Carbonate Factory Research Articles

Relation between stratigraphic architecture and multi-scale heterogeneities in carbonate platforms: The Barremian–lower Aptian of the Monts de Vaucluse, SE France

Understanding the relationship between multi-scale sedimentary heterogeneities and the stratigraphic architectures of carbonate platforms improves the interpretation of paleoenvironmental evolution in time and space. Such an approach improves the prediction of the spatial distribution of reservoir properties within carbonate stratigraphic units.The reservoir-scale stratigraphic anatomy and the growth history of a Barremian–Aptian carbonate platform (Monts de Vaucluse, SE France) was based on 1) high-resolution measured sections on outcrops, 2) detailed microfacies analyses, 3) paleoenvironmental interpretations, 4) biostratigraphy using rudist bivalves, foraminifera, calcareous algae, and locally echinoderm and ammonites, and 5) sequence stratigraphic interpretations with a special focus on the identification of discontinuity surfaces with a significant stratigraphic correlation potential and critical environmental meaning.The platform succession consists of subtidal (mainly infralittoral) with rare inter- to supratidal deposits. Thirty types of elementary facies, grouped in 12 facies associations, have been defined. The vertical and lateral changes in paleoenvironments together with the development of remarkable stratigraphic surfaces, such as subaerial exposures, marine erosional surfaces, and drowning surfaces, allow the definition of five stratigraphic sequences ranging from the late Barremian to the earliest Aptian.These depositional sequences can be grouped into two distinct carbonate sedimentary systems characterized by specific sedimentary profiles and skeletal and non-skeletal components: 1) a carbonate platform with stronger hydrodynamic gradients, rimmed by a narrow belt of high energy deposits with corals and rudistid rudstone facies and 2) a carbonate ramp dominated by orbitolinids fine-grained facies, including an elongated high energy oolitic shoal. The two-dimension architecture of the depositional sequences (dimensioned geometries, thickness and facies variations) and the morphology of the shelf reflect a strong interaction between tectonic (differential subsidence), sediment production and global environmental conditions. At the km-scale, the “Gorges de la Nesque” stratigraphic motif could represent a nested stratigraphic structure within the large-scale model, which could be implemented for up-scaling petrophysical properties from the well to a full-field model. The association of coral facies to the rudist-dominated system in the Provence platform is a very clear proxy of the polarity of the carbonate platform and is critical for subsurface facies-property modeling based on well data. Finally, at the global scale, the stratigraphic architecture and the step-wise drowning events of the Provence platform confirm and document the precursor events associated with environmental perturbations that affected the carbonate factory prior the Oceanic Anoxic Event 1a (OAE1a).

Response of a carbonate platform to the Cenomanian-Turonian Drowning and OAE 2: a case study from the Adriatic Platform (Dalmatia, Croatia)

Abstract Global perturbations during the Cenomanian–Turonian boundary (CTB) interval and the Oceanic Anoxic Event 2 (OAE 2) represent one of the most extensively studied past environmental changes. To explore the response of various carbonate-platform depositional systems to such major environmental perturbations, strata of the intra-Tethyan Adriatic carbonate platform (sensu stricto) from the island of Brac (Adriatic Sea, Croatia) provide excellent exposures and a previously well-established Upper Cretaceous lithostratigraphic framework. Within this context, this study integrated lithostratigraphy, biostratigraphy, and chemostratigraphy to describe a drowned-platform succession underlain and overlain by peritidal carbonates. Carbon-isotope stratigraphy of this succession revealed a shift towards positive δ13C values that reached +4 to +5% VPDB, and represent the CTB interval excursion plateau. We observed variations in thickness of the drowned-platform successions and explained them by three superimposed mechanisms: (1) diachronous drowning of platform relief; (2) intra-platform redeposition of parts of the successions by various mass-gravity transport processes (indicating enhanced instability due to increasing accommodation space related to the late Cenomanian platform drowning and synsedimentary tectonics); and (3) migration of major carbonate factories during the recovery of shallow-platform environments. The results indicate that the CTB interval event caused unusual increase in accommodation space on the carbonate platform enabling open-marine influence and synsedimentary redeposition. However, widespread organic-rich black shales reported from coeval strata of other regions have not been documented from the platform-top successions to date, and were probably accumulated in deeper (anoxic) settings below the rising sea level.

