pensilvaniense Extended author information available on the last page of the article Vol.:(0123456789) 1 3 226 Journal of Iberian Geology (2018) 44:225–241 1 Introduction1.1 Geological setting Carbonate mud mounds are a type of marine buildup which The San Emiliano Formation is located within the Canta- brian Mountains (Fig. 1), León Province, Northern Spain, originate by the successive vertical and lateral accretion of carbonate muds (Monty 1995). This type of buildup became around the small rural town from which it takes its name. The area sits within the Iberian Massif, which itself has increasingly recognised during the 1970s and 1980s when it was found that various mound types, and their associated been informally arranged into six tectonostratigraphic units. These units were based upon differences in facies associa- structures, microfacies and biotas extended a diverse geo- logical range and often included features which were poorly tions, structural styles, metamorphism and magmatism (Lotze 1945). The northern most of these units is where understood or unknown at the time (Wilson 1975; James 1980; Longman 1981; James 1983; Lees 1988; Monty et al. the study area is located and is called the Cantabrian zone (Lotze 1945; Pérez-Estaún et al. 1988). The Cantabrian zone 1988; Swennen 1988). Mud mounds are complex structures which respond to facies changes and have been found to consists of an incomplete Cambrian–Ordovician sedimen- tary sequence, a Silurian and Devonian sequence which record shifts in lithofacies and biofacies (Monty 1995). A wide variety of reef and mound types, ranging from is complete toward the west but is missing elsewhere and several thousand metres of strata spanning the Tournaisian Waulsortian-type mounds, biodetrital and microbial mounds, stromatactoid mounds, Tepee Buttes and a variety of oth- to the Gzhelian Stages (Bahamonde et al. 2002). The Can- tabrian zone (Lotze 1945; Pérez-Estaún et al. 1988) is fur- ers, have been reported both temporally and spatially (e.g. Monty 1995; Pratt 1995; Kaufmann et al. 1996; Neuweiler ther subdivided into five tectonic units: the Fold and Nappe, Central Asturian Coalfield, Ponga Nappe, Picos de Europa et al. 2001; Krause et al. 2004; Hebbeln and Samankassou 2015). Riding (2002) reviewed the historical classifica- and Pisuerga-Carrión provinces. The San Emiliano Forma- tion belongs to the La Sobia-Bodón/Aramo units of the Fold tion of organic reefs, in an attempt at the standardisation of what is a fraught nomenclature. The mounds described here and Nappe Province (Pérez-Estaún et al. 1988; Bahamonde et al. 2002). The formation was deposited within the fore- are classified according to that standardisation. Individual populations of mounds are generally distinguished by their deep depozone of a Variscan foreland basin. The San Emiliano Fm. (Figure  2) consists of at least varying biological, sedimentological and tectonic settings, a distinction between Palaeozoic and Mesozoic mounds also 1800 m (Bowman 1982) of Pennsylvanian aged (Bashkirian to Moscovian), alternating deltaic clastics and shallow exists (Schmidt et al. 2001). Due to the polygenetic nature of reefs and mud mounds, analysis of individual popula- marine carbonates, with rare, thin coal bands (Bowman 1979). At the San Emiliano type locality (i.e. the San tions is fundamental to understanding the factors influencing their presence (e.g. Flajs et al. 1995). The nomenclature of Riding (2002) is based on reef structure. Presented here is Fig. 1 a A geological map showing the area of study. Samples were a population of mounds from the Pennsylvanian aged San obtained from carbonate units within the San Emiliano Formation Emiliano Formation, Cantabrian Mountains, Spain. These (i.e. the blue bands), coordinates are in UTM. Specific localities, mounds have a unique growth dynamic controlled by two where individual mounds were sampled, are indicated by white stars, competing ecological assemblages which has not yet been with the mound imaged in B shown as a red star. The inset shows the location of the study area (indicated by asterisk symbol) in regards described. The mounds observed fall into the category, as to the Spanish and Portuguese landmasses. (Modified after Suárez defined by Riding (2002), of Cluster mounds, although small et  al. 1991). Key in right-hand column bellow. UTM coordinates volumes of the mounds, dominated by encrusting biota are of the named villages (from west to east: Robledo de Babia—29T considered as Agglutinated Microbial mound components. 738542.31 4761967.82; Cospedal—29T 740471.87 4762102.02; La Majúa—29T742542.18 4763067.04; San Emiliano—29T Due to this overlap, the term ‘Composite mound’ is used to 744642.62 4762013.24; Candemuela—30T 255531.56 4763579.80; encapsulate the differing structures observed. The nucleation Villargusan—30T 2565587.05 4764801.92; Pinos—30T 257036.76 of, growth and subsequent demise of these San Emiliano 4763119.49. b Picture and annotated field sketch of a typical mound Fm. mounds is also discussed. from the San Emiliano Formation, due to this mounds location at the side of a road, and the cut through of the mound provided, it is the Pennsylvanian ‘carbonate mud mounds’ from the Canta- most accessible outcrop that displays basal, mound, capping and off- brian Mountains have been described by Bowman (1979), mound facies in the Valley (29T 744467.52 4763114.34). c A field Riding (1979) and Samankassou (2001) additionally, Hensen sketch highlighting the geometrical relationships between mounds. et al. (1995) described both primary and diagenetic mounds The sketch is of 1 km of the carbonate unit that stretches continuously from just south of the village of Cospedal, to the village La Majúa. from the area. The sketch begins 500  m south-east of Cospedal (29T 741166.91 4761689.94) and encompasses the ridge, eastward toward La Majúa (29T 742631.65 4762673.57). The sketch was taken from the north- ern-most point of the carbonate ridge, approx. 200  m south of the mound bearing unit (29T 742640.14 4761171.31) 1 3 ◂ Journal of Iberian Geology (2018) 44:225–241 227 29T 30T N S Capping Unit 1m Mound Clastic sequence Studied mound locations, mound imaged in ‘B’ Clastic Mound? sequence Bedding laps Fractured/ Poorly Bedded Mounds Mounds Trees ‘rubble’ onto Mounds 25m Mounds Mounds Poorly Bedded Mounds Mounds Far fewer Mounds, more bedding 1 3 232/41/NW Mound? Bedded Limestone Bedded Limestone 200/49/W Vegetation 228 Journal of Iberian Geology (2018) 44:225–241 WestphalianA (W/SW/WB) Moscovian (C) WestphalianA/ Namurian C (W) early Verisky/ Kashirsky (VGV/R) late Westphalian A (WB) late Asatausky (VGV) early Westphalian A(GN) early WestphalianA (GN) WesphalianA (WB) early Westphalian A(GN) middle/late Bashkirian (MCWP) Tashatansky/Asatausky boundary (VGV) early Westphalian A(GN) earlyTashastinsky (VGV) early Westphalian (B) Namurian/ Westphalian boundary (WB) earlyTashastinsky (VGV) 1* Key 300 late Yeadonian (N) Coal band Plant fragment Deltaic Fusulinid foram clastics Miospore *bold numbers refer to Limestone the limestone units of Brachiopods unit the La Majúa Member Fig. 2 Litho- and biostratigraphy of the San Emiliano Formation tínez Chacón and Winkler Prins (1979); N Neves in Bowman (1982); at the type point, San Emiliano. The age of the Formation is based R Rumjantseva in Wagner and Bowman (1983); SW Stockmans and upon various authors’ stratigraphic studies, alluding to a Bashkirian Williér (1965); VGV Van Ginkel and Villa (1996); W Wagner (1959); to early Moscovian age for the La Majúa Member. Specimen bear- WB Wagner and Bowman (1983). Miss. Mississippian, Upp. Upper, ing limestone units are labelled 1–8. B Bowman (1982); C Carballeira Serp. Serpurkhovian, Fm formation, Mb member. Modified after Van et al. (1985); GN Gueinn and Neves in Bowman (1982); MCWP Mar- Ginkel and Villa (1996) Emiliano valley) Bowman (1982) subdivided the formation (Fig. 1) are not observed within all eight of the limestone into three constituent members, based upon the proportions units of the La Majúa Member and seem to be confined to of clastic and carbonate sediments. These are the [1] Pinos, units 1, 4 and 8 (Fig. 2). Non-mound bearing units ranged [2] La Majúa and [3] Candemuela Members. The [1] Pinos from off-mound carbonates typical of the formation or ‘Phyl- Member consists of black shales passing into siltstones loid’ algal bearing wackestones. The [3] Candemuela Mem- with thin sandstone interbeds; the member is approximately ber consists of deltaic and shallow marine clastics with coal 250 m thick. The [2] La Majúa Member consists of alter- beds and seatearths; the member is approximately 500 m nating regressive deltaic and shallow marine clastics (with thick. some thin coal bands) separated by eight major limestone Thirty kilometres east of the San Emiliano Valley, in the units (Bowman 1979); the member is approximately 1050 m Bernesga Valley, a Moscovian aged succession, comprising in thickness. Fusulinid Foraminifera collected from the mainly turbidites, carbonate debris flows and shallow water upper part of the La Majúa Member have indicated a late siltstones can be found exposed in the northern flank of the Bashkirian age (van Ginkel and Villa 1996). Mud mounds Cármenes syncline (Bowman 1982; van Ginkel and Villa 1 3 Carboniferous Miss. Pennsylvanian Upp. Lower Middle Serp. Bashkirian Moscovian Late Verisky Kashirsky Early Namurian (Yeadonian) Westphalian A San Emiliano Formation Valdeteja Fm Pinos Mb La Majúa Member Candemuela Mb Journal of Iberian Geology (2018) 44:225–241 229 1996). This sedimentary succession is commonly known as Carbonate units from the San Emiliano Fm. which are bar- the Lois-Cigüera Formation (Rácz 1964; Brouwer and van ren of carbonate mud mounds are also, briefly, considered. Ginkel 1964; de Meijer 1971; Van De Graff, 1971; Bow - man 1982, 1985), and is a stratigraphic equivalent to the 2.1 The basal facies San Emiliano Fm. The Lois-Cigüera Fm. consists of the basal Villanueva Beds, the middle “Caliza masiva” unit and The basal facies often consists of approximately 1  m of the top Villamanín Beds. Brouwer and van Ginkel (1964) bedded limestones, individual beds range from 13 to 32 cm erected the Lois-Cigüera Fm. on the basis that the sediments thick. In hand specimen the basal facies appears as a blue- were mostly detrital (tubiditic and debris flow). Moore et al. grey colour. Numerous macro fossils are visible, with shelly (1971) reported the San Emiliano Formation as missing or fauna, rugose and auloporid corals and algae identified. The reduced along the northern limb of the Cármenes syncline, limestones are micritic and exhibit a porcelaneous fracture. and reported that the “rhythmic units” of the San Emiliano The basal microfacies is a skeletal-microbial bafflestone Fm. (as observed in the vicinity of San Emiliano) are not consisting of intertwining (often bio-cemented) thickets of found in the Villamanín area and are absent (or reduced) as branching Donezella (with some Beresella) thalli, associated a result of uplift and erosion, the responsible uplift believed with birdseye, fenestral and some poorly developed primary to be associated with the Cantabrian Block (as defined by cavity fabrics filled with sparite. Calcimicrobes found within Radig 1962). Bowman (1982) reports several lithologies some of the cavities occur as clumps of sheaths within a (turbidites, shallow water siltstones and carbonate debris) homogenous and peloidal micrite. The primary cavities which are recognised as being the same age as the San Emil- appear to be part of an irregular, porous network within the iano Fm. Due to the sufficiently distinct facies and distribu- Donezella thickets. Areas around the Donezella thickets are tion the Lois-Cigüera Fm. is separated from their San Emil- dominated by bodies of encrusting Claracrusta, Donezella, iano valley counterparts (Bowman 1982; van Ginkel and Girvanella and probably encrusting foraminifera (Fig. 3a). Villa 1996). The San Emiliano Fm. was often mistakenly The encrusting appears non-selective and coats algae, reported from the Villamanín area previous to Moore et al. foraminifera, Girvanella and mudchips. Cortoids (Fig. 3b) (1971) and Bowman (1982). The distinction is still not rec- and cyanoliths are present within the samples particularly ognised by some authors (Riding1979; Samankassou 2001). where mudchips have undergone encrustation. Donezella Outcrops that were investigated and sampled for this thalli are mostly well preserved can be observed as either: study are along the well exposed limestone ridges of the in situ as a three dimensional “shrub” like framework with La Majúa Mb. located within the San Emilliano Valley. 277 abundant baffled micritic sediment and free floating micro- Samples were obtained from a total of 24 locations across fossils (Fig. 3b), or as broken fragments, these fragments the valley. The majority of samples were taken across the show little sign of wear. The micrite-walled calcimicrobe laterally extensive exposure of the uppermost carbonate Girvanella is commonly well preserved as small tangled unit of the La Majúa Member (unit 8, Fig.  2) of the San clumps within thin layers of microspar which acts as a bio- Emiliano Fm. (Figure 1c) in addition to detailed sampling cement between algal thalli, a micritic envelope is present at localities where mounds are well exposed and accessible on most bioclasts (Fig. 3a, b). Other than the dominant Don- (Fig. 1a). Sampling included coverage of the basal, mound, ezellaceans and associated encrusting calcimicrobes (Girva- capping and off-mound facies, where possible. The results nella and Rothpletzella), Archaeolithophyllum, Petschoria, presented are based on a detailed microfacies analysis of Komia (and Ungdarella), Beresellids, Tuberitinidae and rare, 177 thin sections. small foraminifera’s (including Tetrataxis) and ostracods are found. 