Multistage dolomitization and distribution of dolomitized bodies in Early Jurassic carbonate platforms (Southern Alps,Italy)

The Early Jurassic dolomitized carbonates are a hydrocarbon exploration target in Northern Italy. Of these carbonates, the Liassic Albenza Formation platform and the overlying Sedrina Formation shelf were studied to define a pervasive dolomitization model and to shed light on dolomite distribution in the sub‐surface. Field work, as well as analyses of well cores, stable isotopes, trace elements and fluid inclusions, was carried out on the outcropping thrust belt and sub‐surface deformed foreland of the Southern Alps. Petrographic analyses showed a first, pervasive, replacement dolomitization phase (D1) followed by volumetrically less important dolomite cement precipitation phases (D2, D3 and D4). The δ¹⁸O values fall between ?8·2‰ and 0·1‰ Vienna‐Pee Dee Belemnite with the more depleted samples belonging to dolomite cement‐rich dolostones; the δ¹³C ranges from 2·6‰ to 3·7‰ Vienna‐Pee Dee Belemnite. Analysis of trace elements showed different Fe and Mn contents in the sub‐surface and outcropping dolostones, and a higher Fe in the younger dolomite cements. An increase in the precipitation temperature (up to 130 °C from fluid inclusion data) and a decrease in diagenetic fluid salinity (from sea water to brackish) are observed from the first pervasive replacement dolomite to the dolomite cement phases. Field observations indicate that, in the Albenza Formation, dolomitization was limited to palaeohighs or faulted platform margins in the Early Jurassic carbonates. The pervasive replacement phase is interpreted based on a ‘compaction model’; the formation fluids expelled from compacting basinal carbonates could have funnelled along faults into permeable palaeohighs. The high homogenization temperature of the dolomite cements and decreased salinities indicate precipitation at great depth with an influx of meteoric water. These data, along with the thermal history, suggest that the dolomite cements precipitated according to the ‘tectonic squeegee’ dolomitization model. The dolomite precipitation temperature was set against the thermal history of the carbonate platform to interpret the timing of dolomite precipitation. The dolomite precipitation temperatures (90 to 100 °C) were reached in the studied formations first in the thrust fold belt (Early Tertiary, 60 Ma), and then in the foreland succession during the Late Tertiary (10 Ma). This observation suggests that the dolomite precipitation fronts moved southwards over time, recording a ‘diagenetic wave’ linked to the migration of the orogenic system. Observations suggest that the porosity increased during the first phase of replacement dolomitization while the dolomite cementation phases partially occluded the pores. The distribution of porous dolomitized bodies is therefore linked to the ‘compaction dolomitization’ model.     

Anatomy of an Upper Triassic continental to marginal‐marine system: the mixed siliciclastic–carbonate Travenanzes Formation (Dolomites,Northern Italy)

The Travenanzes Formation is a terrestrial to shallow‐marine, siliciclastic–carbonate succession (200 m thick) that was deposited in the eastern Southern Alps during the Late Triassic. Sedimentary environments and depositional architecture have been reconstructed in the Dolomites, along a 60 km south–north transect. Facies alternations in the field suggest interfingering between alluvial‐plain, flood‐basin and shallow‐lagoon deposits, with a transition from terrestrial to marine facies belts from south to north. The terrestrial portion of the Travenanzes Formation consists of a dryland river system, characterized by multicoloured floodplain mudstones with scattered conglomeratic fluvial channels, merging downslope into small ephemeral streams and sheet‐flood sandstones, and losing their entire discharge subaerially before the shoreline. Calcic and vertic palaeosols indicate an arid/semi‐arid climate with strong seasonality and intermittent discharge. The terrestrial/marine transition shows a coastal mudflat, the flood basin, which is usually exposed, but at times is inundated by both major river floods and sea‐water storm surges. Locally coastal sabkha deposits occur. The marine portion of the Travenanzes Formation comprises carbonate tidal‐flat and shallow‐lagoon deposits, characterized by metre‐scale shallowing‐upward peritidal cycles and subordinate intercalations of dark clays from the continent. The depositional architecture of the Travenanzes Formation suggests an overall transgressive pattern organized in three carbonate–siliciclastic cycles, corresponding to transgressive–regressive sequences with internal higher‐frequency sedimentary cycles. The metre‐scale sedimentary cyclicity of the Travenanzes Formation continues without a break in sedimentation into the overlying Dolomia Principale. The onset of the Dolomia Principale epicontinental platform is marked by the exhaustion of continental sediment supply.     