Anatomy of carbonate mounds from the Middle Anisian of Nakhlak (Central Iran): architecture and age of a subtidal microbial-bioclastic carbonate factory

The Anisian succession of Nakhlak (in Central Iran) is characterized by a siliciclastic succession with minor carbonate units, with massive carbonate mounds up to 50 m thick in its upper part. The mounds, constrained in age to the late Bithynian (Ismidicus Zone) by ammonoids and conodonts, are characterized by a flat top and a lateral pinch-out marked by clinostratified slopes (about 15° in dip). Stratigraphic and microfacies analyses document an inner part of the mound characterized by massive microbial carbonates with open-space structures (stromatactis) filled with fine-grained internal sediments and marine cements. Isolated sponges (up to 5 cm), serpulids and bryozoans are present, which grew on the calcimicrobial limestone. A narrow bioclastic margin (mainly with crinoids and brachiopods) produces most of the slope facies (consisting of bioclastic grainstone and packstone, with intraclasts from the inner part of the mounds) which interfinger basinward with volcaniclastic sandstones. The demise of carbonate productivity is marked on the top of the carbonate mounds by a condensed surface, rich in ammonoids, glaucony grains, and articulated crinoids, documenting a rapid drowning. Paleolatitude data support deposition in a tropical setting, and sedimentological constraints indicate deposition close to the fair-weather wave base, within the photic zone. The late Bithynian Nakhlak carbonate mounds developed before the appearance (documented since the Pelsonian in different parts of the world) of scleractinians which, despite the favorable environmental conditions, are absent at Nakhlak. The Nakhlak mounds thus represent one of the last occurrences of the microbial factories (which developed after the Permo-Triassic extinction event and persisted for most of the Middle Triassic, but with a gradually increasing role played by scleractinians) before the first appearance of the Mesozoic corals.

Coralline algae as environmental indicators: a case study from the Attard member (Chattian, Malta)

We investigated the distribution of the red algae assemblages along the depositional profile of the Attard carbonate ramp of Malta (Chattian). The Attard member is ascribed to the Lower Coralline Limestone Formation characterized by 4 members: Maghlaq, Attard, Xlendi and Il Mara. Coralline algae are present in the inner and middle ramp environments of the Attard member. Sporolithaceans and melobesioids dominate the inner ramp, while mastophoroids and peyssonneliaceans are subordinate. In the middle ramp the association of red algae is characterized by an increase of sporolithacenas and a decrease of melobesioids, mastophoroids and peyssonneliaceans. These assemblages are related to the depth gradient existing from the inner relatively shallow to the progressively deeper middle ramp. However, transportation of red algae down slope may have had an effect on the red algae associations. The shape, morphology and structure of rhodoliths in the inner ramp environment are indicative of high-energy conditions. Nevertheless localized sectors of inner ramp are characterized by morphologies typical of low energy probably related to the presence of areas colonized by seagrass. Rhodoliths from the middle ramp have characteristics of moderately highenergy. The presence of Sporolithon Heydrich and Lithoporelk (Foslie) Foslie indicates that the production of carbonate took place under tropical conditions. We suggest that the Mg/Ca ratios may had a control on the flourishing of coralline algae in the Oligocene carbonate factories situated in oligo- to mesotrophic conditions, whereas during Early to Middle Miocene times the trophic conditions were one of the main controlling factor.

Rudist-bearing rhodalgal facies in the post-Turonian recovery of peri-Tethyan carbonate systems: a case history from the Nurra region (northwestern Sardinia, Italy)

ABSTRACT The presence of non-geniculate coralline red algae and bryozoans (rhodalgal lithofacies) in association with rudists has been mentioned only rarely in the literature. Nevertheless, because of the significance of rhodalgal facies in the characterization of shallow-water carbonate factories, a correct interpretation of the related ecological factors may improve the palaeo-environmental reconstruction of some rudist-bearing carbonate depositional systems. Uppermost Coniacian-Santonian rhodolith-rich rudist-bearing carbonate successions in the Nurra region (northwestern Sardinia, Italy) record several discrete depositional settings, from autochthonous shallow-water mobile skeletal deposits including coralline algal fragments and rhodoliths, to re-mobilized deposits rich in skeletal components with rhodoliths. The rudist-bearing rhodalgal limestone studied is part of an uppermost Turonian-Campanian transgressive sequence that covers a tectonically-modelled Lower Cretaceous substrate. The recovery of t...

