Sedimentology

Föllmi, Karl B.; Schlunegger, Fritz; Weissert, Helmut

doi: 10.1111/sed.12014pmid: N/A

Wetzel, Andreas; Weissert, Helmut; Schaub, Monika; Voegelin, Andrea R.; Föllmi, Karl

doi: 10.1111/sed.12007pmid: N/A

The Middle Jurassic Burgundy carbonate platform occupied a central part of the Central European Epeiric Sea during the Middle Jurassic. The facies architecture of the oolitic calcarenite bodies was affected by tidal currents on the platform and relative sea‐level changes. The δ13C‐values of inorganic carbonates, sampled in biostratigraphic‐defined intervals, do not match very well between closely spaced sections and, hence, are of restricted use for stratigraphic purposes. It appears that the platform interior might have been decoupled from the global carbon pool. Although deposited in a rapidly accumulating setting, the recorded isotope signatures might be affected by some local stratigraphic gaps. Nonetheless, the carbon isotope data imply lateral changes of the platform waters; these appear to be related to the position on the platform and to the sediment dispersal pattern, as evidenced by clay minerals. Adjacent to the eastern margin of the platform, detrital chlorite and illite occur in considerable proportions, both ascribed to a boreal source to the east and the north‐east. In contrast, smectite‐rich mixed‐layer clay mineral content increases significantly towards the platform interior, pointing to a delivery from the north‐west. All these data are suggestive of an overall clockwise current pattern in the Central European Epeiric Sea during the Middle Jurassic.

Morales, Chloe; Gardin, Silvia; Schnyder, Johann; Spangenberg, Jorge; Arnaud‐Vanneau, Annie; Arnaud, Hubert; Adatte, Thierry; Föllmi, Karl B.; Weissert, Helmut

doi: 10.1111/sed.12019pmid: N/A

Valanginian sedimentary archives display a positive anomaly in the carbon‐isotope record which is associated with a crisis in neritic and pelagic carbonate production. This study aims to reconstruct the evolution of palaeoenvironments in the Jura area and the Vocontian Basin during late Berriasian and early Valanginian times, in order to better understand the controlling factors leading to the Valanginian episode of major environmental change. Three sections along a transect through the Jura platform (Switzerland and France) and the Vocontian Basin (France) have been studied. Stratigraphic correlations have been established by combining biostratigraphy (benthic foraminifera, ammonites and calpionellids), with geochemistry (δ13C trends) and sequence stratigraphy. A change from a rimmed ramp to a swell‐dominated ramp morphology is observed through the Berriasian–Valanginian boundary. The disappearance of the barrier appears to have been controlled by large sea‐level variations in combination with the arrival of significantly more humid climate conditions. This barrier played a major role in the distribution of geochemical, mineralogical and palynological fluxes towards the basin: during the Berriasian, continental fluxes (detrital particles, nutrients, pollen and spores) were buffered by the platform morphology, whereas in the early Valanginian they were more readily transported towards the basin. The initiation of a major transgression onto lateritic soils, leading to their intense reworking, instigated a fertilization of ocean waters during the earliest Valanginian and with that a change towards heterozoan carbonate production, and subsequently to the demise of the already weakened carbonate platform.

