Sedimentology

Götte, Thomas; Ramseyer, Karl; Pettke, Thomas; Koch‐Müller, Monika; Porta, Giovanna Della

doi: 10.1111/sed.12024pmid: N/A

A petrographic investigation revealed polyphase quartz cementation in the Finefrau Sandstone (Upper Carboniferous, Western Germany) and the Solling Sandstone (Lower Triassic, Central Germany). Three different cements could be distinguished in each sandstone based on their cathodoluminescence and trace element composition. The first quartz generation is suggested to have been formed during eogenesis due to dissolution and replacement of feldspar. The mesogenetic paragenesis comprises two generations of quartz and illite, which are accompanied by albite in the Solling Sandstone. Sharp luminescence zoning in quartz overgrowths points to distinct episodes of cementation in both sandstones. Significant amounts of Al, Li and H and traces of Ge and B have been detected in the quartz overgrowths. The Al‐content of the quartz cements in the Finefrau Sandstones exceeds that in the quartz cements in the Solling Sandstone by a factor of five. It is suggested that this compositional variation reflects the conditions in the pore‐water, such as temperature and pH. The Al‐concentration is generally correlated to the Li‐content with the exception of the latest quartz generation in the Finefrau Sandstones which is also most enriched in trace elements. The ratio of Li/Al varies between 0·11 and 0·25 in the two sandstones. The Li/H‐ratio, which ranges from 0·12 to 0·3, is controlled by the activity ratio of Li and H in the pore fluid. Clay minerals are the most important source for Li and high salinities favour the mobilization of Li during diagenesis. Thus, a relatively low salinity and low pH are responsible for the low Li/H‐ratio in the Finefrau Sandstone, while high salinity and neutral to alkaline pH results in a high Li/H‐ratio for the Solling Sandstone. The Ge‐contents are generally near the average of detrital quartz and indicate that pressure dissolution is a major source for quartz cementation. Different chemical compositions of distinct quartz generations indicate changes in the physico‐chemical conditions and point to mobilization of silica from different sources (for example, pressure solution and clay mineral transformations).

Anadón, Pere; Canet, Carles; Friedrich, Walter L.; Ariztegui, Daniel

doi: 10.1111/sed.12025pmid: N/A

The pyroclastic deposits of the Minoan eruption (ca 3600 yr bp) in Santorini contain abundant xenoliths. Most of these deposits are calcareous blocks of laminated‐botryoidal, stromatolite‐like buildups that formed in the shallow waters of the flooded pre‐Minoan caldera; they consist of (i) light laminae, of fibrous aragonite arranged perpendicular to layering, and (ii) dark laminae, with calcified filaments of probable biological origin. These microstructures are absent in the light laminae, suggesting a predominant inorganic precipitation of aragonite on substrates probably colonized by microbes. Internal cavities contain loose skeletal grains (molluscs, ostracods, foraminifera and diatoms) that comprise taxa typical of shallow marine and/or lagoon environments. Most of these forms are typical of warm water environments, although no typical taxa from hydrothermal vents have been observed. Past gasohydrothermal venting is recorded by the occurrence of barite, pyrolusite and pyrite traces. The most striking features of the stable isotopic data set are: (i) an overall wide range in δ13CPDB (0·16 to 12·97‰) with a narrower variation for δ18OPDB (−0·23 to 4·33‰); and (ii) a relatively uniform isotopic composition for the fibrous aragonite (δ13C = 12·40 ± 0·43‰ and δ18O = 2·42 ± 0·77‰, n = 21). The δ13C and δ18O values from molluscs and ostracods display a covariant trend, which reflects a mixing between sea water and a fluid influenced by volcano‐hydrothermal activity. Accordingly, 87Sr/86Sr from the studied carbonates (0·708758 to 0·709011 in fibrous aragonite and 0·708920 to 0·708991 in molluscs) suggests that the aragonite buildups developed in sea water under the influence of a hydrothermal/volcanic source. Significant differences in trace elements have been detected between the fibrous aragonite and modern marine aragonite cements. The caldera water from which the fibrous aragonite crusts formed received an input from a volcano‐hydrothermal system, probably producing diffuse venting of volcanogenic CO2 gas and of a fluid enriched in Ca, Mn and Ba, and depleted in Mg and probably in Sr.