Temporal changes in biotic and abiotic composition of shallow-water carbonates on submerged seamounts in the northwestern Pacific Ocean and their controlling factors

The lithology of Cretaceous to Pleistocene shallow-water carbonates, which were collected from 29 sites on 24 submerged seamounts in the northwestern Pacific Ocean using the Deep-sea Boring Machine System, are described. The shallow-water carbonate deposits examined in the present study can be roughly divided into three types based on their composition: Cretaceous, Eocene (to lowest Oligocene?), and Oligocene to Pleistocene. The Cretaceous type is characterized by an abundance of molluscs (including rudists), smaller foraminifers, microencrusters, non-skeletal grains (e.g., peloids, cortoids, and intraclasts), and microbial sediments. Most components have been micritized and possess thick micrite envelopes. The Eocene type is characterized by the dominance of larger foraminifers, Halimeda spp., nongeniculate and geniculate coralline algae, bryozoans, and dasycladacean algae. Scleractinian corals are very minor components. The Oligocene to Pleistocene type is similar in composition to the Eocene type, but it differs from the latter by the abundant occurrence of scleractinian corals and nongeniculate coralline algae. Corals, nongeniculate coralline algae, and Halimeda spp., which precipitate carbonates within closed to semi-closed spaces in and around their bodies (intra-tissue), are major components of the Eocene and Oligocene to Pleistocene types. In contrast, the Cretaceous-type sediments contain relatively more carbonates of extra-tissue origin (i.e. carbonates deposited in relatively open spaces around the bodies of organisms, such as rudists, as well as microbialite and ooids) than the Eocene and Oligocene to Pleistocene types. The changes in the major constituents of the carbonate factory depend on local environments, such as nutrient availability, as well as a global factor: seawater chemistry in the surface waters. Temporal variations in the abundance of the shallow-water carbonates on the examined seamounts suggest that carbonate accumulation was not necessarily controlled by climatic conditions; instead, it was related to the volcanism and tectonics that served as the foundations for reef/carbonate-platform formation.

Lacustrine carbonates in rift settings: the interaction of volcanic and microbial processes on carbonate deposition

Abstract The relatively simplistic facies models for lacustrine carbonates do not currently incorporate either the diversity of microbialite carbonate development or the influence of volcanic-related processes found in rift settings. The basic nature of the carbonate factories in these systems, whether microbial, macrophytic, skeletal or abiogenic, is not resolved. Lacustrine microbialites can develop in shallow lakes as concentrations of microbialite mounds covering many hundreds of square kilometres, or as bathymetrically controlled facies belts, but in many rift settings vent-related thermal and non-thermal carbonates (travertines and tufas) are a major component. Subaqueous vent-related carbonates, with evidence of microbial activity, can produce seismic-scale carbonate build-ups in deeper lakes or apparently more stratiform accumulations in shallow lakes. In lakes with only volcanic catchments, Mg and silica activity, coupled with high carbonate alkalinity and microbial influences, can potentially generate a complex set of mineral–microbe interactions and products, creating a unique set of challenges for predicting and understanding reservoirs in such settings.

A delayed carbonate factory breakdown during the Tethyan-wide Carnian Pluvial Episode along the Cimmerian terranes (Taurus, Turkey)

This paper presents a detailed facies analysis and paleo-depth reconstruction of a latest Early Carnian platform drowning-sequence from the Anatolian terrane (Turkey, Taurus Mountains). A total of eight sedimentary microfacies zones were recorded. An open platform margin passes through a deeper shelf margin setting into a basinal environment influenced by more open-marine conditions. The analysis demonstrates an unexpected, pure carbonate depositional system through the so-called Carnian Pluvial Episode (CPE), which has previously been associated with dramatic climate changes throughout the Tethys region. One main finding, based on sedimentological and paleontological analyses, is a much later drowning of the carbonate platforms in Turkey than in other places. The termination of the Kartoz platform postdates the onset of the CPE in the western Tethys by one ammonite zone, corresponding to about 2 million years. The distinctly earlier demise of (north)western Tethyan carbonate platforms and reefs points to a diachrony of this event throughout the Tethys. The decline of carbonate productivity clearly occurs earlier at higher paleolatitudes and later in equatorial areas. Interpreting the CPE as the result of a global or at least Tethys-wide climatic event is therefore highly ambiguous. The described facies changes at Asagiyaylabel probably mirror different coupling effects and, only minimally, the Tethyan-wide climate changes during the Carnian Pluvial Episode.