Planktonic foraminiferal turnover across the Cretaceous-Tertiary boundary in the Vajont valley (Southern Alps, northern Italy)

Planktonic foraminiferal analysis of the Erto section in the Vajont valley (Southern Alps, northern Italy) reveals a relatively complete succession across the Cretaceous–Tertiary (K–T) boundary. The turnover of planktonic foraminiferal fauna was studied for a stratigraphic interval spanning theAbathomphalus mayaroensisZonep.p., Pseudotextularia deformisZone,Guembelitria cretaceaZone,Parvularugoglobigerina eugubinaZone,Eglobigerina eobullioidesSubzone, andParasubbotina pseudobulloidesZonep. p.The extinction of most large, ornate, late Maastrichtian species occurs below a black ‘boundary clay’ (2–4 cm thick); however, part of the Late Cretaceous species, mainly heterohelicidids and hedbergellids, were found over an interval of more than 100 cm above the boundary. Although a relatively high number of species occur for the last time in the main extinction phase, the abundance of these outgoing species is less than 20% of the total population; unkeeled or weakly keeled, simple-shaped forms (heterohelicids, globotruncanellids, hedbergellids) constitute the bulk of the planktonic foraminiferal population both in uppermost Maastrichtian and lowermost Danian beds. The first Tertiary species (‘Globigerina’minutulaand ‘Globigerina’fringa) appear just above the ‘boundary clay’;Parvularuglobigerina eugubinaoccurs a few centimeters above. A marked increase in abundance and diversity in the Tertiary planktonic foraminiferal population occurs at the base of theEoglobigerina eobulloidesSubzone.     

Late Neogene–Quaternary evolution of the intermontane Clusone Basin (Southern Alps,Italy): integration of seismic and geological data

In the Clusone Basin (a large intermontane basin filled by thick late Neogene–Quaternary sediments in the Middle Val Seriana, Southern Alps), two high‐resolution seismic profiles have been acquired in order to reconstruct the geometries of the sediments that fill the depression, with a maximum thickness of more than 200 m as documented by available well data, and to define their relationships with the bedrock, consisting of Late Triassic carbonates. In addition to standard seismic reflection processing, a seismic refraction inversion technique has been applied. The integration of geological (both surface and well data) and seismic data indicates a complex history of the drainage patterns of the Clusone Basin, documenting a shift of the Serio River from a palaeodrainage toward the southeast (Val Borlezza) to the present situation, toward the south (Val Seriana): between the older and the present‐day drainages an important depositional stage occurred, as documented by the thick sediments that fill the Clusone Basin, controlling the capture of the Serio River along the Val Seriana. Copyright © 2004 John Wiley & Sons, Ltd.     

Carbonate platform evolution: from a bioconstructed platform margin to a sand-shoal system (Devonian, Guilin, South China)