2 Microfacies analysis 2.2 The mound facies Mound bearing units exhibit similar vertical profiles, con- The Composite mounds of the San Emiliano Fm. are a union sisting of approximately 30  cm to 3  m of bedded lime- of mound structures: carbonate mud with in-place skeletal stones (basal facies), followed by 2–50  m of carbonate elements (Cluster mound component) with subordinate mud mound/s (mound facies) lacking in primary geological volumes of laminar sediment trapping and binding micro- features, which is in turn overlain by further bedded (up bial elements (Agglutinated Microbial mound component). to 10 m) limestones (capping facies). In places, a bed of The sediments are best described as a skeletal-microbial 10–30 cm thick siliciclastic rich carbonate can be observed pack-wackestone often associated with fenestral cavities to onlap and drape over the mounds (see Fig. 1b for a typical (Fig. 3c, e) and occasional stromatactoid cavities (Fig. 3f). example). The basal facies, mound facies and capping facies The mounds vary in size ranging from 2 to 50 m in thickness and the relationship between them are all considered here. and are steep sided, lens to mound shaped. Mounds appear 1 3 230 Journal of Iberian Geology (2018) 44:225–241 1 3 Journal of Iberian Geology (2018) 44:225–241 231 ◂Fig. 3 Photomicrographs of the basal and mound facies of carbonate or ‘shrub-like’ growths are (Fig.  3a). Encrusting sheets mud mounds from the San Emiliano Formation. a Claracrusta speci- occur in a repetitive manor in association with alternat- mens creating encrusting sheets of tubes which can be seen to branch, ing Donezellacean growths. Biodiversity observed within the sheets encrust several substrates showing no preference. The this facies is low; associated with the highly dominant specimen exhibits the characteristic growth form of Claracrusta, the first few crusts are flat with few undulations, with subsequent crusts Donezellaceans are rare Tuberitinidae and foraminifera, becoming increasingly convoluted. Encrusted substrates include: mud plus Komia, Archaeolithoporella and encrusting Clarac- chips, Donezellacean thickets, Petschoria (a red ungdarellid algae) rusta, Rothpletzella and others (Fig. 4a). Some selective and Girvanella. b Entwined Donezellacean algae, several thalli are dolomitisation has occurred, often as contact or floating cemented together with clotted micrite and bio-cement. A cortoid (centre) exhibiting a characteristically uneven surface with microbor- equigranular rhombic to non-rhombic crystals within the ings and encrusters. c Several fenestral cavities. Homogenous micrite blocky cement filling the fenestral cavities. is the dominant sediment with some peloidal and clotted micrite. The Microfacies investigation of the mounds revealed two cavities have a sparitic fill with no evidence of a stromatactoid fab- sub-facies that occur in a repetitive pattern. The first ric. The geometry of the cavities (flat base, undulating roofs) in the bottom half of the figure may suggest some sort of hard ground or sub-facies (sub-facies a) is a fenestral pack-wackestone surface on which the base of the cavity network has formed, with the characterised by non-laminated peloidal and clotted roof being defined by algal growth. d Sub-rounded micritic grains. micrite and encrusted Donezellacean thalli (Fig.  3c, d). Some areas consist of poorly sorted grains with distinct size differ - The sedimentary framework consists of both sparitic and ences. e Several examples of laminar stromatactis cavities, with char- acteristic flat bases, undulose roofs, internal sedimentary fill and both micritic peloids and although they are non-laminated fibrous and blocky calcite cements. In this example the cavities are they are often thromboidal or ordered. Clotted micrite very closely spaced, with the top of one set of cavities forming the occurs in places, as does homogenous micrite, both are base of the next. f A stromatactoid cavity. This example of stromatac- commonly found as cavity fill. Donezella (and accessory tis appears surrounded by Donezellacean algae, many examples of cavity fabrics from this study show the same relationship, with the Beresella) are well preserved and relatively large, form- algae directly responsible for the construction of the cavity frame- ing isolated and aligned thickets, sub-facies a forms the work Cluster mound (Riding 2002) component of the mounds. The second sub-facies (sub-facies b) is characterised as either isolated buildups or as superimposed complexes by encrusting calcimicrobes, namely, Rothpletzella and of two or more mounds (Fig. 1c). Individual mounds do not Claracrusta with occasional Gir vanella and Wethere- show any evidence of asymmetry, preferred orientation or della growths (Fig. 4a–c). These encrustations occur as thickness changes. Mounds are micritic and in hand speci- laminar sheets of branching filaments; both tightly, and men are lighter in colour than the basal and capping facies loosely packed growths with homogenous and peloidal counterparts. Rare occurrences of siliciclastic conglomerates micrite and some sparry cement between individual lami- were observed within mounds (an accessible outcrop can be nae occur. Rare foraminifera (mostly Tetrataxis, Lasio- found at 29T 738867.33 461336.25) towards the west of the discus and very rare Bradyina) have been observed, often formation (near the village of Robledo de Babia.) between entwined Donezella thali. Sub-facies b forms the The mound facies is a fenestral packstones (Fig.  3c) Agglutinated Microbial mound (Riding 2002) component characterised by non-laminated peloidal and clotted mic- of the mounds. Sub-facies b is volumetrically less signifi- rite (Fig. 3d) associated with variously sized fenestral cav- cant than sub-facies a, with encrusting sheets often being ities and bioclasts. Cavities are spar-filled and range from micron to millimetre in scale (Fig. 4a–c). small ‘birdseye’ to larger, irregular voids (Fig. 3c, e). Sev- eral cavities have a stromatactoid fabric; flat based voids 2.3 The capping facies and off‑mound facies with undulating roofs, often with fine sediment fill at the bottom (Fig. 3f). Isolated clasts of sedimentary framework The capping facies and off-mound facies (those carbonates can be found within some of the larger cavities, as can a flanking or occupying space between mound bearing strata) rare, sparse, coating of pyrite. The sedimentary framework consist of bedded carbonates. The beds directly above the consists of both sparitic and mictitic peloids, the latter mounds often have a high clastic component, they are a associated with a relative abundance of encrusted Donez- dark grey in colour and are observed to drape over/onlap ella (with some accessory Beresella and Dvinella) thalli, the mounds. The contact between the capping facies and peloids are often thromboidal or aligned. Homogenous the mound facies is sharp. Beds are thinner than those of micrite occurs exhibiting no obvious fossil content and the basal facies, ranging from 5 to 25 cm thick. The clas- little in the way of sedimentary fabrics,in association with tic-rich bed directly above the mounds are grain supported Donezellaceans (Fig. 3d). In places a clotted texture can be packstones consisting of broken and transported fossil observed within the micrite (Fig. 3d–f). Several cyanolith fragments, sparry calcite crystals and an insoluble clastic/ like growths and encrusting sheets are observed; individual organic material sedimentary framework (Fig. 4d). Calcite algae thalli are rarely encrusted, although in situ thickets crystals occur interspersed within a poorly laminated very 1 3 232 Journal of Iberian Geology (2018) 44:225–241 AB C D E F fine quartz and insoluble organic framework. Bioclasts are further sedimentation, shell fragments also appear slightly dominantly broken Donezellacean fragments, the orientation flattened. Above these beds the carbonates are classified of which is slightly imbricated. Tuberitina are observed as between a wacke- and packstone. These are characterised well as some encrusting Komia and large fragments of bryo- by a homogenous to slightly peloidal micrite sedimentary zoan fronds. Several flattened, opaque lenses occur, these framework, partially baffled by branching Donezellaceans are possibly plant fragments which have been compacted by and partly supporting large bioclasts (over 1 mm in size) and 1 3 Journal of Iberian Geology (2018) 44:225–241 233 ◂Fig. 4 Photomicrographs of the mound facies, capping facies and encrusted clast in Fig.  4c). Other macro-fossils include selected biota of carbonate mud mounds from the San Emiliano rugose and auloporid corals, brachiopods and gastropods. Formation. a A community of microscopic encrusters consisting of Archaeolithophyllum, Petschoria, Komia (and Ungda- Wetheredella (We.), Girvanella (Gi.), Rothpletzella (Ro.) and Sha- rella), Donezella and several types of encrusting calcimi- movella (Sh.) in association with the dominant encruster, the algae incertae sedis, Claracrusta (Cl.). The dark micritic patches at the crobe are present. centre of the calcimicrobial dome exhibits some branching features and several components that most resemble micritized cyanobacte- rial/calcimicrobial tubes. b Rothpletzella encrusting a Donezella 3 Discussion thicket. Note that successive rows of Rothpletzella are concordant with the previous row. c A calcimicrobial community encrusting a ‘phylloid’ algae thallus. The encrusters comprise of Rothpletzella, The basal facies is characterised by well-preserved, dense and Claracrusta. Some Archaeolithoporella which is characterised by thickets of Donezella and Beresella, which were common in couples of dark and light laminae is present as are globular, cystose Bashkirian and Moscovian age carbonates (see Sect. 3.1). growths which resemble Wetheredella (black arrow). d Broken algal thalli and other bioclasts forming a weakly laminated packstone. e These carbonates typically occur as low relief mounds or Dvinella. Note the thin micritic envelope surrounding the bioclast and banks within marine lagoonal settings (Flügel 2004). Cor- the peloids that appear to be stacked vertically from the flat, upper toids and cyanoliths suggest a shallow subtidal back-reef surface of the specimen. Several of the peloids appear thrombolitic or environment—in the geological setting of these sediments aligned in nature and resemble the calcified bacteria Renalcis. Faint, but discernible filaments can be observed attached to the Dvinella (i.e. a foreland basin) it is envisaged that a tectonic struc- clast, these are possibly calcimicrobial sheaths. The evidence from ture (e.g. a blind thrust or the forebulge) may act as a bar- this photomicrograph suggests that the clotted peloidal material may rier in the place of a ‘traditional’ reef. The Barcaliente and well be micritised remains of cyanobacterial colonies. f Girvanella Valdeteja Formations (which were deposited to the north and south of the San Emiliano Fm.) may also have acted cortoids. Bioclasts are densely packed and consist mainly as a barrier. Behind these, a lagoonal (or lagoon-like) envi- of Donezella and Beresella thalli. Donezella occurs as both ronment is interpreted to have formed. The low-diversity of a branching three dimensional network and as broken frag- the microfacies would suggest a relatively stressed environ- ments. Some algal thalli are micritised with wall structures ment, the high number of algae and little else may suggest destroyed, leaving only outlines of the thalli. Bryozoans a brackish environment. Although rare, the occurrence of are relatively common in these samples and are often the attached Tetrataxis is further evidence for a stressed envi- largest bioclasts found. Bioclasts are frequently enveloped ronment (Cossey and Mundy 1990). The low diversity and by a micritic rim. Foraminifera and calcimicrobes are well simple nature of the taxa observed may indicate a pioneer preserved and appear with relatively high biodiversity, both community. The biological niche the organisms have filled mobile and attached forms of Tuberitinidae are also well pre- is certainly newly opened—as evident in the shift in lithol- served and relatively abundant. A few rare echinoid plates ogy (from clastic dominated to carbonate dominated). The are present. Calcimicrobial assemblages range from small disparity of microfacies evident between the basal and isolated clumps, to larger encrustations involving several mounds facies indicates a change in environmental condi- organisms. The cortoids in these samples are spherical in tions or possibly ecological tiering, whereby the Donezella nature and resemble ooids but for the lack of any laminae, dominated mounds successfully establish themselves in the they generally consist of a recrystallized spherical nucleus newly opened niche. with a micritic rim exhibiting calcimicrobial growth. These The light colour of the mounds (when compared to the samples show a biodiversity higher than samples from the darker basal, capping and off-mound beds) may indicate a mound facies, the microfossils present within these samples lower Total Organic Content value, perhaps alluding to a are well preserved. shorter formation period. The shape of the mounds doesn’t suggest any asymmetry. The boundary between mounds and 2.4 Non‑mound bearing units onlapping (or draping) beds—may also indicate that the mound accumulated faster than the surrounding capping Several of the carbonate units of the La Majúa Mb. were and basal facies. The mound facies is dominated by vari- observed to be barren of mud mounds. These units resem- ous packstones, containing mostly Donezellaceans. Peloidal bled either the basal facies or consist of bedded whole micrite and homogenous micrite are more common than in fossil wackestones and bioclastic packstones. The latter the basal beds, and in some cases are the major constituents. units appear as dark buff grey beds approximately 20 cm Much of