Stratal, sedimentary and faunal relationships in the Coniacian 3rd-order sequence of the Iberian Basin, Spain

The Coniacian 3rd-order sequence in the Iberian Basin is represented by a carbonate ramp-like open platform. The biofacies is mainly dominated by nekto-benthic (such as ammonites) and benthic organisms (such as bivalves, mainly rudists) with scarce solitary corals (hermatypics are absent), showing major differences among the Transgressive System Tract (TST) and Highstand Normal Regression (HNR). During the TST, platform environments were dominated by Pycnodonte, other oysters and molluscs (with only subordinate rudists) and ammonites, which were represented by ornamented platycones ( Tissotioides and Prionocycloceras), and by smooth oxycones ( Tissotia and Hemitissotia). During the HNR, shallow water depositional areas were occupied by rudist-dominated associations. Storm- and wind-induced currents and waves acting on these associations produced large amounts of loose bioclastic debris that covered outer platform areas. This facies belt graded landwards into protected, lower-energy settings (inner platform, lagoon and littoral environments). Rudist biostromes were preserved in seaward areas of these protected shallow environments of overall moderate to low hydrodynamic gradient, which was punctuated by storms. In this environment and landwards, large areas of marly substrate favoured the presence of gastropods, other bivalves, echinoderms, benthic foraminifera and solitary corals. Because of the input of siliciclastics and, probably, the lack of nutrients in suspension, the establishment of rudist communities was difficult in more landward areas of the lagoon and in tidal environments. This heterozoan carbonate factory was thus controlled by warm-water conditions and high energy levels, which were responsible for high-nutrient contents in suspension.

The effects of the Late Pliensbachian-Early Toarcian climate changes on the carbonate factory of the Apenninic Platform

The effects of the Late Pliensbachian-Early Toarcian climate changes on the carbonate factory of the Apenninic Platform

Decay and recovery of the mid-Aptian carbonate factory in the Southern Apennine Carbonate Shelf (southern Italy)

Decay and recovery of the mid-Aptian carbonate factory in the Southern Apennine Carbonate Shelf (southern Italy)

Temperate bioerosion: ichnodiversity and biodiversity from intertidal to bathyal depths (Azores)

In the temperate Azores carbonate factory, a substantial fraction of the calcareous skeletal components is recycled by a remarkable biodiversity of biota producing bioerosion traces (incipient trace fossils). To study this biodiversity, experimental carbonate substrates were exposed to colonisation by epilithic and endolithic organisms along a bathymetrical gradient from 0 to 500 m depth, during 1 and 2 years of exposure. The overall bioerosion ichnodiversity is very high and comprises 56 ichnotaxa and ichnoforms attributed to cyanobacteria, chlorophytes, fungi, other micro-chemotrophs, macroborers, grazers and epilithic attachment scars. In the intertidal, hydrodynamic force, partial emersion and strong temperature fluctuations lead to the lowest ichnospecies richness. This contrasts with the highest ichnodiversity found at 15 m under the most favourable environmental conditions. Towards aphotic depths, a gradual depletion in ichnodiversity is observed, most probably because of the restricted light availability and a slowdown in ichnocoenosis development. Analysis of similarity (ANOSIM), in combination with non-metrical multidimensional scaling (NMDS), was used to highlight variability in the relative abundance of traces among depths, substrate orientations and exposure times. Ichnodiversity and abundance of traces decrease significantly with depth and are higher on up-facing versus down-facing substrates, whereas differences between years were not as pronounced. This study demonstrates that statistical methods of biodiversity analysis are not per se restricted to biotaxa but may well be applied also to ichnotaxa. In the analysis of trace fossil assemblages, this approach supports the recognition of diversity patterns and their relation to environmental gradients.