Sena, Claire M.; John, Cédric M.; Reijmer, John

doi: 10.1111/sed.12026pmid: N/A

This study, based in the Haushi‐Huqf area of central east Oman, aims to characterize the controls on facies distribution and geometries of some of the best preserved examples of Lower Cretaceous tidal flat facies within the Tethyan epeiric platform. Field, petrographic and geochemical data were acquired from the Barremian–Aptian Jurf and Qishn formations that crop out in a 500 × 1000 m2 butte, thus allowing for pseudo three‐dimensional quantitative data acquisition of the dimensions and spatial distributions of discontinuity surfaces and sedimentary bodies. The interpretation presented here suggests that the main processes impacting sedimentation in the Lower Cretaceous peritidal environment of the Haushi‐Huqf were transport and erosion processes related to storm waves and currents. The vertical evolution of the carbonate system is organized into six types of metre‐scale depositional sequences, from subtidal dominated sequences to supratidal‐capped sequences, which are bounded by regional discontinuity surfaces. At subaerial exposure and submarine erosion surfaces associated with a base level shift, sedimentary horizons along the entire depositional profile are cut by scours possibly created by storm events. Chemostratigraphy allows correlation between the Haushi‐Huqf and the age‐equivalent sections logged in the interior of the platform in Oman. The correlation suggests that the change from subtidal to intertidal depositional sequences during the late highstand is coeval with the development of rudist dominated shoals on the shelf. This study is the first to discuss the controls on Lower Cretaceous peritidal carbonate cyclicity of the Arabian epeiric platform. The results presented here also offer a unique quantitative dataset of the distribution and dimensions of peritidal carbonate shoals and storm scours in a regional sequence stratigraphic context.

Taylor, Paul D.; James, Noel P.

doi: 10.1111/sed.12032pmid: N/A

Ever since their first radiation in the Ordovician, bryozoans have contributed significantly to carbonate sedimentation. Most of the numerous colony‐forms developed by bryozoans have evolved repeatedly in different taxonomic groups and vary in their sediment‐producing potential. There are nine basic bryozoan colony‐forms: encrusting, dome‐shaped, palmate, foliose, fenestrate, robust branching, delicate branching, articulated and free‐living. The proportion of these morphotypes in bryozoan faunas period by period is shown to change significantly through the Phanerozoic. Notable patterns include: (i) steady increase in the number and proportion of encrusting species through time, interrupted by a transient drop in the Late Palaeozoic; (ii) post‐Triassic decrease in robust branching colonies; (iii) rise in the proportion of fenestrate colonies through the Palaeozoic, followed by their absence in the Triassic and Jurassic, rarity in the Cretaceous and reappearance in smaller proportions in the Cenozoic; and (iv) scarcity of articulated colonies and absence of free‐living colonies until the Cretaceous. Most Palaeozoic bryozoan sediments come from two architecturally distinct groups of colonies: (i) domal, delicate branching, robust branching and palmate; and (ii) fenestrate. The former generate coarse particles both as sediment and components of stromatoporoid‐coral reefs in the Early and mid Palaeozoic, whereas the delicate lacy fans of the latter create both prolific coarse sediment and form the cores of Late Palaeozoic deep‐water, sub‐photic biogenic mounds. Nearly all post‐Palaeozoic bryozoan sediments comprise cyclostomes and cheilostomes with many of the same growth forms but with the addition of free‐living colonies and significant numbers of articulated colonies. The latter produced sand and mud‐sized bryozoan sediment via disarticulation for the first time. In contrast to the Palaeozoic, post‐Palaeozoic bryozoans generated sediment varying more widely across the grain‐size spectrum, from mud to sand to gravel. This article highlights the need to consider evolutionary changes in carbonate‐producing organisms when interpreting facies changes through time.

Weill, Pierre; Mouazé, Dominique; Tessier, Bernadette

doi: 10.1111/sed.12027pmid: N/A

Beach ridges in macrotidal environments experience strong multi‐annual to multi‐decennial fluctuations of tidal inundation. The duration of tide flooding directly controls the duration of sediment reworking by waves, and thus the ridge dynamics. Flume modelling was used to investigate the impact of low‐frequency tidal cycles on beach ridge evolution and internal architecture. The experiment was performed using natural bioclastic sediment, constant wave parameters and low‐frequency variations of the mean water level. The morphological response of the beach ridge to water level fluctuations and the preservation of sedimentary structures were monitored by using side‐view and plan‐view photographs. Results were compared with the internal architecture of modern bioclastic beach ridges in a macrotidal chenier plain (Mont St. Michel Bay, France) surveyed with ground‐penetrating radar. The experimentally obtained morphologies and internal structures matched those observed in the field, and the three ridge development stages identified in ground‐penetrating radar profiles (early transgressive, late transgressive and progradational) were modelled successfully. Flume experiments indicate that flat bioclastic shapes play a key role in sediment sorting in the breaker zone, and in sediment layering in the beach and washover fans. Water level controls washover geometry, beach ridge evolution and internal structure. Low water levels allow beach ridge stabilization and sediment accumulation lower on tidal flats. During subsequent water level rise, accumulated sediment becomes available for deposition of new washover units and for bayward extension of the beach ridges. In the field, low‐frequency water level fluctuations are related to the 4·4 year and 18·6 year tidal cycles. Experimental results suggest that these cycles may represent the underlying factor in the evolution of the macrotidal chenier coast at the multi‐decadal to centennial time scale.