ABSTRACT The depositional organization and architecture of the middle–late Devonian Yangdi rimmed carbonate platform margin in the Guilin area of South China were related to oblique, extensional faulting in a strike‐slip setting. The platform margin shows two main stages of construction in the late Givetian to Frasnian, with a bioconstructed margin evolving into a sand‐shoal system. In the late Givetian, the platform margin was rimmed with microbial buildups composed mainly of cyanobacterial colonies (mostly Renalcis and Epiphyton). These grew upwards and produced an aggradational (locally slightly retrogradational) architecture with steep foreslope clinoforms. Three depositional sequences (S3–S5) are recognized in the upper Givetian strata, which are dominated by extensive microbialites. Metre‐scale depositional cyclicity occurs in most facies associations, except in the platform‐margin buildups and upper foreslope facies. In the latest Givetian (at the top of sequence S5), relative platform uplift (± subaerial exposure) and associated rapid basin subsidence (probably a block‐tilting effect) caused large‐scale platform collapse and slope erosion to give local scalloped embayments along the platform margin and the synchronous demise of microbial buildups. Subsequently, sand shoals and banks composed of ooids and peloids and, a little later, stromatoporoid buildups on the palaeohighs, developed along the platform margin, from which abundant loose sediment was transported downslope to form gravity‐flow deposits. Another strong tectonic episode caused further platform collapse in the early Frasnian (at the top of sequence S6), leading to large‐scale breccia release and the death of the stromatoporoid buildups. Siliceous facies (banded cherts and siliceous shales) were then deposited extensively in the basin centre as a result of the influx of hydrothermal fluids. The platform‐margin sand‐shoal/bank system, possibly with gullies on the slope, persisted into the latest Frasnian until the restoration of microbial buildups. Four sequences (S6–S9), characterized by abundant sand‐shoal deposits on the margin and gravity‐flow and hemipelagic deposits on the slope, are distinguished in the Frasnian strata. Smaller‐scale depositional cyclicity is evident in all facies associations across the platform–slope–basin transect. The distinctive depositional architecture and evolution of this Yangdi Platform are interpreted as having been controlled mainly by regional tectonics with contributions from eustasy, environmental factors, oceanographic setting, biotic and sedimentary fabrics.     

The Bocchigliero Palaeozoic sequence in the context of the Calabrian–Peloritan Hercynian Range (Italy)

The Calabrian–Peloritan Hercynian Range includes three weakly metamorphosed Palaeozoic sequences cropping out in north-eastern Sila (Bocchigliero sequence), southern Sila, Serre and Aspromonte (Stilo sequence), and in the Peloritan Mountains (Peloritan sequence). The work reported here considers the Bocchigliero sequence and comprises part of a geological, petrological and geochemical research programme on the Palaeozoic evolution of the Calabrian–Peloritan Arc. The Bocchigliero sequence constitutes the lower tectonic unit of the Hercynian Caiabrian–Peloritan Range and is overthrusted by the metamorphic Mandatoriccio Unit. The Bocchigliero sequence is a terrigenous–carbonate–volcanic association, is affected byclow grade metamorphism, contains Cambro-Ordovician fossils and extends in age from the Cambrian to the Devonian. The terrigenous material is represented by meta-arenites and metapelites (Cambrian–Devonian); the volcanics include metatuffites (Cambrian and Ordovician), metabasalts (Cambro-Ordovician), metaandesites and metarhyolites (Ordovician and Siluro-Devonian); limestone beds are present in the Devonian. It is believed that the Palaeozoic Bocchigliero basin formed in the Cambrian on a continental crust in which the rocks constituting today's Mandatoriccio Unit were located at 3–8 km depth. The crustal thinning in the Cambro-Ordovician led to fracturing and upwelling of alkaline within-plate basaltic magmas, whereas in the Ordovician the thinning took place under conditions of higher plasticity. In this latter period an increase in temperature resulting from mantle upwelling produced crustal partial melts of andesite and rhyolite composition. In addition, this thermal uprise was responsible for regional metamorphism characterized by low pressures and by the absence of penetrative deformation. The effects of this metamorphism are well developed in the rocks of the Mandatoriccio Unit. In the Silurian and Devonian, progressive closing of the basin took place. The Palaeozoic sequence was then subjected to Variscan low pressure–low temperature metamorphism and Alpine deformation.     