CarboCAT: A cellular automata model of heterogeneous carbonate strata

CarboCAT is a new numerical model of carbonate deposystems that uses a cellular automata to calculate lithofacies spatial distributions and hence to calculate the accumulation of heterogeneous carbonate strata in three dimensions. CarboCAT includes various geological processes, including tectonic subsidence, eustatic sea-level oscillations, water depth-dependent carbonate production rates in multiple carbonate factories, lateral migration of carbonate lithofacies bodies, and a simple representation of sediment transport. Results from the model show stratigraphically interesting phenomena such as heterogeneous strata with complex stacking patterns, laterally discontinuous subaerial exposure surfaces, nonexponential lithofacies thickness distributions, and sensitive dependence on initial conditions whereby small changes in the model initial conditions have a large effect on the final model outcome. More work is required to fully assess CarboCAT, but these initial results suggest that a cellular automata approach to modeling carbonate strata is likely to be a useful tool for investigating the nature and origins of heterogeneity in carbonate strata.

Compositional changes in sigmoidal carbonate clinoforms (Late Tithonian, eastern Sardinia, Italy): insights from quantitative microfacies analyses

AbstractQuantitative analysis of sediment composition was performed on a kilometre wide section of Upper Tithonian low relief (up to 70 m), gently inclined (3° to 15°), sigmoidal carbonate clinoforms (eastern Sardinia) to identify changes in sediment composition along the slope and across the studied succession. These changes may reflect modifications of the carbonate factory and of processes responsible for sediment transport. Point‐count analysis of carbonate microfacies, Q‐mode/R‐mode cluster analysis and Spearman’s rank provided a composition‐based classification of microfacies and highlighted relationships among sediment components. The studied clinoforms are mainly composed of non‐skeletal grains (70%), such as peloids and lithoclasts, together with micrite and cements and only a limited contribution from coated grains (2%). Among skeletal grains (28%), the greatest contribution derives from a coral–stromatoporoid–encruster reef that provided 15% of the components. Crinoids, brachiopods and other along‐slope thriving biota provided nearly 5% of the allochems, whilst fragments of molluscs (gastropods, bivalves and diceratids) from the backreef sourced another 2%. The contribution of platform interior biota is negligible (1%). The association of composition‐based facies varies along the slope. The upper slope beds consist of coral‐stromatoporoid grainstone to rudstone; the middle slope deposits are dominated by encruster‐lithoclast grainstone and packstone. At the lower slope, peloidal lithoclastic packstone as well as brachiopod–crinoidal wackestone prevail. Also the association of skeletal grains changes along the slope. The encruster–frame builder association typifies the upper slope whilst encrusters characterize the middle slope sediments. In the lower slope encrusters are equally represented as the brachiopod–crinoid association. Along‐slope compositional changes evidence a scarce downslope transport of frame builders and a progressive enrichment in along‐slope thriving biota. Quantitative analysis of microfacies allowed the sigmoidal clinoforms to be grouped into six sets. Each set gathers sigmoids with a similar sediment composition. Coated grains are dominant in the first set whilst they are lacking in the overlying sets reflecting a change in the carbonate factory. Other major compositional changes among the sets concern the relative amounts of peloids, micrite, frame builders (corals and stromatoporoids) and encrusters. The contribution of peloids varies inversely to that of cements and micrite as evidenced in the third and fifth sets which, respectively, record the highest occurrence of peloids or cement and micrite. Variations in the amount of frame builders and encrusters are instead non‐linear. High percentages of both frame builders and encrusters, as recorded in the second and fifth sets, are related to low amounts of peloids and lithoclasts that probably reflect episodes of reduced background sedimentation. This study demonstrates that quantitative analysis of carbonate microfacies represents a powerful tool that can improve the reconstruction of the stacking pattern in carbonate slope successions both in outcrop and in subsurface settings.