Eberli, Gregor P.; Föllmi, Karl

doi: 10.1111/sed.12011pmid: N/A

In outcrops, shallow‐water carbonates often form thick, vertically stacked, metre‐scale, exposure‐bounded depositional packages, here called carbonate cycles. Because the facies within carbonate cycles can be tied precisely to water depths, they are considered ideal for reconstructing past sea‐level changes. There is, however, increasing evidence that these depositional cycles are unreliable recorders of both the frequency and the amplitude of orbitally driven sea‐level fluctuations. Carbonate cycles record only a fraction of the amplitude of a sea‐level cycle: a portion of the rise and nothing of the fall. Cores through the Pleistocene cycles on Great Bahama Bank illustrate this shortcoming. Although the amplitudes of the last nine sea‐level changes are each a hundred metres or more, the thickness of the cycles varies from a few metres to ca 15 m. The lack of correlation between sea‐level amplitude and cycle thickness is not eliminated for cycles deposited during times of lower sea‐level amplitudes, for example, the Cretaceous. Upper Cretaceous cycles on the Maiella platform margin document the irregularly filled accommodation space and the resultant variability in cycle thickness and frequency. Uncertainties in assessing the frequencies of sea‐level changes from shallow‐water carbonate cycles are caused by ‘missed beats’ and metre‐scale oscillations of sea‐level within highstands that potentially produce cycles of very short duration. The random amplitude variability during the last 57 glacio‐eustatic sea‐level changes illustrates the difficulty of assessing ‘missed beats’, where a sea‐level fluctuation is not recorded because the sea‐level rise does not reach the platform top. ‘Missed beats’ are also produced by the depositional topography that is created by irregularly filled accommodation space. As a result, variable numbers of cycles are deposited across the platform. Further complicating orbital frequency analyses are decametre‐scale oscillations of the sea‐level during highstands. The amplitude of these sub‐orbital sea‐level oscillations (up to 17 m within the last interglacial, Marine Isotope Stage 5e) are sufficient to expose shallow platforms like Great Bahama Bank and subsequently produce an additional depositional cycle with similar facies successions. The combined effects of missed beats and oscillations within highstands are likely to produce cycles and hiatuses of variable duration that are difficult to extract from the rock record. Consequently, estimates of the orbital forcing mechanisms and frequencies from ancient shallow‐water carbonate cycles carry large uncertainties.

Schlunegger, Fritz; Norton, Kevin P.; Föllmi, Karl

doi: 10.1111/sed.12010pmid: N/A

This study uses the widths, the spacing and the grain‐size pattern of Oligo/Miocene alluvial fan conglomerates in the central segment of the Swiss Alpine foreland to reconstruct the topographic development of the Alps. These data are analysed with models of longitudinal stream profile development, to propose that the Alpine topography evolved from an early transient state where streams adjusted to rock uplift by headward retreat, to a mature phase where any changes in rock uplift were accommodated by vertical incision. The first stage comprises the time interval between ca 31 Ma and 22 Ma, when the Alpine streams deposited many small fans with a lateral spacing of <30 km in the north Alpine foreland. As the range evolved, the streams joined and the fans coalesced into a few large depositional systems with a lateral spacing of ca 80 to 100 km at 22 Ma. The models used here suggest that the overall elevation of the Alps increased rapidly within <5 Myr. The variability in pebble size increased either due to variations in sediment supply, enhanced orographic effects, or preferentially due to a change towards a stormier palaeoclimate. By 22 Ma, only two large rivers carried material into the foreland fans, suggesting that the major Alpine streams had established themselves. This second phase of stable drainage network was maintained until ca 5 Ma, when the uplift and erosion of the Molasse started and streams were redirected both in the Alps and in the foreland. This study illustrates that sedimentological archives of foreland basins can be used to reconstruct the chronology of the topographic development of mountain belts. It is suggested that the finite elevation of mountainous landscapes is reached early during orogeny and can be maintained for millions of years, provided that erosion is efficient.

Allen, Philip A.; Armitage, John J.; Carter, Andrew; Duller, Robert A.; Michael, Nikolas A.; Sinclair, Hugh D.; Whitchurch, Amy L.; Whittaker, Alexander C.; Schlunegger, Fritz