the peloidal micrite is nonlaminated and appears as in thickness. Bedding is wavy in places and the limestones clotted growths and other “shrub” like forms. The presence can often be observed to transition into marls toward the of what are quite possibly calcified filamentous cyanobac- top of the beds. ‘Phylloid’ algae are common, found as terial sheaths suggests that peloids are possibly micritised laminar, to cyathiform and ‘leaf’-like growths (e.g. the remains of these and of the calcareous algae. Homogenous 1 3 234 Journal of Iberian Geology (2018) 44:225–241 micrite is often in association with faint thrombolytic tex- 3.1 Palaeoecology of the mounds tures, perhaps hinting at a microbial or algal origin. The heterolithic nature of the mound facies possibly indicates The mounds have a distinct biological community consist- a considerable variation in current flow, energy conditions, ing of Donezella (Fig. 3b, c) and several encrusting forms sea levels and/or nutrient levels during the deposition of the including: Rothpletzella, Claracrusta, Girvanella and occa- mud mounds. The biota associated with this facies remains sionally Wetheredella and Shamovella (Fig. 4a). In some mostly the same as the basal assemblage and is dominated examples Girvanella (Fig.  4e) can be observed between by non-complex alga and encrusting cyanobacterial forms. entwined Donezella thalli. In other cases, Rothpletzella This association of biota suggest a restricted, shallow, low and Claracrusta can be found to encrust, bind and assum- energy environment. Evidence for changing conditions is edly stabilise Donezella thickets. Evidence of calcimicro- present along with some higher energy events, although the bial sheaths encrusting Donezellaceans is present (Fig. 4f). environment is interpreted to remain dominantly restricted, Donezella, Claracrusta and Rothpletzella are described in shallow and low energy throughout the deposition of the some detail before considering the biological assemblage mounds. The capping facies consists of three distinct lith- as a whole. ologies: a packstone characterised by its lithoclastic compo- Donezella is observed as either whole, bush- or shrub- nent in addition to its bioclastic component which is found like growths often up to several centimetres in diameter, as ‘draped’ over several of the mounds. Above that is a wacke- sheet like encrustations of entwined thalli which appear to packstone with accessory bioclasts which include taxa typi- encrust the subjacent surface or as broken thalli which have cal of metazoan dominated carbonates. The relatively large, been reworked. There are two common species observed; reworked and broken bioclasts within the unit draped over the type species, Donezella lutugini Maslov and Donezella the mounds, plus the presence of increased clastics indi- lunaensis Rácz. Thalli are found to range from 50 to 275 μm cate a higher energy depositional environment than that of in diameter with an average diameter of 140 μm. Donezella the mounds. Some imbrication of bioclasts and lamination is generally observed within the mound and basal facies of is evident, suggesting stronger current activity. The occur- the observed mounds. Where Donezella is found in situ (as rence of several different foraminifera, bryozoans and more either a shrub like growth or as an encrusting sheet) it is Tuberitina may indicate the environment is becoming less always the dominant biota with very few other biotas found restricted. Donezellaceans are absent or found as broken in association. Maslov (1929) originally assigned Donez- and transported clasts within most of the capping facies. ella to the red algae (Johnson 1963), however, in succes- However, the occurrence of a similar biological commu- sive subsequent studies Donezella has been considered nity, and the presence of mud chips, it is envisaged that the to be: green (Codiaceae) alga (Rácz 1964), alga incertae depositional depth of the last unit was similar to the depth sedis (Rich 1967), Foraminifera (Riding 1979), calcareous of mound formation or slightly deeper, but with a higher sponges (Termier et al. 1977), microproblematica (Riding energy level. The higher energy may be a result of the envi- 1979; Chuvashov and Riding 1984), green (Paleosiphono- ronment becoming more open, making it difficult for the cladales) algae (Shuysky 1985; Ivanova 1999), green algae algae to establish and grow into shrub like thickets. The of incertae familiae (Groves, 1986), green (Dasycladales) mounds are thought to have been smothered by the draping algae (Deloffre, 1988) and pseudo-algae (Vachard et  al. (capping) units as a result of an increase of clastic mate- 1989). Recent work regarding Donezella refers to the genus rial to the basin, probably as a result of thrust propagation as green (Chlorophycophyta) algae (Mamet and Villa 2004), (causing the migration of the foreland basin and increased algae incertae sedis (Mamet and Zhu 2005), algospongia clastic supply.) The bedded units on top of the mounds (Vachard and Cózar 2010) or more commonly, microprob- indicate a relative sea-level rise; this is interpreted to be lematica (Samankassou 2001; Della Porta et al. 2002; Choh flexure related basin subsidence. The changes in microfa- and Kirkland 2006; Corrochano et  al. 2012). Donezella cies throughout the mounds suggest a steady sea level rise accumulations are often interpreted to have grown within a throughout deposition. warm, shallow (low to moderate energy, below fair-weather The presence of stromatactoid and fenestral cavities with wave base) environment but have been shown to thrive over internal early marine cement implies that there was constant a range of water depths up to 200 m (Della Porta et al. 2002; movement of water through the carbonates when they were Choh and Kirkland 2006; Corrochano et al. 2012). Envi- partially lithified, which would suggest the presence of a ronmental conditions from quiet to quite agitated have been current or wave activity. The peloids, bioclasts, microfos- interpreted (Rácz 1964; Bowman 1979). Donezella growths sils and cavities show little sign of compression, which may have been found to form in a range of environments rang- indicate an early lithification. ing from platform interiors to deep-slopes (Della Porta et al. 2002). This cosmopolitan habit and ability of Donezella to occupy a variety of environments, quickly establishing itself 1 3 Journal of Iberian Geology (2018) 44:225–241 235 and growing to maturity suggest that Donezella may be an Filaments branch often and can form fan like structures. example of the “opportunist” species of Connell and Slatyer Filaments have a thin micritic wall and are filled with (1977). The dominant occurrence of Donezella in the studied clear sparry cement. Rothpletzella grows as thin crusts samples indicates that it played a direct role in the forma- and oncoids (Feng et al. 2010) and platy growths (Wood tion of the carbonates of the San Emiliano Formation, it is 2000). Growths are laminar and undulate to mimic the observed to both baffle and bind (in association with several subjacent micro-topography (Fig. 4b). encrusting organisms, specifically Claracrusta, Rothpletzella Rothpletzella, Wetheredella and Girvanella in combi- and Girvanella) and at a millimetre scale may be considered nation have been known to grow together symbiotically to be a constructor (Fig. 3c). In association with microscopic (Wood 1948) forming what is essentially a microbial organ- encrusters, Donezella was a successful baffler, binder and ism where boundaries between the various components are constructor, in regards to mound formation, Donezella must impossible to see with the naked eye/optical microscope. be considered as an integral component and directly linked This microbial organism was mistakenly interpreted as a sin- to the nucleation, growth and success of carbonate mud gle fossil and named Sphaerocodium (Wood 1948; Adachi mound formation within the San Emiliano Fm. Beresella et al. 2007). Rothpletzella is a commonly observed calci- and Dvinella are both found in association with Donezella, microbe in this study and is often found as well preserved although they are rare. This important skeletal element of the laminations of juxtaposed ovoid filaments. The genus is mounds leads to the classification of this mound component often found in association with other encrusting alga and to be that of a Cluster mound (Riding 2002). calcimicrobes, especially Claracrusta and Girvanella and Specimens of Claracrusta (Fig.  3a) are observed as more rarely Wetheradella. These calcimicrobes often form laminar rows of irregular, often bean and ovoid to sub- variously layered growths. Several examples have a micro- quadratic shaped cells which have a honey or yellow sparitic wall and are filled with micrite with the filaments coloured calcite wall. The cells have been referred to as measuring up to 50 μm in height (width was not measured as tubular (Cózar 2005, and elsewhere), but here, appear to it is unsure if the cutting angle is completely perpendicular be ovoid or subquadratic, agreeing with Mamet and Villa to the filament). Several of the laminar growths of Rothplet- (2004). Individual cells have been observed to range from zella reach up to a millimetre in height. The genus is usually 10 to 110 μm but the average cell size is around 50 μm. found encrusting various bioclasts, in particular Donezella Encrustations have the characteristic growth form of a con- thickets (Fig. 4b) and ‘phylloid’ algae (Fig. 4c). This associ- cordant first lamination with subsequent encrusting lami- ation of encrusting forms is interpreted to play an important nations becoming more and more undulating; the space role in the binding and stabilisation of Donezella thickets between laminations is generally filled with homogenous within the San Emiliano Fm. mounds, although minimal in micrite or more rarely, other encrusting forms. Claracrusta volume, these micron to millimetre build-ups play a vital appears to be non-selective in what it encrusts, although role, and are considered as Agglutinated Microbial mound Donezella and Archaeolithophyllum appear to be the most (Riding 2002) elements. Both of the biological assemblage’s commonly encrusted objects. Several examples of entire present form build-ups of sediment with unique characteris- Donezella florets being encrusted have been observed and tics. These are individually interpreted as a Cluster mound the preservation level is usually particularly high for these element and an Agglutinated Microbial mound element. The Donezella specimens (Fig.  3a). These florets are gener - introduction, and use of the term, ‘Composite mound’ to ally associated with homogenous micrite and constructed encapsulate this overlapping of mound structure allows for cavities which have been selectively dolomitised. Clarac- the appreciation of both of these important structural fea- rusta in association with Rothpletzella, Girvanella and to tures of the mounds, without giving gratuitous emphasis to a lesser extent Wetheredella (Fig. 4a), are considered to be the volumetrically dominant element (i.e. in this case, the important binders and stabilisers of Donezella growths and Cluster mound element). therefore the role they play within the formation of carbon- The association of biotas presumably formed wave ate mounds within the San Emiliano Formation (particu- resistant, topographic mounds. The presence of bound larly within the San Emiliano Valley) is very important. and stabilised Donezella/encruster assemblages may have Rothpletzella (Fig.  4a) is characterised by laminar encouraged consecutive colonisation of the same area by sheet-like growths of branching tubes which lie parallel subsequent Donezella and encrusters by acting as a substrate to the substrate. In transverse sections Rothpletzella often (Fig. 4b, 5). Other biota observed consist of low diversity resembles a chain or bead like structure with individual assemblages and often consist of biota which may represent filaments ovoid in shape, often resembling beans or sau- organisms living in a stressed environment (e.g. attached sages. Individual filaments have been reported to range Tetrataxis). The concentration of Donezella and its ally taxa from 13 to 37 μm (Adachi et al. 2007) in diameter. Lon- may have directly contributed to the low biological diver- gitudinal cuts reveal dichotomous branching of filaments. sity; low diversity assemblages characterised by dominant 1 3 236 Journal of Iberian Geology (2018) 44:225–241 Encrusting forms (Agglutinated Microbial mound element) e.g. Rothpletzella ,, Claracrust aG irvanella Donezella thicket (Cluster mound element) Cavity Fig. 5 A schematic representation of mound formation. (1) Donezella interaction of Donezella growths and laminar encrusters. Note that it grows and is broken and abraded by local wave action and currents. is not individual Donezella thalli that are encrusted, but the ‘shrub’ (2) Donezella grows on top of the accumulated, broken Donezella. like growths as a whole. The approximate scales for the individual (3) Environmental conditions deteriorate and laminar encrusters col- components differ, Cluster mound elements range from millimetre onise the substrate provided by the Donezella thickets. (4, 5 and 6) to centimetre in vertical profile, Agglutinated Microbe mound com- Environmental conditions continue to fluctuate resulting in the repeti- ponents are often micron to millimetre in vertical profile. The area tion of Donezella growth, and stabilisation by encrusting forms. (7) shown in 7 is approx. 