Large-scale progradation, demise and rebirth of a high-relief carbonate platform (Triassic, Lombardy Southern Alps, Italy)

The Upper Anisian to Early Carnian succession of the Middle Val Brembana–Pegherolo Massif (Central Southern Alps of Italy) records a complete depositional cycle from platform inception to growth, demise and rebirth. The depositional architecture of this system reflects different evolutionary stages: an inception stage which postdates a previous drowning of an Anisian carbonate platform with progradation of the carbonate platform from the nucleation areas, an aggradational stage with increasing water depth in the basins, a progradational stage where steep slopes composed of margin-derived breccias develop and a final crisis corresponding to the subaerial exposure of the platform top, followed by the deposition of shales in the basin before the rebirth of a different type of carbonate factory. The record of this evolution reflects the effects of the change in accommodation space (interplay of subsidence and eustacy), which controls the type and storage sites of the sediments produced by the carbonate factory. The effects of the changes in accommodation space are recorded in the shallow water platform as well as in the intraplatform basins, where the sediments, delivered at different rates from the platform top are stored. As a consequence, the aggradational stage corresponds to reduced sedimentation in the basins (i.e. sediments are stored on the platform top) whereas during progradation resedimented limestones are more common in the basin. Subaerial exposure rapidly halted the carbonate production on the platform top, while a major input of shales (probably reflecting a climate change and/or lowering of the base level) is recorded in the basin, where shales onlap the slope of the previous carbonate system. The rebirth of the carbonate factory after subaerial exposure of the platform top is characterized by a different composition of the carbonate factory, probably reflecting changes of the environmental conditions. The step-by-step recording of the evolution of the carbonate system represents a unique opportunity to record a seismic-scale complete evolutionary cycle of a carbonate system in its different sub-environments, from the platform top to the basin.

Carbonate sedimentation and hydrodynamic pattern on a modern temperate shelf: The strait of Bonifacio (western Mediterranean)

The sedimentary features of the inner-middle shelf of the strait of Bonifacio (western Mediterranean) were analyzed to evaluate the relationship between the production and transport of biogenic carbonate sediments and the basin morphology and hydrodynamics. A three-dimensional hydrodynamic modeling was performed in order to simulate the influence of waves and currents at seabed level. Superficial sediments were collected at depths ranging from 5 to 80 m and were analyzed for grain size, mineralogical composition and skeletal carbonate composition. Posidonia oceanica seagrass meadows border the coasts in a narrow strip on both sides of the strait down to a depth of 40 m. At greater depths, the seabed is characterized by the presence of plateaus and ridges which are controlled by outcropping bedrock morphology. Waves and seabed currents are driven by the prevailing northwest and northeast winds. For both wind directions, higher values for the seabed current velocity, associated with wind-storm events, were detected in shallower sectors and along an east–west-oriented belt that connects the western Mediterranean and the Tyrrhenian Seas. The sediments range from sand to gravel and show a mixed biogenic carbonate/siliciclastic composition. This is due to the carbonate production associated with benthic ecosystems and the mixing of modern carbonate with relict sediments. Biogenic gravelly sands were found in association with Posidonia oceanica seagrass meadows and extended down to the circalittoral zone outside the deeper boundary of the meadows. This sedimentary facies was derived from the modern carbonate production associated with the P. oceanica ecosystem. Sediments collected outside the deeper limit of the meadows were identified as sediments deposited during the stand of the meadows at a deeper level during lower sea level conditions. Maërl (free-living calcareous red algae) beds are the main carbonate factory and are mainly located at the top of the rocky plateaus (at ∼50–55 m) formed by the outcropping of the bedrock. Downward, the increased currents at the seabed level in the east–west-oriented belt, which connects the western Mediterranean and the Tyrrhenian Seas, limit the extension of this carbonate factory. This results in a mixed sedimentary facies composed of biogenic carbonate and relict siliciclastic sandy gravel. Compared to other Mediterranean shelves, the strait of Bonifacio is characterized by a distinct oceanographic setting. This is due to the connection between two basins. The currents at the seabed play a crucial role in controlling the distribution of the carbonate factories.