doi: 10.1111/sed.12015pmid: N/A

Although the stratigraphy of sedimentary basins depends on the balance between the magnitude and grain‐size characteristics of the sediment supply (Qs) and the spatial distribution of tectonic subsidence generating accommodation σ(x), Qs is problematical to measure in present‐day sediment routing systems and formidably difficult to predict in their ancient counterparts. This challenge was tackled by treating the sediment discharge from the outlet of mountain catchments as the result of incision by a drainage network with a bulk diffusivity based on the length over which the mean annual rainfall is concentrated. The size, relief and slope of palaeo‐catchments acting as feeders for sediment routing systems are used to run simulations of sediment discharge and bulk diffusivity for a range of annual precipitation values. A wide range of observable geological phenomena can be used to converge on the most likely solutions for Qs, including depositional volumes in the basin, and bedrock thermochronology and detrital cosmogenic nuclide dating to constrain catchment erosion rates. Modelled sediment discharges can be checked with estimates derived from global regressions. The sediment efflux of mountain catchments serves as a boundary condition for down‐system sediment transport and deposition. Variations in the volumetric ratio of sediment supply to available accommodation, Qs/σ(x), determines patterns of transverse versus longitudinal (axial) sediment dispersal. The volumetric ratio may change as a result of variations in climatic parameters, tectonic uplift rate and catchment expansion. An abrupt climate change to higher precipitation values promotes higher Qs/σ(x), but transient landscape response causes a return to values close to the baseline, generating a distinctive down‐system extension of a gravel ‘spike’. Catchment expansion has a similar, but more prolonged, effect on gravel progradation. In contrast, a change in tectonic forcing, such as an increase in slip rate on a border fault, causes little change in Qs/σ(x), because increased subsidence compensates for the increased sediment supply. Studies of mid Eocene–Oligocene sediment routing systems in the south‐central Pyrenees allow the discrimination of different types of proximal wedge‐top sedimentary systems on the basis of the volumetric ratio of Qs to accommodation σ(x): (i) small, steep, local fan systems in tectonically ponded, underfilled basins, supplied by low sediment discharges; (ii) tectonically guided, long‐range, axial systems fed by large sediment discharges from widely spaced palaeovalleys; and (iii) large, shallow‐sloping transverse megafans burying underlying defunct or active tectonic structures, supplied by high to very high sediment discharges. Understanding the role of variations in Qs helps to explain the syntectonic evolution of proximal foreland basin systems. The Oligocene–Miocene North Alpine Foreland Basin, Switzerland, is qualitatively identified as a high‐Qs example, the Miocene–Recent northern Apennines of Italy as a low‐Qs example and the Eocene‐Oligocene southern Pyrenees of Spain as intermediate in character.

FÖllmi, Karl B.; Godet, Alexis; Weissert, Helmut

doi: 10.1111/sed.12004pmid: N/A

Shallow‐water carbonate platforms evolve in an interplay of local, regional and global dynamics, thereby generating a detailed record of environmental conditions. Platform sediments constitute important archives of past environmental change and, as such, they are increasingly analysed for the information they contain on wider‐scale patterns. The integration of this information with palaeoceanographic data obtained from deeper‐water sediments provides a powerful tool to develop a more diverse and complete image of past environmental change. In this contribution, the remains of the northern Tethyan carbonate platform of Early Cretaceous age cropping out in the western Swiss Jura and the Helvetic Alps are used to illustrate the advantage of a palaeoceanographic approach in their interpretation. The tools applied are biostratigraphy and chemostratigraphy (carbon and strontium isotopes) in order to obtain a precise age model for correlation with (hemi‐)pelagic sediments in adjacent basins, sequence stratigraphy as a means to reconstruct the impact of sea‐level change, phosphorus stratigraphy as a proxy for trophic levels, facies and microfacies analyses as a gateway to gain better insight into the carbonate factory, and the detailed analysis of platform‐drowning unconformities as an indicator of wider‐scale palaeoenvironmental change. It is shown that changes from periods of predominantly photozoan to predominantly heterozoan carbonate production near the Berriasian–Valanginian boundary and middle early Aptian coincided with substantial increases in phosphorus burial rates and major carbon‐isotope excursions. Furthermore, the majority of the platform‐drowning episodes occurred just prior to and during episodes of major palaeoenvironmental change and, last but not least, the platform itself may have influenced the chemistry of adjacent basins and the carbon‐isotope composition of associated carbonates by the export of periplatform ooze.

Godet, Alexis; Föllmi, Karl B.; Spangenberg, Jorge E.; Bodin, Stéphane; Vermeulen, Jean; Adatte, Thierry; Bonvallet, Lucie; Arnaud, Hubert; Weissert, Helmut

doi: 10.1111/sed.12008pmid: N/A

Drowning unconformities are a frequent feature of carbonate platforms and generally express the incapacity of shallow‐marine ecosystems to adapt to abrupt sea‐level rise and/or palaeoenvironmental change. During the late Hauterivian and early Barremian, the Helvetic carbonate platform experienced a major drowning phase documented by the phosphate and glauconite‐rich Altmann Member. It has been shown that a drowning unconformity in the form of a hardground at the base of the Altmann Member on the Pilatus summit (central Switzerland) results from a complex, polyphased diagenetic history that includes two main phases of phosphogenesis. Using carbon‐isotope stratigraphy, biostratigraphy and sequence stratigraphy, the correlation of this drowning unconformity from the Helvetic domain with more distal and more complete sedimentary archives allows reconstruction of the sequence‐stratigraphic context of the drowning surface, and also an estimation of the amount of time represented by the episodes of condensation. Unconformities associated with the Altmann Member drowning phase developed during transgressive episodes, when strong currents arrived onto the previously subaerially exposed platform, and periods of phosphogenesis may have lasted up to 2·8 Myr. Variations in trophic levels and hitherto less well‐known intervening emersion phases played an essential triggering role for the Altmann drowning phase. High‐resolution studies of drowning unconformities unravel the diversity of palaeoenvironmental parameters involved in the unfolding of such crises during the evolution of carbonate platforms.