2 mm across A close up of the area circled in (6), schematic representation of the algal communities have been attributed to the exclusion of of mounds from the Bernesga Valley (Samankassou 2001) other biota from the living space via both chemical defence are absent, or greatly reduced, from the San Emiliano Valley. (poisons produced by the algae) and through the occupation Several, distinct palaeoecological assemblages are rec- of all available living space (Toomey 1976, 1991; Forsythe ognisable from the studied material. Mound and off-mound et al. 2002; Samankassou and West 2003; Enpu et al. 2007). carbonates (i.e. those carbonates found within the same unit Petschoria, Archaeolithophyllum, Komia and Ungdarella as mounds) exhibit similar biological communities, however, were all observed to be present within the mound facies, in distinct variations were observed between the abundance of far lesser numbers than the dominant biotas described above. certain biota between the communities. Bedded lithologies There appears to be a reduced population or complete lack of are associated with a palaeoecological assemblage distinct grazers within environments where they would normally be to those from mound bearing strata. expected. Bryozoans, corals and other biota often associated with Pennsylvanian aged carbonate deposits are generally 3.2 Initiation, growth and demise of the mounds restricted to the mixed clastic and bedded carbonate lith- ologies and are rarely seen within mounds within the San The Composite mounds from the type locality of the San Emiliano Valley. Dasyclad algae, foraminifera, bryozoans, Emiliano Fm. initiated as the result of several interacting corals and other shelly fauna are more common within the factors. The combination of a newly opened niche within a Bernesga Valley mounds (Samankassou 2001). Thartharella restricted, warm and shallow basin encouraged the growth and calcisponges, which were reported as a main component of pioneer or opportunist organisms. The absence of detri- tal clastic input allowed for carbonate producers to fully 1 3 Journal of Iberian Geology (2018) 44:225–241 237 Table 1 Summary of documented occurrences of Donezella related buildups Mound description Age Location References Low-energy buildups Lower Permian Central Texas (USA) Wiggins (1986) Donezella mounds with siliceous sponges, Lower Permian Ouachita Mountains (Oklahoma, USA) Choh and Kirkland (2000) Archaeolithophyllum and worm tubes Donezella-Siliceous sponge dominated Lower Pennsylvanian Ouachita Mountains (Oklahoma, USA) Choh and Kirkland (2006) carbonate buildup with worm tubes Donezella low bank to mound controlled Moscovian Hueco Mountains (W Texas and SE New Lambert (1986) and Lambert by water depth and energy Mexico) and Stanton (1988) Skeletal microbial Donezella—Tubiphytes Bashkirian California (USA) Watkins (1999) (= Shamovella) biostromes Donezella baffled mounds associated with Bashkirian—Moscovian Cantabrian Mountains (Spain) Rácz (1964) Petschoria, Komia and Archaeolitho- phyllum Donezella mounds with Komia, Ungda- Bashkirian—Moscovian Cantabrian Mountains (Spain) Bowman (1979) rella, encrusting foraminifers, ‘phylloid algae’ and dasyclads Mounds sparse of Donezella (5–10%). Bashkirian—Moscovian Cantabrian Mountains (Spain) Riding (1979) Donezella mounds with Petschoria, Bashkirian—Moscovian Cantabrian Mountains (Spain) Eichmüller (1985) Komia, Ungdarella, encrusting foramini- fers, Archaeolithophyllum and bryozoans Skeletal-microbial boundstone. Don- Bashkirian—Moscovian Cantabrian Mountains (Spain) Samankassou (2001) ezella, agglutinated worm tubes and calcisponges Skeletal microbial Donezella—Tubiphytes Bashkirian—Moscovian Cantabrian Mountains (Spain) Samankassou (2001) (= Shamovella) mounds Clotted microbial peloids with Donezella Pennsylvanian Cantabrian Mountains (Spain) Samankassou et al. (2013) and ‘phyloid algae’ Donezella pack- wackestones mounds with Bashkirian—Moscovian Cantabrian Mountains (Spain) This study Rothpletzella and Claracrusta (Compos- ite mound) Donezella mounds with Beresella, Late Surpukhovian to Ellesmere Island (Canadian Arctic Archi- Davies and Nassichuk (1989) Komia/Ungdarella, bryozoans, crinoids, early Bashkirian pelago) encrusting foraminifers and ostracods Skeletal Donezella—Tubiphytes (= Sham- Carboniferous Kazakhstan Cook et al. (2007) ovella) mounds Donezella and Ungdarella buildups Bashkirian Western Urals (Russia) Proust et al. (1996) Mounds, bioherms and biostromes are all included in this table. It should be noted that Riding (1979) concluded that Donezella was not respon- sible for the formation of the studied mounds. Mounds are grouped into geographical locations establish themselves, resulting in an ‘in house’ production Donezella growth when environmental conditions became of carbonate mud. Growth of the mounds resulted from the more tolerable. The mounds ceased to form when detrital interaction of the genus Donezella and several encrusting sediment deposition returned to the basin. Microfacies anal- forms including the genera Rothpletzella and Claracrusta. ysis indicated that when detrital input increased the environ- Fluctuating environmental conditions allowed the encrust- ment became less restricted with encrusting forms becoming ing forms to bind and stabilise the framework building and less common whilst (relatively) large and mobile metazoans carbonate mud baffling Donezella growths. Donezella grew become more common. The Donezella/encruster relation- during ‘normal’ environmental conditions within the basin ship ceased and mounds could no longer form. The major whilst the encrusters, which were direct competitors for the factors responsible for mound formation can be attributed to: environmental niche, flourished during deteriorating condi- a] biological community present, b] a restricted basin and c] tions (Fig. 5). Adachi et al. (2007) observed a similar rela- a lack of clastic input. tionship between Graticula-like red algae, Rothpletzella and laminar stromatoporoids in Lower Devonian age bindstones. The resulting stabilised (and likely wave/current resistant), encrusted Donezella thickets provided a substrate for further 1 3 238 Journal of Iberian Geology (2018) 44:225–241 relationship controlled by fluctuating environmental condi- 3.3 Comparison to other mounds tions. Off mound sediments (basal and capping facies) were commonly associated with higher biodiversity, including Throughout the Pennsylvanian Series, carbonate units asso- ciated with a dominance of Donezella growths are relatively biota such as bryozoans, corals and foraminifera. Sedi- mentary deposition of the mounds was characterised by common. Several examples also exist from the Permian, although these are far less abundant. These carbonates autochthonous micrite, in the “in-house” manner, typical of many microbial mounds. The process identified as being have been reported to form a wide variety of geometries and features including mud mounds (of various classifica- responsible for the nucleation and growth of the mounds is a dynamic relationship between the Donezella dominated tions), bioherms, biostromes and as carbonate ramps and platforms (Della Porta et al. 2002; Choh and Kirkland 2006 community and the laminar encrusting form dominated community. As indicated by microfacies analysis the envi- and references within). Other mounds (including bioherms and biostromes) containing Donezella are summarised in ronmental conditions of deposition fluctuated. Where con - ditions were near ‘normal’, Donezella flourished, acting as Table 1 and are loosely grouped into geographical location. Several common biological associations are apparent. An a microscopic framework builder as well as a baffler. Envi- ronmental ‘degradation’ led to the encruster guild becom- association with Petschoria, Archaeolithophyllum, Komia and Ungdarella is plain to see within the majority of these ing dominant. Laminar encrusters grew in a non-selective manner covering the majority of grain types. Commonly mounds. Several of the mounds, including those from the Bernesga Valley, are associated with worm tubes and sili- the encrusters could be found atop of Donezella thickets, stabilising and protecting the (supposedly) delicate Donez- ceous sponges. No other mounds (other than those in this study), bioherms or biostromes have a reported biological ella thalli. When environmental conditions returned to suit Donezella the laminar encrusters provide a suitable substrate association between Donezella, Claracrusta, Rothpletzella, Girvanella and other microscopic encrusting forms. The for new growth. The Composite mounds ceased to form due to drowning by a deepening sea-level or by an increase in Composite mounds of the San Emiliano Valley have com- positional and formational mechanisms that are unique when clastic sediment resulting in a clastic-rich bed draped over the mound. The mechanism responsible for mud mound pro- compared to other mounds with a dominant Donezella com- ponent, as well as other known populations of mounds from duction within the San Emiliano Fm. is unique for known Donezella dominated mounds, and comparable Carbonifer- the Carboniferous. The depth and/or environmental setting at which the San Emiliano mounds formed may be the cause ous mounds in general. The term Composite mound is used here to describe mounds bearing two or more of the struc- for this unique biological assemblage, most other Donezella mounds have been interpreted to form at a greater depth tural characteristics defined by Riding (2002). below sea level. Acknowledgements The author would like to express his gratitude to the technical staff in GGE at Keele University for the preparation of all thin sections used. This study forms part of a PhD completed in 2015 at the University of Keele under the supervision of Dr Michael Montenari. The author is grateful to Sergio Rodriguez for providing 4 Conclusions constructive comments which greatly improved the manuscript. Javier Martín-Chivelet is thanked for his editorial assistance when this manu- Three distinctive microfacies are recognised between the script became a ghost in the system. Alex Nobajas is thanked for his basal, mound and capping material of the carbonate units assistance with the Spanish translation of the abstract. of the San Emiliano Fm. at its type point, plus an addition a Open Access This article is distributed under the terms of the Crea- fourth facies; carbonate units which are barren of mounds. tive Commons Attribution 4.0 International License (http://creat iveco Composite mounds from the San Emiliano Fm. are inter- mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- preted as skeletal-microbial pack-wackestones with various tion, and reproduction in any medium, provided you give appropriate types of cavity networks. The dominant palaeontological credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. communities were low diversity assemblages representing a pioneer or opportunist community living in a newly opened (relatively restricted) niche. 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Upper Paleozoic biostromes in island-arc carbon- Rich, M. (1967). Donezella and Dvinella, widespread algae in Lower ates of the eastern Klamath terrane, California. Paleontological and Middle Pennsylvanian rocks in East-Central Nevada and Research, 3, 151–161. West-Central Utah. Journal of Paleontology, 41, 973–980. Wiggins, W. D. (1986). Geochemical signature in carbonate matrix and Riding, R. (1979). Donezella bioherms in the Carboniferous of the their relation to deposition and diagenesis, Pennsylvanian Marble southern Cantabrian Mountains, Spain. Bulletin des Centres de Falls Limestone, Central Texas. Journal of Sedimentary Petrol- Recherches Exploration-Production Elf-Aquitaine, 3, 787–794. ogy, 56, 771–783. Riding, R. (2002). Structure and composition of organic reefs and Wilson, J. L. (1975). Carbonate Facies in Geologic History (p. 379). carbonate mud mounds: concepts and categories. Earth Science Berlin: Springer. Reviews, 58, 163–231. Wood, A. (1948). “Sphaerocodium”, a misinterpreted fossil from the Wen- Samankassou, E. (2001). Internal structure and depositional environ- lock Limestone. Proceedings of the Geological Association, 59, 9–22. ment of Late Carboniferous mounds from the San Emiliano For- Wood, R. (2000). Novel paleoecology of a post extinction reef: Famen- nian (Late Devonian) of the Canning basin, northwestern Aus- mation, Cármenes Syncline, Cantabrian Mountains, North Spain. tralia. Geology, 28, 987–990. Sedimentary Geology, 145, 235–252. 1 3 Journal of Iberian Geology (2018) 44:225–241 241 Affiliations Steven L. Rogers * Steven L. Rogers School of Geography, Geology and the Environment, s.l.rogers@keele.ac.uk University of Keele, William Smith Building, Keele, Staffordshire ST5 5BG, UK 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Iberian Geology Springer Journals

A novel population of composite mounds: their initiation, growth and demise. San Emiliano Formation, Cantabrian Mountains, Spain

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Earth Sciences; Earth Sciences, general; Geology; Structural Geology
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Abstract