Détriché, Sébastien; Bréhéret, Jean‐Gabriel; Karrat, L'houcine; Hinschberger, Florent; Macaire, Jean‐Jacques; Ariztegui, Daniel

doi: 10.1111/sed.12028pmid: N/A

The Lake Afourgagh sediment record and facies successions provide an outstanding example of environmentally controlled carbonate sedimentation. Afourgagh is a small, shallow permanent lake located in the Middle‐Atlas Mountains in Morocco in a karstic context. It is fed by ground waters that are relatively enriched in Mg resulting from the leaching of the Jurassic dolomitic bedrock of the catchment. This eutrophic lake is episodically restricted and characterized by alkaline waters with a fluctuating high Mg/Ca ratio. The maximum extension of the Holocene shoreline coincides with evidence of a lake stabilization level corresponding to the outflow of the lake through a wadi. Lakeshore terrace sediments deposited on an alluvial fan siltstone during the past ca 2500 cal yr bp comprise four main facies: a littoral crust, palaeosols, palustrine silts and charophyte tufas, which reflect different environments from the shoreline toward the deeper water. In the more distal parts, the charophyte tufas display a well‐expressed lamination punctuated by the development of microstromatolites on algae thalli. The mineralogical composition of the carbonates is linked to the facies. While the charophyte tufas are characterized by a relatively high content in aragonite, in addition to low‐Mg calcite, the littoral crust is mainly composed of magnesite. This pattern is related to the evolving chemistry of water due to the influence of charophyte proliferation during dry summers. Calcium‐carbonate precipitation on algae thalli (both bioinduced and microbially mediated) progressively induces an increase in the Mg/Ca ratio of the lake water, while the capillary evaporation of shallow ground waters causes precipitation of a magnesite precursor on the shoreline, producing magnesite during early diagenesis. This effect is characteristic of two episodes: part of the Roman Warm Period and the beginning of the Dark Age Cold Period. The carbonate mineralogy of the different depositional sequences at Afourgagh indicates lake‐level and water‐chemistry fluctuations under a climatic influence. Therefore, among other regional records, the Lake Afourgagh sedimentary record provides useful evidence for reconstructing these environmental changes.

Ventra, Dario; Dìaz, Guillermo Chong; boer, Poppe l.

doi: 10.1111/sed.12029pmid: N/A

Research on colluvial depositional systems has recently emphasized periglacial and high‐altitude settings, and the relations between Quaternary slope stratigraphy and climate change. This article examines the role of variable slope morphology, surface hydrology and microclimate in controlling colluvial sedimentation along a coastal tract of the hyperarid Atacama Desert in northern Chile. Direct accessibility of active surfaces is accompanied by uninterrupted stratigraphic exposures along the base of slopes, allowing direct comparisons between surface processes and the resulting sedimentary record. Four slope sectors are identified, based on differences in morphology and processes over active surfaces. Colluvial sedimentation is controlled by complex interactions of slope gradients and profiles, exposure to dominant winds, and potential runoff pathways, which vary considerably between different sectors. Major differences are evident between these hyperarid deposits and slope sedimentation in periglacial and temperate settings, including the complete absence of pedogenic activity and clay minerals; the volume of aeolian deposits and their role in controlling processes which redistribute sediment downslope, extending colluvial aprons; and the occurrence of runoff processes only where favoured by particular topographic configurations. Depositional surfaces range from steep talus cones, to debris‐flow‐dominated and aeolian‐dominated colluvial aprons, to an aeolian ramp subject to reworking by mass flows and flash floods. Consequently, facies associations and architectures at outcrop are highly variable and highlight the importance of spatial variations in slope morphology and processes in producing distinct, coeval colluvial stratigraphies within a single environmental context. Discrepancies between active processes and the corresponding stratigraphic signatures are also evident in some sectors; for example, preservation of alluvial and aeolian facies in stratigraphic sections does not always reflect the dominant processes over active slopes. Together with the spatial variability in processes and deposits along these slopes, this suggests that caution is required when extracting palaeoenvironmental information from analyses of colluvial successions.