Sequence development in an isolated carbonate platform (Lower Jurassic, Djebel Bou Dahar, High Atlas, Morocco): influence of tectonics, eustacy and carbonate production

AbstractSix sequences, each with a distinct shape, stratal anatomy, bounding surfaces, lithofacies composition and stacking patterns, were identified in the Djebel Bou Dahar isolated carbonate platform which developed in a half‐graben in the High Atlas marine rift basin during the Early Jurassic. Sequences are believed to have a complex, multi‐process origin as the result of tectonic processes (regional subsidence and fault‐block rotation) overprinting the eustatic signal. In addition, changing carbonate producers with shifting centres of deposition and relative accumulation potential influenced the filling and anatomy of the accommodation space that had been created by the hybrid tectonic and eustatic processes. Sequences I and II are tabular in shape and interpreted to primarily reflect the combined processes of uniform tectonic subsidence and eustacy which resulted in a ramp to shelf carbonate system with lagoon and tidal flat lithofacies. Onset of a rifting episode around the base of the Lotharingian and fault block rotation were responsible for the development of the angular unconformity with the underlying Sequence II, and the progressive development of a shallow‐water isolated platform and the wedge‐shaped depositional Sequences III to VI during the Lotharingian and Pliensbachian. Concurrent variations in palaeoceanographic conditions and in the carbonate factory caused a shift towards a flat‐topped platform geometry with high‐energy coated grains and sub‐photic sponge‐microbial deposition around the margin. The aggradational to retrogradational character of Sequences III, IV and VI is inferred to be a result of the combined effect of in situ carbonate production coupled with high rates of creation of accommodation space in the hanging wall. Environmental effects on carbonate production were the most probable cause for drowning of the Djebel Bou Dahar platform during the Toarcian but also influenced the filling and geometry of Sequences IV to VI. This study suggests that the commonly accepted model of pulse‐driven sequence‐development in half‐grabens may not be fully applicable for the Djebel Bou Dahar, where a more continuous rotation at sequence scale, associated with subsidence pulses of the Djebel Bou Dahar block, seems to have interacted with eustacy and carbonate production to generate sequences. Continuous rotation is suggested by the presence of regular patterns of diverging strata and numerous minor exposure surfaces in lagoonal deposits of Sequences V and VI. In addition, this study demonstrates that eustacy and variations in the carbonate factory might be as significant as tectonics in sequence development and platform drowning in carbonate platforms within rift basins. This observation could imply that the effects of eustacy and the carbonate factory might not have been correctly recognized in other similar carbonate platforms due to a tectonic‐biased interpretation.

Depositional processes of the Zhushadong and Mantou formations (Early to Middle Cambrian), Shandong Province, China: roles of archipelago and mixed carbonate–siliciclastic sedimentation on cycle genesis during initial flooding of the North China Platform

AbstractTo understand the depositional processes and environmental changes during the initial flooding of the North China Platform, this study focuses on the Lower to Middle Cambrian Zhushadong and Mantou formations in Shandong Province, China. The succession in the Jinan and Laiwu areas comprises mixed carbonate and siliciclastic deposits composed of limestone, dolostone, stromatolite, thrombolite, purple and grey mudstone, and sandstone. A detailed sedimentary facies analysis of seven well‐exposed sections suggests that five facies associations are the result of an intercalation of carbonate and siliciclastic depositional environments, including local alluvial fans, shallowing‐upward carbonate–siliciclastic peritidal cycles, oolite dominant shoals, shoreface and lagoonal environments. These facies associations successively show a transition from an initially inundated tide‐dominated carbonate platform to a wave‐dominated shallow marine environment. In particular, the peritidal sediments were deposited during a large number of depositional cycles. These sediments consist of lime mudstone, dolomite, stromatolite and purple and grey mudstones. These shallowing‐upward cycles generally resulted from carbonate production in response to an increase of accommodation during rising sea‐level. The carbonate production was, however, interrupted by frequent siliciclastic input from the adjacent emergent archipelago. The depositional cycles thus formed under the influence of both autogenetic changes, including sediment supply from the archipelago, and allogenic control of relative sea‐level rise in the carbonate factory. A low‐relief archipelago with an active tidal regime allowed the development of tide‐dominated siliciclastic and carbonate environments on the vast platform. Siliciclastic input to these tidal environments terminated when most of the archipelago became submerged due to a rapid rise in sea‐level. This study provides insights on how a vast Cambrian carbonate platform maintained synchronous sedimentation under a tidal regime, forming distinct cycles of mixed carbonates and siliciclastics as the system kept up with rising relative sea‐level during the early stage of basin development in the North China Platform.