Purpose Mounds from the Pennsylvanian aged San Emiliano Formation (Cantabrian Mountains, Spain) are commonly well exposed. These mounds range from 2 to 50 m in height and are observed to be primary geological features. The mounds are described and classified and the factors and controls of mound nucleation, growth and demise have been established. Methods Microfacies analysis of 177 thin sections has revealed the composition of the mounds and surrounding carbonates. Results Composite mounds, exhibiting characteristic components of both Cluster mounds and Agglutinated Microbial mounds are described. The mounds are skeletal-microbial/pack-wackestones. Peloidal, homogenous and clotted micrites are the main sedimentological constituents of the mounds. Microfossils are dominant with Donezella, Claracrusta, Rothpletzella and Girvanella being common. Small foraminifera, bryozoans, corals and algae are all present within the mounds, but are more common within off-mound carbonates. Conclusions The formation of the mounds was controlled by a dynamic relationship between Donezellacean algae, and microscopic encrusters, a bio-mechanism not observed in mud mounds elsewhere. Fluctuating environmental conditions lead to the alternate dominance between the two groups, resulting in accretion and stabilisation of carbonate muds. These mounds are compositionally different to their Pennsylvanian counterparts. Keywords Composite mound · Agglutinated microbial mound · Cluster mound · Donezella · Encrusting biota · Microfacies analysis · Pennsylvanian Resumen Propósito Los montículos de la Formación San Emiliano de edad Pensilvaniense (Cantabrian Mountains, España) están comúnmente bien bexpuestos. Estos montículos tienen una altura de 2 a 50 m y se observa que son rasgos geológicos prima- rios. El los montículos se describen y clasifican y los factores y controles de la nucleación, el crecimiento y la fallecimiento tienen que ver con los siguientes factores se ha establecido. Métodos El análisis de microfacias de 177 sección delgada ha revelado la composición de los montículos y carbonatos circundantes. Resultados Montículos compuestos, que exhiben los componentes característicos de ambos montículos Cluster y Microbial aglutinado. Se describen los montículos. Los montículos son esqueléticos-microbianos lodogranular/lodosa. Peloide, homo- géneo y coagulado micrites son los principales componentes sedimentológicos de los montículos. Los microfósiles dominan con Donezella, Claracrusta, Rothpletzella y Girvanella son comunes. Pequeños foraminíferos, briozoarios, corales y algas son todos ellos presente dentro de los montículos, pero son más comunes dentro de los carbonatos fuera del montículos. Conclusiones La formación de los montículos estaba controlada por una relación dinámica entre las algas donezellaceanas, y incrustaciones microscópicas, un mecanismo biológico no observado en montículos de lodo en otras partes. Medio ambiente fluctuante condiciones conducen a la dominación alternada entre los dos grupos, resultando en la creación y estabilización de lodos de carbonato. Estos montículos son compositivamente diferentes a sus contrapartes de Pensilvaniense. Palabras claves Montículo compuesto · Montículo microbiano aglutinado · Montículo de cúmulos · Donezella · Biota incrustante · Análisis de microfacias · Pensilvaniense Extended author information available on the last page of the article Vol.:(0123456789) 1 3 226 Journal of Iberian Geology (2018) 44:225–241 1 Introduction1.1 Geological setting Carbonate mud mounds are a type of marine buildup which The San Emiliano Formation is located within the Canta- brian Mountains (Fig. 1), León Province, Northern Spain, originate by the successive vertical and lateral accretion of carbonate muds (Monty 1995). This type of buildup became around the small rural town from which it takes its name. The area sits within the Iberian Massif, which itself has increasingly recognised during the 1970s and 1980s when it was found that various mound types, and their associated been informally arranged into six tectonostratigraphic units. These units were based upon differences in facies associa- structures, microfacies and biotas extended a diverse geo- logical range and often included features which were poorly tions, structural styles, metamorphism and magmatism (Lotze 1945). The northern most of these units is where understood or unknown at the time (Wilson 1975; James 1980; Longman 1981; James 1983; Lees 1988; Monty et al. the study area is located and is called the Cantabrian zone (Lotze 1945; Pérez-Estaún et al. 1988). The Cantabrian zone 1988; Swennen 1988). Mud mounds are complex structures which respond to facies changes and have been found to consists of an incomplete Cambrian–Ordovician sedimen- tary sequence, a Silurian and Devonian sequence which record shifts in lithofacies and biofacies (Monty 1995). A wide variety of reef and mound types, ranging from is complete toward the west but is missing elsewhere and several thousand metres of strata spanning the Tournaisian Waulsortian-type mounds, biodetrital and microbial mounds, stromatactoid mounds, Tepee Buttes and a variety of oth- to the Gzhelian Stages (Bahamonde et al. 2002). The Can- tabrian zone (Lotze 1945; Pérez-Estaún et al. 1988) is fur- ers, have been reported both temporally and spatially (e.g. Monty 1995; Pratt 1995; Kaufmann et al. 1996; Neuweiler ther subdivided into five tectonic units: the Fold and Nappe, Central Asturian Coalfield, Ponga Nappe, Picos de Europa et al. 2001; Krause et al. 2004; Hebbeln and Samankassou 2015). Riding (2002) reviewed the historical classifica- and Pisuerga-Carrión provinces. The San Emiliano Forma- tion belongs to the La Sobia-Bodón/Aramo units of the Fold tion of organic reefs, in an attempt at the standardisation of what is a fraught nomenclature. The mounds described here and Nappe Province (Pérez-Estaún et al. 1988; Bahamonde et al. 2002). The formation was deposited within the fore- are classified according to that standardisation. Individual populations of mounds are generally distinguished by their deep depozone of a Variscan foreland basin. The San Emiliano Fm. (Figure  2) consists of at least varying biological, sedimentological and tectonic settings, a distinction between Palaeozoic and Mesozoic mounds also 1800 m (Bowman 1982) of Pennsylvanian aged (Bashkirian to Moscovian), alternating deltaic clastics and shallow exists (Schmidt et al. 2001). Due to the polygenetic nature of reefs and mud mounds, analysis of individual popula- marine carbonates, with rare, thin coal bands (Bowman 1979). At the San Emiliano type locality (i.e. the San tions is fundamental to understanding the factors influencing their presence (e.g. Flajs et al. 1995). The nomenclature of Riding (2002) is based on reef structure. Presented here is Fig. 1 a A geological map showing the area of study. Samples were a population of mounds from the Pennsylvanian aged San obtained from carbonate units within the San Emiliano Formation Emiliano Formation, Cantabrian Mountains, Spain. These (i.e. the blue bands), coordinates are in UTM. Specific localities, mounds have a unique growth dynamic controlled by two where individual mounds were sampled, are indicated by white stars, competing ecological assemblages which has not yet been with the mound imaged in B shown as a red star. The inset shows the location of the study area (indicated by asterisk symbol) in regards described. The mounds observed fall into the category, as to the Spanish and Portuguese landmasses. (Modified after Suárez defined by Riding (2002), of Cluster mounds, although small et  al. 1991). Key in right-hand column bellow. UTM coordinates volumes of the mounds, dominated by encrusting biota are of the named villages (from west to east: Robledo de Babia—29T considered as Agglutinated Microbial mound components. 738542.31 4761967.82; Cospedal—29T 740471.87 4762102.02; La Majúa—29T742542.18 4763067.04; San Emiliano—29T Due to this overlap, the term ‘Composite mound’ is used to 744642.62 4762013.24; Candemuela—30T 255531.56 4763579.80; encapsulate the differing structures observed. The nucleation Villargusan—30T 2565587.05 4764801.92; Pinos—30T 257036.76 of, growth and subsequent demise of these San Emiliano 4763119.49. b Picture and annotated field sketch of a typical mound Fm. mounds is also discussed. from the San Emiliano Formation, due to this mounds location at the side of a road, and the cut through of the mound provided, it is the Pennsylvanian ‘carbonate mud mounds’ from the Canta- most accessible outcrop that displays basal, mound, capping and off- brian Mountains have been described by Bowman (1979), mound facies in the Valley (29T 744467.52 4763114.34). c A field Riding (1979) and Samankassou (2001) additionally, Hensen sketch highlighting the geometrical relationships between mounds. et al. (1995) described both primary and diagenetic mounds The sketch is of 1 km of the carbonate unit that stretches continuously from just south of the village of Cospedal, to the village La Majúa. from the area. The sketch begins 500  m south-east of Cospedal (29T 741166.91 4761689.94) and encompasses the ridge, eastward toward La Majúa (29T 742631.65 4762673.57). The sketch was taken from the north- ern-most point of the carbonate ridge, approx. 200  m south of the mound bearing unit (29T 742640.14 4761171.31) 1 3 ◂ Journal of Iberian Geology (2018) 44:225–241 227 29T 30T N S Capping Unit 1m Mound Clastic sequence Studied mound locations, mound imaged in ‘B’ Clastic Mound? sequence Bedding laps Fractured/ Poorly Bedded Mounds Mounds Trees ‘rubble’ onto Mounds 25m Mounds Mounds Poorly Bedded Mounds Mounds Far fewer Mounds, more bedding 1 3 232/41/NW Mound? Bedded Limestone Bedded Limestone 200/49/W Vegetation 228 Journal of Iberian Geology (2018) 44:225–241 WestphalianA (W/SW/WB) Moscovian (C) WestphalianA/ Namurian C (W) early Verisky/ Kashirsky (VGV/R) late Westphalian A (WB) late Asatausky (VGV) early Westphalian A(GN) early WestphalianA (GN) WesphalianA (WB) early Westphalian A(GN) middle/late Bashkirian (MCWP) Tashatansky/Asatausky boundary (VGV) early Westphalian A(GN) earlyTashastinsky (VGV) early Westphalian (B) Namurian/ Westphalian boundary (WB) earlyTashastinsky (VGV) 1* Key 300 late Yeadonian (N) Coal band Plant fragment Deltaic Fusulinid foram clastics Miospore *bold numbers refer to Limestone the limestone units of Brachiopods unit the La Majúa Member Fig. 2 Litho- and biostratigraphy of the San Emiliano Formation tínez Chacón and Winkler Prins (1979); N Neves in Bowman (1982); at the type point, San Emiliano. The age of the Formation is based R Rumjantseva in Wagner and Bowman (1983); SW Stockmans and upon various authors’ stratigraphic studies, alluding to a Bashkirian Williér (1965); VGV Van Ginkel and Villa (1996); W Wagner (1959); to early Moscovian age for the La Majúa Member. Specimen bear- WB Wagner and Bowman (1983). Miss. Mississippian, Upp. Upper, ing limestone units are labelled 1–8. B Bowman (1982); C Carballeira Serp. Serpurkhovian, Fm formation, Mb member. Modified after Van et al. (1985); GN Gueinn and Neves in Bowman (1982); MCWP Mar- Ginkel and Villa (1996) Emiliano valley) Bowman (1982) subdivided the formation (Fig. 1) are not observed within all eight of the limestone into three constituent members, based upon the proportions units of the La Majúa Member and seem to be confined to of clastic and carbonate sediments. These are the [1] Pinos, units 1, 4 and 8 (Fig. 2). Non-mound bearing units ranged [2] La Majúa and [3] Candemuela Members. The [1] Pinos from off-mound carbonates typical of the formation or ‘Phyl- Member consists of black shales passing into siltstones loid’ algal bearing wackestones. The [3] Candemuela Mem- with thin sandstone interbeds; the member is approximately ber consists of deltaic and shallow marine clastics with coal 250 m thick. The [2] La Majúa Member consists of alter- beds and seatearths; the member is approximately 500 m nating regressive deltaic and shallow marine clastics (with thick. some thin coal bands) separated by eight major limestone Thirty kilometres east of the San Emiliano Valley, in the units (Bowman 1979); the member is approximately 1050 m Bernesga Valley, a Moscovian aged succession, comprising in thickness. Fusulinid Foraminifera collected from the mainly turbidites, carbonate debris flows and shallow water upper part of the La Majúa Member have indicated a late siltstones can be found exposed in the northern flank of the Bashkirian age (van Ginkel and Villa 1996). Mud mounds Cármenes syncline (Bowman 1982; van Ginkel and Villa 1 3 Carboniferous Miss. Pennsylvanian Upp. Lower Middle Serp. Bashkirian Moscovian Late Verisky Kashirsky Early Namurian (Yeadonian) Westphalian A San Emiliano Formation Valdeteja Fm Pinos Mb La Majúa Member Candemuela Mb Journal of Iberian Geology (2018) 44:225–241 229 1996). This sedimentary succession is commonly known as Carbonate units from the San Emiliano Fm. which are bar- the Lois-Cigüera Formation (Rácz 1964; Brouwer and van ren of carbonate mud mounds are also, briefly, considered. Ginkel 1964; de Meijer 1971; Van De Graff, 1971; Bow - man 1982, 1985), and is a stratigraphic equivalent to the 2.1 The basal facies San Emiliano Fm. The Lois-Cigüera Fm. consists of the basal Villanueva Beds, the middle “Caliza masiva” unit and The basal facies often consists of approximately 1  m of the top Villamanín Beds. Brouwer and van Ginkel (1964) bedded limestones, individual beds range from 13 to 32 cm erected the Lois-Cigüera Fm. on the basis that the sediments thick. In hand specimen the basal facies appears as a blue- were mostly detrital (tubiditic and debris flow). Moore et al. grey colour. Numerous macro fossils are visible, with shelly (1971) reported the San Emiliano Formation as missing or fauna, rugose and auloporid corals and algae identified. The reduced along the northern limb of the Cármenes syncline, limestones are micritic and exhibit a porcelaneous fracture. and reported that the “rhythmic units” of the San Emiliano The basal microfacies is a skeletal-microbial bafflestone Fm. (as observed in the vicinity of San Emiliano) are not consisting of intertwining (often bio-cemented) thickets of found in the Villamanín area and are absent (or reduced) as branching Donezella (with some Beresella) thalli, associated a result of uplift and erosion, the responsible uplift believed with birdseye, fenestral and some poorly developed primary to be associated with the Cantabrian Block (as defined by cavity fabrics filled with sparite. Calcimicrobes found within Radig 1962). Bowman (1982) reports several lithologies some of the cavities occur as clumps of sheaths within a (turbidites, shallow water siltstones and carbonate debris) homogenous and peloidal micrite. The primary cavities which are recognised as being the same age as the San Emil- appear to be part of an irregular, porous network within the iano Fm. Due to the sufficiently distinct facies and distribu- Donezella thickets. Areas around the Donezella thickets are tion the Lois-Cigüera Fm. is separated from their San Emil- dominated by bodies of encrusting Claracrusta, Donezella, iano valley counterparts (Bowman 1982; van Ginkel and Girvanella and probably encrusting foraminifera (Fig. 3a). Villa 1996). The San Emiliano Fm. was often mistakenly The encrusting appears non-selective and coats algae, reported from the Villamanín area previous to Moore et al. foraminifera, Girvanella and mudchips. Cortoids (Fig. 3b) (1971) and Bowman (1982). The distinction is still not rec- and cyanoliths are present within the samples particularly ognised by some authors (Riding1979; Samankassou 2001). where mudchips have undergone encrustation. Donezella Outcrops that were investigated and sampled for this thalli are mostly well preserved can be observed as either: study are along the well exposed limestone ridges of the in situ as a three dimensional “shrub” like framework with La Majúa Mb. located within the San Emilliano Valley. 