Macdonald, Robert G.; Alexander, Jan; Bacon, John C.; Cooker, Mark J.; Dey, Subhasish

doi: 10.1111/sed.12035pmid: N/A

Sediment accumulation downstream of hydraulic jumps can occur in many settings but the architectures of such deposits are poorly documented. Here, three flume runs were used to examine the influence of sediment grain size and transport rate on the characteristics of hydraulic‐jump unit bars. In one of these runs six hydraulic‐jump unit bars formed a hydraulic‐jump bar complex. In another, the same sediment was supplied more quickly and only two unit bars formed. In the third run with the same sediment supply rate, but different grain size, only one large unit bar formed. All unit bars developed in a similar way but their size and internal architecture differed; they all resulted from a reduction in sediment transport capacity at the transition from supercritical flow to subcritical flow in the hydraulic jump. After initial onset of sedimentation and unit bar formation, generation of subsequent unit bars may be: (i) related to small changes in sediment flux; and (ii) independent of changes in the hydraulic jump. Continued sedimentation caused changes from oscillating to weak hydraulic jumps and hydraulic‐jump unit bars formed in both circumstances. The flow of water and suspended sediment becomes shallower over the lee of the bar complex. This leads to flow acceleration and a return to supercritical flow conditions. In turn, a chain of such features can form and generate a chute and pool bed morphology. There is an inherent upper size limit to a hydraulic‐jump bar complex due to the changing flow conditions over the growing deposit as the water above it becomes shallower. There is also an amplitude minimum for the development of foresets and subsequent unit bar growth. Hydraulic‐jump unit bars have architectures that should be recognizable in the rock record and because their size is constrained by the flow conditions, their identification should be useful for interpreting palaeoenvironment.

Legler, Berit; Johnson, Howard D.; Hampson, Gary J.; Massart, Benoit Y.G.; Jackson, Chris A‐L.; Jackson, Matthew D.; El‐Barkooky, Ahmed; Ravnas, Rodmar; Uličný, David

doi: 10.1111/sed.12037pmid: N/A

Existing facies models of tide‐dominated deltas largely omit fine‐grained, mud‐rich successions. Sedimentary facies and sequence stratigraphic analysis of the exceptionally well‐preserved Late Eocene Dir Abu Lifa Member (Western Desert, Egypt) aims to bridge this gap. The succession was deposited in a structurally controlled, shallow, macrotidal embayment and deposition was supplemented by fluvial processes but lacked wave influence. The succession contains two stacked, progradational parasequence sets bounded by regionally extensive flooding surfaces. Within this succession two main genetic elements are identified: non‐channelized tidal bars and tidal channels. Non‐channelized tidal bars comprise coarsening‐upward sandbodies, including large, downcurrent‐dipping accretion surfaces, sometimes capped by palaeosols indicating emergence. Tidal channels are preserved as single‐storey and multilateral bodies filled by: (i) laterally migrating, elongate tidal bars (inclined heterolithic strata, 5 to 25 m thick); (ii) forward‐facing lobate bars (sigmoidal heterolithic strata, up to 10 m thick); (iii) side bars displaying oblique to vertical accretion (4 to 7 m thick); or (iv) vertically‐accreting mud (1 to 4 m thick). Palaeocurrent data show that channels were swept by bidirectional tidal currents and typically were mutually evasive. Along‐strike variability defines a similar large‐scale architecture in both parasequence sets: a deeply scoured channel belt characterized by widespread inclined heterolithic strata is eroded from the parasequence‐set top, and flanked by stacked, non‐channelized tidal bars and smaller channelized bodies. The tide‐dominated delta is characterized by: (i) the regressive stratigraphic context; (ii) net‐progradational stratigraphic architecture within the succession; (iii) the absence of upward deepening trends and tidal ravinement surfaces; and (iv) architectural relations that demonstrate contemporaneous tidal distributary channel infill and tidal bar accretion at the delta front. The detailed facies analysis of this fine‐grained, tide‐dominated deltaic succession expands the range of depositional models available for the evaluation of ancient tidal successions, which are currently biased towards transgressive, valley‐confined estuarine and coarser grained deltaic depositional systems.