Late Pleistocene and Holocene cool‐water carbonates of the Western Mediterranean Sea

AbstractPost‐glacial, neritic cool‐water carbonates of the Western Mediterranean Sea were examined by means of hydroacoustic data, sediment surface sampling and vibrocoring to unravel geometries and to reconstruct sedimentary evolution in response to the last sea‐level rise. The analysed areas, located on the Alboran Ridge, in the Bay of Oran, and at the southern shelf of the island of Mallorca, are microtidal and bathed by oligotrophic to weakly mesotrophic waters. Seasonal water temperature varies between 13 °C and 27 °C. Echosounder profiles show that the Bay of Oran and the southern shelf of Mallorca are distally steepened ramps, while the Alboran Ridge forms a steep‐flanked rugged plateau around the Alboran Island. In the three areas, an up to 10 m thick post‐glacial sediment cover overlies an unconformity. In Oran and Mallorca, stacked lowstand wedges occur in water depths of 120 to 130 m. On the Alboran Ridge and in the Bay of Oran, highstand wedges occur at 35 to 40 m. Up to 5 m long cores of upper Pleistocene to Holocene successions were recovered in water depths between 40 and 81 m. Deposits contain more than 80% carbonate, with mixed carbonate‐volcaniclastics in the lower part of some cores in Alboran. The carbonates consist of up to 53% of aragonite and up to 83% of high magnesium calcite. Radiocarbon dating of bivalve shells, coralline algae and serpulid tubes indicates that deposits are as old as 12 400 cal yr bp. The carbonate factories in the three areas are dominated mostly by red algae, but some intervals in the cores are richer in bivalves. A facies rich in the gastropod Turritella, reflecting elevated surface productivity, is restricted to the Mallorca Shelf. Rhodoliths occur at the sediment surface in most areas at water depths shallower than 70 m; they form a 10 to 20 cm thick veneer overlying rhodolith‐poor bioclastic sediments which, nonetheless, contain abundant red algal debris. This rhodolith layer has been developing for the past 800 to 1000 years. Similar layers at different positions in the cores are interpreted as reflecting in situ growth of rhodoliths at times of reduced net sedimentation. Sedimentary successions in the cores record the post‐glacial sea‐level rise and the degree of sediment exposure to bottom currents. Deepening‐upward trends in the successions are either reflected by shallow to deep facies transitions or by a corresponding change of depth‐indicative red algae. There are only weak downcore variations of carbonate mineralogy, which indicate that no dissolution or high magnesium to low magnesium calcite neomorphism occurs in the shallow subsurface. These new data support the approach of using the Recent facies distribution for interpretation of past cool‐water, low‐energy, microtidal carbonate depositional systems. Hydroacoustic data show that previous Pleistocene transgressive and highstand inner ramp deposits and wedges were removed during sea‐level lowstands and accumulated downslope as stacked lowstand wedges; this suggests that, under conditions of high‐amplitude sea‐level fluctuations, the stratigraphic record of similar cool‐water carbonates may be biased.

Stromatoporoid palaeoecology in the Frasnian (Upper Devonian) Belgian platform, and its applications in interpretation of carbonate platform environments

Abstract: Stromatoporoid faunas in the Frasnian of southern Belgium are abundant in the carbonate platform environments present in this area. Stromatoporoids dominate the large skeletal organisms and occur principally in biostromes. The stromatoporoid assemblage is represented by a small number of taxa. Stromatoporoid genera include Actinostroma, Amphipora, Atelodictyon, Clathrocoilona, Salairella, Stachyodes, Stictostroma, Stromatopora and Trupetostroma which are present in environments ranging from the outer, outer intermediate, inner intermediate and inner zones and associated biostromes. Most large skeletal stromatoporoids are low profile, which reinforces the conclusions of previous studies that low‐profile growth forms were the most successful stromatoporoid forms. These low‐profile forms are likely to have been important sediment stabilisers that may have led to expansion of the carbonate factory. Growth forms vary between facies, indicating some degree of environmental control on form; for example, laminar in the intermediate zone, bulbous and domical in the inner and outer zones. Stromatoporoid taxa vary in occurrence across the environmental gradient from shallow to deep. There is some taxonomic control on growth forms, with some taxa showing more variability than others in different environments.