277 abundant baffled micritic sediment and free floating micro- Samples were obtained from a total of 24 locations across fossils (Fig. 3b), or as broken fragments, these fragments the valley. The majority of samples were taken across the show little sign of wear. The micrite-walled calcimicrobe laterally extensive exposure of the uppermost carbonate Girvanella is commonly well preserved as small tangled unit of the La Majúa Member (unit 8, Fig.  2) of the San clumps within thin layers of microspar which acts as a bio- Emiliano Fm. (Figure 1c) in addition to detailed sampling cement between algal thalli, a micritic envelope is present at localities where mounds are well exposed and accessible on most bioclasts (Fig. 3a, b). Other than the dominant Don- (Fig. 1a). Sampling included coverage of the basal, mound, ezellaceans and associated encrusting calcimicrobes (Girva- capping and off-mound facies, where possible. The results nella and Rothpletzella), Archaeolithophyllum, Petschoria, presented are based on a detailed microfacies analysis of Komia (and Ungdarella), Beresellids, Tuberitinidae and rare, 177 thin sections. small foraminifera’s (including Tetrataxis) and ostracods are found. 2 Microfacies analysis 2.2 The mound facies Mound bearing units exhibit similar vertical profiles, con- The Composite mounds of the San Emiliano Fm. are a union sisting of approximately 30  cm to 3  m of bedded lime- of mound structures: carbonate mud with in-place skeletal stones (basal facies), followed by 2–50  m of carbonate elements (Cluster mound component) with subordinate mud mound/s (mound facies) lacking in primary geological volumes of laminar sediment trapping and binding micro- features, which is in turn overlain by further bedded (up bial elements (Agglutinated Microbial mound component). to 10 m) limestones (capping facies). In places, a bed of The sediments are best described as a skeletal-microbial 10–30 cm thick siliciclastic rich carbonate can be observed pack-wackestone often associated with fenestral cavities to onlap and drape over the mounds (see Fig. 1b for a typical (Fig. 3c, e) and occasional stromatactoid cavities (Fig. 3f). example). The basal facies, mound facies and capping facies The mounds vary in size ranging from 2 to 50 m in thickness and the relationship between them are all considered here. and are steep sided, lens to mound shaped. Mounds appear 1 3 230 Journal of Iberian Geology (2018) 44:225–241 1 3 Journal of Iberian Geology (2018) 44:225–241 231 ◂Fig. 3 Photomicrographs of the basal and mound facies of carbonate or ‘shrub-like’ growths are (Fig.  3a). Encrusting sheets mud mounds from the San Emiliano Formation. a Claracrusta speci- occur in a repetitive manor in association with alternat- mens creating encrusting sheets of tubes which can be seen to branch, ing Donezellacean growths. Biodiversity observed within the sheets encrust several substrates showing no preference. The this facies is low; associated with the highly dominant specimen exhibits the characteristic growth form of Claracrusta, the first few crusts are flat with few undulations, with subsequent crusts Donezellaceans are rare Tuberitinidae and foraminifera, becoming increasingly convoluted. Encrusted substrates include: mud plus Komia, Archaeolithoporella and encrusting Clarac- chips, Donezellacean thickets, Petschoria (a red ungdarellid algae) rusta, Rothpletzella and others (Fig. 4a). Some selective and Girvanella. b Entwined Donezellacean algae, several thalli are dolomitisation has occurred, often as contact or floating cemented together with clotted micrite and bio-cement. A cortoid (centre) exhibiting a characteristically uneven surface with microbor- equigranular rhombic to non-rhombic crystals within the ings and encrusters. c Several fenestral cavities. Homogenous micrite blocky cement filling the fenestral cavities. is the dominant sediment with some peloidal and clotted micrite. The Microfacies investigation of the mounds revealed two cavities have a sparitic fill with no evidence of a stromatactoid fab- sub-facies that occur in a repetitive pattern. The first ric. The geometry of the cavities (flat base, undulating roofs) in the bottom half of the figure may suggest some sort of hard ground or sub-facies (sub-facies a) is a fenestral pack-wackestone surface on which the base of the cavity network has formed, with the characterised by non-laminated peloidal and clotted roof being defined by algal growth. d Sub-rounded micritic grains. micrite and encrusted Donezellacean thalli (Fig.  3c, d). Some areas consist of poorly sorted grains with distinct size differ - The sedimentary framework consists of both sparitic and ences. e Several examples of laminar stromatactis cavities, with char- acteristic flat bases, undulose roofs, internal sedimentary fill and both micritic peloids and although they are non-laminated fibrous and blocky calcite cements. In this example the cavities are they are often thromboidal or ordered. Clotted micrite very closely spaced, with the top of one set of cavities forming the occurs in places, as does homogenous micrite, both are base of the next. f A stromatactoid cavity. This example of stromatac- commonly found as cavity fill. Donezella (and accessory tis appears surrounded by Donezellacean algae, many examples of cavity fabrics from this study show the same relationship, with the Beresella) are well preserved and relatively large, form- algae directly responsible for the construction of the cavity frame- ing isolated and aligned thickets, sub-facies a forms the work Cluster mound (Riding 2002) component of the mounds. The second sub-facies (sub-facies b) is characterised as either isolated buildups or as superimposed complexes by encrusting calcimicrobes, namely, Rothpletzella and of two or more mounds (Fig. 1c). Individual mounds do not Claracrusta with occasional Gir vanella and Wethere- show any evidence of asymmetry, preferred orientation or della growths (Fig. 4a–c). These encrustations occur as thickness changes. Mounds are micritic and in hand speci- laminar sheets of branching filaments; both tightly, and men are lighter in colour than the basal and capping facies loosely packed growths with homogenous and peloidal counterparts. Rare occurrences of siliciclastic conglomerates micrite and some sparry cement between individual lami- were observed within mounds (an accessible outcrop can be nae occur. Rare foraminifera (mostly Tetrataxis, Lasio- found at 29T 738867.33 461336.25) towards the west of the discus and very rare Bradyina) have been observed, often formation (near the village of Robledo de Babia.) between entwined Donezella thali. Sub-facies b forms the The mound facies is a fenestral packstones (Fig.  3c) Agglutinated Microbial mound (Riding 2002) component characterised by non-laminated peloidal and clotted mic- of the mounds. Sub-facies b is volumetrically less signifi- rite (Fig. 3d) associated with variously sized fenestral cav- cant than sub-facies a, with encrusting sheets often being ities and bioclasts. Cavities are spar-filled and range from micron to millimetre in scale (Fig. 4a–c). small ‘birdseye’ to larger, irregular voids (Fig. 3c, e). Sev- eral cavities have a stromatactoid fabric; flat based voids 2.3 The capping facies and off‑mound facies with undulating roofs, often with fine sediment fill at the bottom (Fig. 3f). Isolated clasts of sedimentary framework The capping facies and off-mound facies (those carbonates can be found within some of the larger cavities, as can a flanking or occupying space between mound bearing strata) rare, sparse, coating of pyrite. The sedimentary framework consist of bedded carbonates. The beds directly above the consists of both sparitic and mictitic peloids, the latter mounds often have a high clastic component, they are a associated with a relative abundance of encrusted Donez- dark grey in colour and are observed to drape over/onlap ella (with some accessory Beresella and Dvinella) thalli, the mounds. The contact between the capping facies and peloids are often thromboidal or aligned. Homogenous the mound facies is sharp. Beds are thinner than those of micrite occurs exhibiting no obvious fossil content and the basal facies, ranging from 5 to 25 cm thick. The clas- little in the way of sedimentary fabrics,in association with tic-rich bed directly above the mounds are grain supported Donezellaceans (Fig. 3d). In places a clotted texture can be packstones consisting of broken and transported fossil observed within the micrite (Fig. 3d–f). Several cyanolith fragments, sparry calcite crystals and an insoluble clastic/ like growths and encrusting sheets are observed; individual organic material sedimentary framework (Fig. 4d). Calcite algae thalli are rarely encrusted, although in situ thickets crystals occur interspersed within a poorly laminated very 1 3 232 Journal of Iberian Geology (2018) 44:225–241 AB C D E F fine quartz and insoluble organic framework. Bioclasts are further sedimentation, shell fragments also appear slightly dominantly broken Donezellacean fragments, the orientation flattened. Above these beds the carbonates are classified of which is slightly imbricated. Tuberitina are observed as between a wacke- and packstone. These are characterised well as some encrusting Komia and large fragments of bryo- by a homogenous to slightly peloidal micrite sedimentary zoan fronds. Several flattened, opaque lenses occur, these framework, partially baffled by branching Donezellaceans are possibly plant fragments which have been compacted by and partly supporting large bioclasts (over 1 mm in size) and 1 3 Journal of Iberian Geology (2018) 44:225–241 233 ◂Fig. 4 Photomicrographs of the mound facies, capping facies and encrusted clast in Fig.  4c). Other macro-fossils include selected biota of carbonate mud mounds from the San Emiliano rugose and auloporid corals, brachiopods and gastropods. Formation. a A community of microscopic encrusters consisting of Archaeolithophyllum, Petschoria, Komia (and Ungda- Wetheredella (We.), Girvanella (Gi.), Rothpletzella (Ro.) and Sha- rella), Donezella and several types of encrusting calcimi- movella (Sh.) in association with the dominant encruster, the algae incertae sedis, Claracrusta (Cl.). The dark micritic patches at the crobe are present. centre of the calcimicrobial dome exhibits some branching features and several components that most resemble micritized cyanobacte- rial/calcimicrobial tubes. b Rothpletzella encrusting a Donezella 3 Discussion thicket. Note that successive rows of Rothpletzella are concordant with the previous row. c A calcimicrobial community encrusting a ‘phylloid’ algae thallus. The encrusters comprise of Rothpletzella, The basal facies is characterised by well-preserved, dense and Claracrusta. Some Archaeolithoporella which is characterised by thickets of Donezella and Beresella, which were common in couples of dark and light laminae is present as are globular, cystose Bashkirian and Moscovian age carbonates (see Sect. 3.1). growths which resemble Wetheredella (black arrow). d Broken algal thalli and other bioclasts forming a weakly laminated packstone. e These carbonates typically occur as low relief mounds or Dvinella. Note the thin micritic envelope surrounding the bioclast and banks within marine lagoonal settings (Flügel 2004). Cor- the peloids that appear to be stacked vertically from the flat, upper toids and cyanoliths suggest a shallow subtidal back-reef surface of the specimen. Several of the peloids appear thrombolitic or environment—in the geological setting of these sediments aligned in nature and resemble the calcified bacteria Renalcis. Faint, but discernible filaments can be observed attached to the Dvinella (i.e. a foreland basin) it is envisaged that a tectonic struc- clast, these are possibly calcimicrobial sheaths. The evidence from ture (e.g. a blind thrust or the forebulge) may act as a bar- this photomicrograph suggests that the clotted peloidal material may rier in the place of a ‘traditional’ reef. The Barcaliente and well be micritised remains of cyanobacterial colonies. f Girvanella Valdeteja Formations (which were deposited to the north and south of the San Emiliano Fm.) may also have acted cortoids. Bioclasts are densely packed and consist mainly as a barrier. Behind these, a lagoonal (or lagoon-like) envi- of Donezella and Beresella thalli. Donezella occurs as both ronment is interpreted to have formed. The low-diversity of a branching three dimensional network and as broken frag- the microfacies would suggest a relatively stressed environ- ments. Some algal thalli are micritised with wall structures ment, the high number of algae and little else may suggest destroyed, leaving only outlines of the thalli. Bryozoans a brackish environment. Although rare, the occurrence of are relatively common in these samples and are often the attached Tetrataxis is further evidence