Sequential and climatic framework of the growth and demise of a carbonate platform: implications for the peritidal cycles (Late Jurassic,North‐eastern France)

The Middle Oxfordian of the eastern Paris Basin constitutes a remarkable example of the growth and demise of a carbonate platform. Fischer plots, sedimentary and diagenetic features allow the identification of four depositional cycles (S5 to S8) in the Transversarium Zone; they are inserted in a lower frequency cycle of increased/decreased accommodation space (SoIII). The long‐term period of accommodation creation occurred during the older S5 and S6 cycles, the maximum accommodation zone being located in the lower part of the S6 cycle. This high accommodation period was tectonically controlled and was coeval with local distensive activity of a Hercynian fault. A major minimum accommodation zone exists during the S8 cycle. At that time, the platform was isolated and presented both a windward and a leeward margin. The growth of the platform was favoured by a warm and arid climate, oligotrophic conditions and reduced siliciclastic input during a highstand in relative sea‐level. These palaeoenvironmental features favoured the proliferation of phototrophic organisms producing carbonate material. The death of the platform was generated by a reduction in the carbonate production surface during a lowstand in relative sea‐level and by the appearance of mesotrophic conditions induced by the increase in siliciclastic inputs at the beginning of a period with a cooler and more humid climate. In the eastern Paris Basin, during the Middle Oxfordian, the parasequences are ordered and present ‘greenhouse’ characteristics. In contrast, at the beginning of the S8 cycle, the randomness in the thickness of contiguous parasequences increased. Decreased carbonate production during the lowstand caused by a transition from photozoan to heterozoan benthic communities certainly favoured this randomness and the appearance of catch‐down parasequences.     

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

Quantitative 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.     

Three orders of regional sea-level changes control facies and stacking patterns of shallow platform carbonates in the Maestrat Basin (Tithonian-Berriasian,NE Spain)

Middle Tithonian-Berriasian shallow platform carbonates of the Maestrat Basin (Salzedella and Montanejos sections, NE Spain) are stacked in sequence stratigraphic units of different orders. Higher-order sequences (parasequences and subunits) have a shallowing or deepening-shallowing evolution. They have been related to the short-term eccentricity and precession cycles. Major facies changes and stacking pattern of parasequences reveal the presence of two 2nd-order sequences. The Lower Sequence is middle Tithonian-mid early Berriasian in age. The Upper Sequence extends up to the mid-late Berriasian. It is suggested that local subsidence changes along with regional sea-level changes controlled the long-term evolution of accommodation in the Maestrat Basin. Facies evolution, stacking pattern and sharp lithological changes have allowed the definition of five 3rd-order sequences in the Lower Sequence in Montanejos. The transgressive deposits are characterised by the progressive absence of the restricted lagoon facies, and the presence of deepening-upward intervals in the parasequences. The highstand deposits display an increase in siliciclastics and a progressive predominance of restricted lagoon facies. Some of the 3rd-order sequence boundaries match the sequence boundaries identified in other European basins and may be related to sea-level falls (induced by the long-term eccentricity cycle) enhanced during periods of long-term loss of accommodation.     

Facies architecture and cyclicity in a mosaic carbonate platform: effects of fault-block tectonics (Lower Lias, Causses platform, south-east France)

In the Causses platform (south‐east France), Late Hettangian to Sinemurian deposits were interpreted previously as shallow‐water carbonate ramp deposits. A new look at these deposits has shown a fault‐controlled mosaic carbonate platform that is different from the carbonate ramp models. Within the platform mosaic, 15 lithofacies have been recognized, which are organized in four facies associations, including peritidal, restricted shallow sub‐tidal, sand dunes and sub‐tidal shelf facies associations. The rapid lateral and vertical facies changes, and the lack of consistent landward or seaward direction indicated by the pattern of facies changes, question the existence of a shoreline suggested by the traditional models for this region. Instead, the facies organization and cycle stacking pattern suggest deposition in a mosaic of intertidal islands between which sub‐tidal restricted or open conditions could coexist in very close proximity. Such a platform mosaic would have been defined by tectonic activities along normal faults which segmented the shallow‐water Causses platform. The facies and facies associations are arranged into metre‐scale, peritidal and sub‐tidal cycles that are also variable. Certain cycles show the same stacking pattern in all the sections and seem to be traceable over tens of kilometres. On the contrary, other cycles cannot be correlated; they are present only in specific sections and have a maximum lateral extension of 1 or 2 km. These metre‐scale cycles stack to form four medium‐scale cycles bounded by surfaces that display sub‐aerial exposure features. Medium‐scale cycles stack into two larger‐scale cycles (tens of metres thick) and are bounded by well‐defined karstic surfaces. Based on their lateral continuity and their stacking pattern, the metre‐scale cycles are controlled probably by high frequency eustatic variations overprinting the topographic irregularities formed by differential subsidence of fault‐bounded blocks. Episodic fault activities may reorganize the topography so that, even if eustatic changes may still be the major control of cycles, the expression and number of cycles could be different. Cycles of medium and large‐scale are interpreted as being allogenic, controlled by changes in eustasy and/or subsidence rates as evidenced by their lateral continuity and the correlations of the large‐scale cycles with third‐order depositional sequences.     