Masini, Emmanuel; Manatschal, Gianreto; Mohn, Geoffroy; Weissert, Helmut

doi: 10.1111/sed.12017pmid: N/A

This study focuses on the assessment of the stratigraphic architecture of the proximal and distal Jurassic Alpine Tethys rifted margins. The aim of the study was to reconcile the major observations performed in the Alps through time with what is observed in present‐day magma‐poor rifted margins. The proximal and distal rifted margins reflect two evolutionary phases of rifting involving different structures and isostatic evolutions leading to different stratigraphic records. The proximal rifted margins formed from Upper Triassic to Pliensbachian times and resulted in the formation of classical half graben basins. Rift evolution changed drastically during Pliensbachian to Toarcian times when extension started to localize in the future distal rifted margins. Low‐angle detachment faults become the new elementary structure controlling final crustal tapering and eventually mantle exhumation to the sea floor. New structural elements controlling the stratigraphic architecture of the distal rifted margins include extensional allochthons and breakaway blocks, both resulting from the delamination of a keystone block, also referred to as hangingwall block, during hyper‐extension processes. To define the stratigraphic architecture of the proximal and distal rifted margins, three stratigraphic marker horizons are used, namely the top of the pre‐rift sequences, the top of the proximal margin syn‐tectonic sequence and the base of post‐rift sequences. Based on the distribution of these stratigraphic marker horizons across the Alpine units and a structural re‐evaluation of the rift structures, a synthetic tectono‐stratigraphic evolution of the Alpine Tethys domain is proposed that reconciles old and new ideas about how rift systems may evolve in space and time.

Beltrán‐Triviño, Alejandro; Winkler, Wilfried; Quadt, Albrecht; Schlunegger, Fritz

doi: 10.1111/sed.12006pmid: N/A

Alpine provenance studies based on conventional methods such as sandstone framework grain and heavy mineral analyses are now enhanced by improved techniques in laser ablation inductively coupled plasma mass spectrometry detrital zircon analysis. Although the conventional methods appear to have reached their limits of resolution in palaeogeographic problems, laser ablation inductively coupled plasma mass spectrometry U–Pb dating of detrital zircons adds the time dimension to the provenance analysis. Hafnium‐isotope ratios measured on dated zircons give further information on the origin of the magmas in which the detrital zircons have grown. This study reports detrital zircon U–Pb dating and Hf‐isotope results from sandstone formations related to rifting, drifting and subduction settings at different stages of the Alpine Tethys development. This study is a first evaluation of the correlation between U–Pb age and isotopic features of detrital zircons aimed at describing source terranes in different palaeogeographic domains in the Alpine Tethys area. Pan‐African/Cadomian (Ediacaran–Ordovician), Variscan (Middle Devonian–Carboniferous) and Post‐Variscan (Permian) detrital zircon populations are present in nearly all palaeogeographic settings, but in varying amounts. Single Mesoproterozoic and Palaeoproterozoic detrital zircons are found as minor populations. When comparing the northern and southern margins of the Alpine Tethys, the southern margin detrital sources are characterized mostly by a decreased occurrence or by the absence of Silurian–Devonian zircons. A major distinction between northern (Helvetic, North and Middle Penninic domains) and southern (Austro‐Alpine and South Alpine domains) detrital sources is the occurrence of Triassic zircons at the southern Alpine Tethys margin during rifting and subduction stage sedimentation. Hafnium‐isotope ratios measured on uppermost Permian–Triassic zircons from the South Alpine domain suggest a continental crust derivation of the hosting magmas, as expected in a continental rift environment. In the late stage of Alpine convergence (Late Cretaceous–Palaeogene), the Permian–Triassic zircons are reworked into basins situated on the northern Alpine Tethys margin.