for a stressed envi- largest bioclasts found. Bioclasts are frequently enveloped ronment (Cossey and Mundy 1990). The low diversity and by a micritic rim. Foraminifera and calcimicrobes are well simple nature of the taxa observed may indicate a pioneer preserved and appear with relatively high biodiversity, both community. The biological niche the organisms have filled mobile and attached forms of Tuberitinidae are also well pre- is certainly newly opened—as evident in the shift in lithol- served and relatively abundant. A few rare echinoid plates ogy (from clastic dominated to carbonate dominated). The are present. Calcimicrobial assemblages range from small disparity of microfacies evident between the basal and isolated clumps, to larger encrustations involving several mounds facies indicates a change in environmental condi- organisms. The cortoids in these samples are spherical in tions or possibly ecological tiering, whereby the Donezella nature and resemble ooids but for the lack of any laminae, dominated mounds successfully establish themselves in the they generally consist of a recrystallized spherical nucleus newly opened niche. with a micritic rim exhibiting calcimicrobial growth. These The light colour of the mounds (when compared to the samples show a biodiversity higher than samples from the darker basal, capping and off-mound beds) may indicate a mound facies, the microfossils present within these samples lower Total Organic Content value, perhaps alluding to a are well preserved. shorter formation period. The shape of the mounds doesn’t suggest any asymmetry. The boundary between mounds and 2.4 Non‑mound bearing units onlapping (or draping) beds—may also indicate that the mound accumulated faster than the surrounding capping Several of the carbonate units of the La Majúa Mb. were and basal facies. The mound facies is dominated by vari- observed to be barren of mud mounds. These units resem- ous packstones, containing mostly Donezellaceans. Peloidal bled either the basal facies or consist of bedded whole micrite and homogenous micrite are more common than in fossil wackestones and bioclastic packstones. The latter the basal beds, and in some cases are the major constituents. units appear as dark buff grey beds approximately 20 cm Much of the peloidal micrite is nonlaminated and appears as in thickness. Bedding is wavy in places and the limestones clotted growths and other “shrub” like forms. The presence can often be observed to transition into marls toward the of what are quite possibly calcified filamentous cyanobac- top of the beds. ‘Phylloid’ algae are common, found as terial sheaths suggests that peloids are possibly micritised laminar, to cyathiform and ‘leaf’-like growths (e.g. the remains of these and of the calcareous algae. Homogenous 1 3 234 Journal of Iberian Geology (2018) 44:225–241 micrite is often in association with faint thrombolytic tex- 3.1 Palaeoecology of the mounds tures, perhaps hinting at a microbial or algal origin. The heterolithic nature of the mound facies possibly indicates The mounds have a distinct biological community consist- a considerable variation in current flow, energy conditions, ing of Donezella (Fig. 3b, c) and several encrusting forms sea levels and/or nutrient levels during the deposition of the including: Rothpletzella, Claracrusta, Girvanella and occa- mud mounds. The biota associated with this facies remains sionally Wetheredella and Shamovella (Fig. 4a). In some mostly the same as the basal assemblage and is dominated examples Girvanella (Fig.  4e) can be observed between by non-complex alga and encrusting cyanobacterial forms. entwined Donezella thalli. In other cases, Rothpletzella This association of biota suggest a restricted, shallow, low and Claracrusta can be found to encrust, bind and assum- energy environment. Evidence for changing conditions is edly stabilise Donezella thickets. Evidence of calcimicro- present along with some higher energy events, although the bial sheaths encrusting Donezellaceans is present (Fig. 4f). environment is interpreted to remain dominantly restricted, Donezella, Claracrusta and Rothpletzella are described in shallow and low energy throughout the deposition of the some detail before considering the biological assemblage mounds. The capping facies consists of three distinct lith- as a whole. ologies: a packstone characterised by its lithoclastic compo- Donezella is observed as either whole, bush- or shrub- nent in addition to its bioclastic component which is found like growths often up to several centimetres in diameter, as ‘draped’ over several of the mounds. Above that is a wacke- sheet like encrustations of entwined thalli which appear to packstone with accessory bioclasts which include taxa typi- encrust the subjacent surface or as broken thalli which have cal of metazoan dominated carbonates. The relatively large, been reworked. There are two common species observed; reworked and broken bioclasts within the unit draped over the type species, Donezella lutugini Maslov and Donezella the mounds, plus the presence of increased clastics indi- lunaensis Rácz. Thalli are found to range from 50 to 275 μm cate a higher energy depositional environment than that of in diameter with an average diameter of 140 μm. Donezella the mounds. Some imbrication of bioclasts and lamination is generally observed within the mound and basal facies of is evident, suggesting stronger current activity. The occur- the observed mounds. Where Donezella is found in situ (as rence of several different foraminifera, bryozoans and more either a shrub like growth or as an encrusting sheet) it is Tuberitina may indicate the environment is becoming less always the dominant biota with very few other biotas found restricted. Donezellaceans are absent or found as broken in association. Maslov (1929) originally assigned Donez- and transported clasts within most of the capping facies. ella to the red algae (Johnson 1963), however, in succes- However, the occurrence of a similar biological commu- sive subsequent studies Donezella has been considered nity, and the presence of mud chips, it is envisaged that the to be: green (Codiaceae) alga (Rácz 1964), alga incertae depositional depth of the last unit was similar to the depth sedis (Rich 1967), Foraminifera (Riding 1979), calcareous of mound formation or slightly deeper, but with a higher sponges (Termier et al. 1977), microproblematica (Riding energy level. The higher energy may be a result of the envi- 1979; Chuvashov and Riding 1984), green (Paleosiphono- ronment becoming more open, making it difficult for the cladales) algae (Shuysky 1985; Ivanova 1999), green algae algae to establish and grow into shrub like thickets. The of incertae familiae (Groves, 1986), green (Dasycladales) mounds are thought to have been smothered by the draping algae (Deloffre, 1988) and pseudo-algae (Vachard et  al. (capping) units as a result of an increase of clastic mate- 1989). Recent work regarding Donezella refers to the genus rial to the basin, probably as a result of thrust propagation as green (Chlorophycophyta) algae (Mamet and Villa 2004), (causing the migration of the foreland basin and increased algae incertae sedis (Mamet and Zhu 2005), algospongia clastic supply.) The bedded units on top of the mounds (Vachard and Cózar 2010) or more commonly, microprob- indicate a relative sea-level rise; this is interpreted to be lematica (Samankassou 2001; Della Porta et al. 2002; Choh flexure related basin subsidence. The changes in microfa- and Kirkland 2006; Corrochano et  al. 2012). Donezella cies throughout the mounds suggest a steady sea level rise accumulations are often interpreted to have grown within a throughout deposition. warm, shallow (low to moderate energy, below fair-weather The presence of stromatactoid and fenestral cavities with wave base) environment but have been shown to thrive over internal early marine cement implies that there was constant a range of water depths up to 200 m (Della Porta et al. 2002; movement of water through the carbonates when they were Choh and Kirkland 2006; Corrochano et al. 2012). Envi- partially lithified, which would suggest the presence of a ronmental conditions from quiet to quite agitated have been current or wave activity. The peloids, bioclasts, microfos- interpreted (Rácz 1964; Bowman 1979). Donezella growths sils and cavities show little sign of compression, which may have been found to form in a range of environments rang- indicate an early lithification. ing from platform interiors to deep-slopes (Della Porta et al. 2002). This cosmopolitan habit and ability of Donezella to occupy a variety of environments, quickly establishing itself 1 3 Journal of Iberian Geology (2018) 44:225–241 235 and growing to maturity suggest that Donezella may be an Filaments branch often and can form fan like structures. example of the “opportunist” species of Connell and Slatyer Filaments have a thin micritic wall and are filled with (1977). The dominant occurrence of Donezella in the studied clear sparry cement. Rothpletzella grows as thin crusts samples indicates that it played a direct role in the forma- and oncoids (Feng et al. 2010) and platy growths (Wood tion of the carbonates of the San Emiliano Formation, it is 2000). Growths are laminar and undulate to mimic the observed to both baffle and bind (in association with several subjacent micro-topography (Fig. 4b). encrusting organisms, specifically Claracrusta, Rothpletzella Rothpletzella, Wetheredella and Girvanella in combi- and Girvanella) and at a millimetre scale may be considered nation have been known to grow together symbiotically to be a constructor (Fig. 3c). In association with microscopic (Wood 1948) forming what is essentially a microbial organ- encrusters, Donezella was a successful baffler, binder and ism where boundaries between the various components are constructor, in regards to mound formation, Donezella must impossible to see with the naked eye/optical microscope. be considered as an integral component and directly linked This microbial organism was mistakenly interpreted as a sin- to the nucleation, growth and success of carbonate mud gle fossil and named Sphaerocodium (Wood 1948; Adachi mound formation within the San Emiliano Fm. Beresella et al. 2007). Rothpletzella is a commonly observed calci- and Dvinella are both found in association with Donezella, microbe in this study and is often found as well preserved although they are rare. This important skeletal element of the laminations of juxtaposed ovoid filaments. The genus is mounds leads to the classification of this mound component often found in association with other encrusting alga and to be that of a Cluster mound (Riding 2002). calcimicrobes, especially Claracrusta and Girvanella and Specimens of Claracrusta (Fig.  3a) are observed as more rarely Wetheradella. These calcimicrobes often form laminar rows of irregular, often bean and ovoid to sub- variously layered growths. Several examples have a micro- quadratic shaped cells which have a honey or yellow sparitic wall and are filled with micrite with the filaments coloured calcite wall. The cells have been referred to as measuring up to 50 μm in height (width was not measured as tubular (Cózar 2005, and elsewhere), but here, appear to it is unsure if the cutting angle is completely perpendicular be ovoid or subquadratic, agreeing with Mamet and Villa to the filament). Several of the laminar growths of Rothplet- (2004). Individual cells have been observed to range from zella reach up to a millimetre in height. The genus is usually 10 to 110 μm but the average cell size is around 50 μm. found encrusting various bioclasts, in particular Donezella Encrustations have the characteristic growth form of a con- thickets (Fig. 4b) and ‘phylloid’ algae (Fig. 4c). This associ- cordant first lamination with subsequent encrusting lami- ation of encrusting forms is interpreted to play an important nations becoming more and more undulating; the space role in the binding and stabilisation of Donezella thickets between laminations is generally filled with homogenous within the San Emiliano Fm. mounds, although minimal in micrite or more rarely, other encrusting forms. Claracrusta volume, these micron to millimetre build-ups play a vital appears to be non-selective in what it encrusts, although role, and are considered as Agglutinated Microbial mound Donezella and Archaeolithophyllum appear to be the most (Riding 2002) elements. Both of the biological assemblage’s commonly encrusted objects. Several examples of entire present form build-ups of sediment with unique characteris- Donezella florets being encrusted have been observed and tics. These are individually interpreted as a Cluster mound the preservation level is usually particularly high for these element and an Agglutinated Microbial mound element. The Donezella specimens (Fig.  3a). These florets are gener - introduction, and use of the term, ‘Composite mound’ to ally associated with homogenous micrite and constructed encapsulate this overlapping of mound structure allows for cavities which have been selectively dolomitised. Clarac- the appreciation of both of these important structural fea- rusta in association with Rothpletzella, Girvanella and to tures of the mounds, without giving gratuitous emphasis to a lesser extent Wetheredella (Fig. 4a), are considered to be the volumetrically dominant element (i.e. in this case, the important binders and stabilisers of Donezella growths and Cluster mound element). therefore the role they play within the formation of carbon- The association of biotas presumably formed wave ate mounds within the San Emiliano Formation (particu- resistant, topographic mounds. The presence of bound larly within the San Emiliano Valley) is very important. and stabilised Donezella/encruster assemblages may have Rothpletzella (Fig.  4a) is characterised by laminar encouraged consecutive colonisation of the same area by sheet-like growths of branching tubes which lie parallel subsequent Donezella and encrusters by acting as a substrate to the substrate. In transverse sections Rothpletzella often (Fig. 4b, 5). Other biota observed consist of low diversity resembles a chain or bead like structure with individual assemblages and often consist of biota which may represent filaments ovoid in shape, often resembling beans or sau- organisms living in a stressed environment (e.g. attached sages. Individual filaments have been reported to range Tetrataxis). The concentration of Donezella and its ally taxa from 13 to 37 μm (Adachi et al. 2007) in diameter. Lon- may have directly contributed to the low biological diver- gitudinal cuts reveal dichotomous branching of filaments. sity; low diversity assemblages characterised by dominant 1 3 236 Journal of Iberian Geology (2018) 44:225–241 Encrusting forms (Agglutinated Microbial mound element) e.g. Rothpletzella ,, Claracrust aG irvanella Donezella thicket (Cluster mound element) Cavity Fig. 5 A schematic representation of mound formation. (1) Donezella interaction of Donezella growths and laminar encrusters. Note that it grows and is broken and abraded by local wave action and currents. is not individual Donezella thalli that are encrusted, but the ‘shrub’ (2) Donezella grows on top of the accumulated, broken Donezella. like growths as a whole. The approximate scales for the individual (3) Environmental conditions deteriorate and laminar encrusters col- components differ, Cluster mound elements range from millimetre onise the substrate provided by the Donezella thickets. (4, 5 and 6) to centimetre in vertical profile, Agglutinated Microbe mound com- Environmental conditions continue to fluctuate resulting in the repeti- ponents are often micron to millimetre in vertical profile. The area tion of Donezella growth, and stabilisation by encrusting forms. (7) shown in 7 is approx. 