The Mid-Cenomanian Event in shallow marine environments: Influence on carbonate producers and depositional sequences (northern Aquitaine Basin,France)

The Mid-Cenomanian Event was a positive carbon-isotope (δ¹³C) excursion recorded in hemipelagic basins of the western Tethyan Sea, North to Tropical Atlantic Ocean, and Japan. It is thought of as a prelude to the Oceanic Anoxic Event 2. However, the Mid-Cenomanian Event has never been studied in detail in shallow marine platform deposits and it is not known how it relates to carbonate production and stratigraphic geometry. To better understand how this carbon cycle disruption influenced the neritic biological communities in shallow carbonates during the Cenomanian, a facies, geochemical, diagenetic, and sequence stratigraphic study of the northern Aquitaine platform has been conducted. Seventy-six δ¹³C and δ¹⁸O measurements have been made on micrite, rudists, and diagenetic cements. Fifteen sedimentary facies have been arranged into four depositional environments. Three third-order sequences (C_B, C_C, C_D) are defined from late early Cenomanian to early late Cenomanian and are well correlated with eustatic cycles in European basins. Two peaks of the Mid-Cenomanian Event (MCE1a, +1.2‰, and MCE1b, +1.7‰) have been identified for the first time in shallow marine carbonates. Analysis of diagenetic blocky calcite cements suggests that diagenesis did not affect the δ¹³C of micrite, which can be discussed in terms of the initial signal. The Mid-Cenomanian Event was synchronous with a turnover in neritic carbonate producers marking a transition from photozoan to heterozoan facies. This facies change resulted from the establishment of mesotrophic to eutrophic conditions at the early/mid-Cenomanian transition, reflecting a clear connection between the Mid-Cenomanian Event and neritic biological communities. Depositional geometry and carbonate production varied with δ¹³C during the Mid-Cenomanian Event on the Aquitaine platform. When δ¹³C values were between 2.5‰ and 3‰, the geometry was a flat platform with a high carbonate sedimentation rate leading to the formation of sandbars and rudist bioherms (Accommodation/Sedimentation ratio less than 1, A/S < 1). When the δ¹³C value exceeded 3‰, a carbonate demise occurred and clays and marls were deposited in the lower offshore environment (A/S >> 1). The general carbonate demise affecting the northern Aquitaine platform during the mid-Cenomanian can be explained by both a eustatic sea-level rise and the establishment of eutrophic conditions. The coincidence of the Mid-Cenomanian Event with both (1) the occurrence of mesotrophic to eutrophic conditions marked by carbonate producer turnover from photozoan to heterozoan facies and (2) the transgressive cycles, suggests that eustatic sea-level rise leading to high trophic conditions could explain this positive δ¹³C excursion in the Atlantic and western Tethyan domain. During the mid-Cenomanian, carbon cycle perturbations largely controlled the neritic biological communities on shallow carbonate platforms in a part of the western Tethyan domain.     