2 mm across A close up of the area circled in (6), schematic representation of the algal communities have been attributed to the exclusion of of mounds from the Bernesga Valley (Samankassou 2001) other biota from the living space via both chemical defence are absent, or greatly reduced, from the San Emiliano Valley. (poisons produced by the algae) and through the occupation Several, distinct palaeoecological assemblages are rec- of all available living space (Toomey 1976, 1991; Forsythe ognisable from the studied material. Mound and off-mound et al. 2002; Samankassou and West 2003; Enpu et al. 2007). carbonates (i.e. those carbonates found within the same unit Petschoria, Archaeolithophyllum, Komia and Ungdarella as mounds) exhibit similar biological communities, however, were all observed to be present within the mound facies, in distinct variations were observed between the abundance of far lesser numbers than the dominant biotas described above. certain biota between the communities. Bedded lithologies There appears to be a reduced population or complete lack of are associated with a palaeoecological assemblage distinct grazers within environments where they would normally be to those from mound bearing strata. expected. Bryozoans, corals and other biota often associated with Pennsylvanian aged carbonate deposits are generally 3.2 Initiation, growth and demise of the mounds restricted to the mixed clastic and bedded carbonate lith- ologies and are rarely seen within mounds within the San The Composite mounds from the type locality of the San Emiliano Valley. Dasyclad algae, foraminifera, bryozoans, Emiliano Fm. initiated as the result of several interacting corals and other shelly fauna are more common within the factors. The combination of a newly opened niche within a Bernesga Valley mounds (Samankassou 2001). Thartharella restricted, warm and shallow basin encouraged the growth and calcisponges, which were reported as a main component of pioneer or opportunist organisms. The absence of detri- tal clastic input allowed for carbonate producers to fully 1 3 Journal of Iberian Geology (2018) 44:225–241 237 Table 1 Summary of documented occurrences of Donezella related buildups Mound description Age Location References Low-energy buildups Lower Permian Central Texas (USA) Wiggins (1986) Donezella mounds with siliceous sponges, Lower Permian Ouachita Mountains (Oklahoma, USA) Choh and Kirkland (2000) Archaeolithophyllum and worm tubes Donezella-Siliceous sponge dominated Lower Pennsylvanian Ouachita Mountains (Oklahoma, USA) Choh and Kirkland (2006) carbonate buildup with worm tubes Donezella low bank to mound controlled Moscovian Hueco Mountains (W Texas and SE New Lambert (1986) and Lambert by water depth and energy Mexico) and Stanton (1988) Skeletal microbial Donezella—Tubiphytes Bashkirian California (USA) Watkins (1999) (= Shamovella) biostromes Donezella baffled mounds associated with Bashkirian—Moscovian Cantabrian Mountains (Spain) Rácz (1964) Petschoria, Komia and Archaeolitho- phyllum Donezella mounds with Komia, Ungda- Bashkirian—Moscovian Cantabrian Mountains (Spain) Bowman (1979) rella, encrusting foraminifers, ‘phylloid algae’ and dasyclads Mounds sparse of Donezella (5–10%). Bashkirian—Moscovian Cantabrian Mountains (Spain) Riding (1979) Donezella mounds with Petschoria, Bashkirian—Moscovian Cantabrian Mountains (Spain) Eichmüller (1985) Komia, Ungdarella, encrusting foramini- fers, Archaeolithophyllum and bryozoans Skeletal-microbial boundstone. Don- Bashkirian—Moscovian Cantabrian Mountains (Spain) Samankassou (2001) ezella, agglutinated worm tubes and calcisponges Skeletal microbial Donezella—Tubiphytes Bashkirian—Moscovian Cantabrian Mountains (Spain) Samankassou (2001) (= Shamovella) mounds Clotted microbial peloids with Donezella Pennsylvanian Cantabrian Mountains (Spain) Samankassou et al. (2013) and ‘phyloid algae’ Donezella pack- wackestones mounds with Bashkirian—Moscovian Cantabrian Mountains (Spain) This study Rothpletzella and Claracrusta (Compos- ite mound) Donezella mounds with Beresella, Late Surpukhovian to Ellesmere Island (Canadian Arctic Archi- Davies and Nassichuk (1989) Komia/Ungdarella, bryozoans, crinoids, early Bashkirian pelago) encrusting foraminifers and ostracods Skeletal Donezella—Tubiphytes (= Sham- Carboniferous Kazakhstan Cook et al. (2007) ovella) mounds Donezella and Ungdarella buildups Bashkirian Western Urals (Russia) Proust et al. (1996) Mounds, bioherms and biostromes are all included in this table. It should be noted that Riding (1979) concluded that Donezella was not respon- sible for the formation of the studied mounds. Mounds are grouped into geographical locations establish themselves, resulting in an ‘in house’ production Donezella growth when environmental conditions became of carbonate mud. Growth of the mounds resulted from the more tolerable. The mounds ceased to form when detrital interaction of the genus Donezella and several encrusting sediment deposition returned to the basin. Microfacies anal- forms including the genera Rothpletzella and Claracrusta. ysis indicated that when detrital input increased the environ- Fluctuating environmental conditions allowed the encrust- ment became less restricted with encrusting forms becoming ing forms to bind and stabilise the framework building and less common whilst (relatively) large and mobile metazoans carbonate mud baffling Donezella growths. Donezella grew become more common. The Donezella/encruster relation- during ‘normal’ environmental conditions within the basin ship ceased and mounds could no longer form. The major whilst the encrusters, which were direct competitors for the factors responsible for mound formation can be attributed to: environmental niche, flourished during deteriorating condi- a] biological community present, b] a restricted basin and c] tions (Fig. 5). Adachi et al. (2007) observed a similar rela- a lack of clastic input. tionship between Graticula-like red algae, Rothpletzella and laminar stromatoporoids in Lower Devonian age bindstones. The resulting stabilised (and likely wave/current resistant), encrusted Donezella thickets provided a substrate for further 1 3 238 Journal of Iberian Geology (2018) 44:225–241 relationship controlled by fluctuating environmental condi- 3.3 Comparison to other mounds tions. Off mound sediments (basal and capping facies) were commonly associated with higher biodiversity, including Throughout the Pennsylvanian Series, carbonate units asso- ciated with a dominance of Donezella growths are relatively biota such as bryozoans, corals and foraminifera. Sedi- mentary deposition of the mounds was characterised by common. Several examples also exist from the Permian, although these are far less abundant. These carbonates autochthonous micrite, in the “in-house” manner, typical of many microbial mounds. The process identified as being have been reported to form a wide variety of geometries and features including mud mounds (of various classifica- responsible for the nucleation and growth of the mounds is a dynamic relationship between the Donezella dominated tions), bioherms, biostromes and as carbonate ramps and platforms (Della Porta et al. 2002; Choh and Kirkland 2006 community and the laminar encrusting form dominated community. As indicated by microfacies analysis the envi- and references within). Other mounds (including bioherms and biostromes) containing Donezella are summarised in ronmental conditions of deposition fluctuated. Where con - ditions were near ‘normal’, Donezella flourished, acting as Table 1 and are loosely grouped into geographical location. Several common biological associations are apparent. An a microscopic framework builder as well as a baffler. Envi- ronmental ‘degradation’ led to the encruster guild becom- association with Petschoria, Archaeolithophyllum, Komia and Ungdarella is plain to see within the majority of these ing dominant. Laminar encrusters grew in a non-selective manner covering the majority of grain types. Commonly mounds. Several of the mounds, including those from the Bernesga Valley, are associated with worm tubes and sili- the encrusters could be found atop of Donezella thickets, stabilising and protecting the (supposedly) delicate Donez- ceous sponges. No other mounds (other than those in this study), bioherms or biostromes have a reported biological ella thalli. When environmental conditions returned to suit Donezella the laminar encrusters provide a suitable substrate association between Donezella, Claracrusta, Rothpletzella, Girvanella and other microscopic encrusting forms. The for new growth. The Composite mounds ceased to form due to drowning by a deepening sea-level or by an increase in Composite mounds of the San Emiliano Valley have com- positional and formational mechanisms that are unique when clastic sediment resulting in a clastic-rich bed draped over the mound. The mechanism responsible for mud mound pro- compared to other mounds with a dominant Donezella com- ponent, as well as other known populations of mounds from duction within the San Emiliano Fm. is unique for known Donezella dominated mounds, and comparable Carbonifer- the Carboniferous. The depth and/or environmental setting at which the San Emiliano mounds formed may be the cause ous mounds in general. The term Composite mound is used here to describe mounds bearing two or more of the struc- for this unique biological assemblage, most other Donezella mounds have been interpreted to form at a greater depth tural characteristics defined by Riding (2002). below sea level. Acknowledgements The author would like to express his gratitude to the technical staff in GGE at Keele University for the preparation of all thin sections used. This study forms part of a PhD completed in 2015 at the University of Keele under the supervision of Dr Michael Montenari. The author is grateful to Sergio Rodriguez for providing 4 Conclusions constructive comments which greatly improved the manuscript. Javier Martín-Chivelet is thanked for his editorial assistance when this manu- Three distinctive microfacies are recognised between the script became a ghost in the system. Alex Nobajas is thanked for his basal, mound and capping material of the carbonate units assistance with the Spanish translation of the abstract. of the San Emiliano Fm. at its type point, plus an addition a Open Access This article is distributed under the terms of the Crea- fourth facies; carbonate units which are barren of mounds. tive Commons Attribution 4.0 International License (http://creat iveco Composite mounds from the San Emiliano Fm. are inter- mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- preted as skeletal-microbial pack-wackestones with various tion, and reproduction in any medium, provided you give appropriate types of cavity networks. The dominant palaeontological credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. communities were low diversity assemblages representing a pioneer or opportunist community living in a newly opened (relatively restricted) niche. 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(1948). “Sphaerocodium”, a misinterpreted fossil from the Wen- Samankassou, E. (2001). Internal structure and depositional environ- lock Limestone. Proceedings of the Geological Association, 59, 9–22. ment of Late Carboniferous mounds from the San Emiliano For- Wood, R. (2000). Novel paleoecology of a post extinction reef: Famen- nian (Late Devonian) of the Canning basin, northwestern Aus- mation, Cármenes Syncline, Cantabrian Mountains, North Spain. tralia. Geology, 28, 987–990. Sedimentary Geology, 145, 235–252. 1 3 Journal of Iberian Geology (2018) 44:225–241 241 Affiliations Steven L. Rogers * Steven L. Rogers School of Geography, Geology and the Environment, s.l.rogers@keele.ac.uk University of Keele, William Smith Building, Keele, Staffordshire ST5 5BG, UK 1 3

Journal

Journal of Iberian GeologySpringer Journals

Published: Mar 28, 2018

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