Integrating outcrop data and forward computer modelling to unravel the development of a Messinian carbonate platform in SE Spain (Sorbas Basin)

Geological mapping, definition of facies distributions and reconstruction of platform‐interior growth geometries of the Messinian Cariatiz carbonate platform (Sorbas basin, South Spain), were performed to evaluate the controlling factors in platform growth and to test a 3‐D computer simulation program. For the simulation with the program REPRO, five platform‐related facies were modelled: (1) the reef crest facies by the numerical solution of a Fisher equation; (2) the lagoonal facies by a function of water depth‐dependent carbonate production; (3) the proximal and middle slope facies (breccia and block facies, calcarenite facies) by a subroutine simulating gravity‐driven particle export from the reef crest; (4) a distal slope; and (5) a basinal facies by a pelagic rain function. Development of a fan delta conglomeratic system is simulated by using a siliciclastic point source and gravity‐driven particle redistribution. A best fit between the observed platform growth geometries and modelling results is achieved by assuming that high‐frequency sea‐level changes superimposed onto a longer term sea‐level fall controlled platform growth. For the modelling, a relative sea‐level curve was reconstructed, which is based on a deep‐sea benthic foraminiferal stable oxygen isotope record at ODP Site 926 with a 45 m eustatic sea‐level fall, and a tectonic uplift component of 20 m. The consistency of 3‐D simulation results is corroborated by the coral growth rates provided by the Fisher‐equation subroutine. These rates of 2–8 mm year⁻¹ compare well to the coral growth rates in Recent fringing reefs. We propose that during the early stage of platform evolution the high‐frequency fluctuations were obliquity‐modulated precessional cycles, whereas precessional cycles control later stages of platform growth. REPRO provides a separate visualization of the different facies bodies as a function of time and space, showing the intrinsic pattern of facies distribution in the platform. This is the result of a combination of platform growth and syndepositional subaerial erosion. For example, only the youngest stages of reef framework facies in the development of the Cariatiz carbonate platform are preserved.     

Facies architecture of an isolated carbonate platform: tracing the cycles of the Latemàr (Middle Triassic, northern Italy)

The 720-m-thick succession of the Middle Triassic Latemàr Massif (Dolomites, Italy) was used to reconstruct the lagoonal facies architecture of a small atoll-like carbonate platform. Facies analysis of the lagoonal sediments yields a bathymetric interpretation of the lateral facies variations, which reflect a syndepositional palaeorelief. Based on tracing of lagoonal flooding surfaces, the metre-scale shallowing-upward cycles are interpreted to be of allocyclic origin. Short-term sea-level changes led to subaerial exposure of wide parts of the marginal zone, resulting in the development of a tepee belt of varying width. Occasional emergence of the entire lagoon produced lagoon-wide decimetre-thick red exposure horizons. The supratidal tepee belt in the backreef area represented the zone of maximum elevation, which circumscribed the sub- to peritidal lagoonal interior during most of the platform's development. This tepee rim, the subtidal reef and a sub- to peritidal transition zone in between stabilized the platform margin. The asymmetric width of facies belts within individual metre-scale cycles was caused by redistribution processes that reflect palaeowinds and storm paths from the present-day south and west. The overall succession shows stratigraphic changes on a scale of tens of metres from a basal subtidal unit, overlain by three tepee-rich intervals, separated by tepee-poor units composed of subtidal to peritidal facies. This stacking pattern reflects two third-order sequences during the late Anisian to early middle Ladinian.     

Oceanographic and eustatic control of carbonate platform evolution and sequence stratigraphy on the Cretaceous (Valanginian–Campanian) passive margin,northern Gulf of Mexico

An integrated sequence stratigraphic study based on outcrop, core and wireline log data documents the combined impact of Cretaceous eustacy and oceanic anoxic events on carbonate shelf morphology and facies distributions in the northern Gulf of Mexico. The diverse facies and abundant data of the Comanche platform serve as a nearly complete global reference section and provide a sensitive record of external processes affecting Cretaceous platform development. Regional cross‐sections across the shoreline to shelf‐margin profile provide a detailed record of mixed carbonate–siliciclastic strata for the Hauterivian to lower Campanian stages (ca 136 to 80 Ma). The study window on the slowly subsiding passive margin allows the stratigraphic response to external forcing mechanisms to be isolated from regional structural processes. Three second‐order supersequences comprised of eight composite sequences are recognized in the Valanginian–Barremian, the Aptian–Albian and the Cenomanian–Campanian. The Valanginian–Barremian supersequence transitioned from a siliciclastic ramp to carbonate rimmed shelf and is a product of glacial ice accumulation and melting, as well as variable rates of mid‐ocean ridge volcanism. The Aptian–Albian supersequence chronicles the drowning and recovery of the platform surrounding oceanic anoxic events 1a and 1b. The Cenomanian–Campanian supersequence similarly documents shelf drowning following oceanic anoxic event 1d, after which the platform evolved to a deep‐subtidal system consisting of anoxic/dysoxic shale and chalk in the time surrounding oceanic anoxic event 2. Each period of oceanic anoxia is associated with composite sequence maximum flooding, termination of carbonate shelf sedimentation and deposition of condensed shale units in distally steepened ramp profiles. Composite sequences unaffected by oceanic anoxic events consist of aggradational to progradational shelves with an abundance of grain‐dominated facies and shallow‐subtidal to intertidal environments. Because they are products of eustacy and global oceanographic processes, the three supersequences and most composite sequences defined in the south Texas passive margin are recognizable in other carbonate platforms and published eustatic sea‐level curves.     

From platform to basin: the evolution of a Paleocene carbonate margin (Eastern Desert, Egypt)

In this study, progradation and the subsequent retrogradation of a late Paleocene isolated carbonate platform (Galala Mountains, Eastern Desert, Egypt) is demonstrated by variations of distinct facies associations from the platform margin in the north to the hemipelagic basin in the south. A combination of a sea-level drop and tectonic uplift at around 59 Ma (calcareous nannofossil biozone NP5) favored the initiation of the carbonate platform. From this time onwards, the facies distribution along the platform–basin transect can be subdivided into five facies belts comprising nine different facies associations. Their internal relationships and specific depositional settings are strongly coupled with the Maastrichtian–Paleocene seafloor topography, which resulted from local tectonic movements. Patch reefs and reef debris were deposited at the platform margin and the horizontally bedded limestones on the upper slope. Slumps and debris flows were stored on the lower slope. In the subhorizontal toe-of-slope facies belt, mass-flow deposits pass into calciturbidites. Further southwards in the basin, only hemipelagic marls were deposited. Between 59 and 56.2 Ma (NP5–NP8), the overall carbonate platform system prograded in several pulses. Distinct changes in facies associations from 56.2 to 55.5 Ma (NP9) resulted from rotational block movements. They led to increased subsidence at the platform margin and a coeval uplift in the toe-of-slope areas. This resulted in the retrogradation of the carbonate platform. Furthermore the patch-reef and reef-debris facies associations were substituted by the larger foraminifera shoal association. The retrogradation is also documented by a significant decrease in slump and debris-flow deposits on the slope and calciturbidites at the toe of slope.     

Facies, cycles, and controls on the evolution of a keep-up carbonate platform (Kimmeridgian, Swiss Jura)

During the Late Jurassic, accelerated ocean-floor spreading and associated sea-level rise were responsible for a worldwide transgression, which reached its maximum in the Late Kimmeridgian. In many Western European basins, this major sea-level rise led to the formation of marly and condensed sections. In the Swiss Jura, however, a shallow carbonate platform kept growing and only subtle changes in the stratigraphic record suggest an increasingly open-marine influence. Field observations and thin-section analyses reveal that the central Swiss Jura was at that time occupied by tidal flats and by more or less open marine lagoons where shoals and bioherms developed. The evolution through time of sedimentary facies and bed thicknesses permits the definition of small-, medium-, and large-scale depositional sequences. The diagnostic features of these sequences are independent of scale and seem largely controlled by the Kimmeridgian second-order transgression. A high-resolution sequence-stratigraphic correlation with biostratigraphically well-dated hemipelagic and pelagic sections in the Vocontian Basin in France reveals that: (i) The most important increase in accommodation recorded in the Kimmeridgian of the central Swiss Jura occurs in the Eudoxus ammonite zone (Late Kimmeridgian) and corresponds to the second-order maximum flooding recognized in many sedimentary basins. (ii) The small- and medium-scale sequences have time durations corresponding to the first and second orbital eccentricity cycle (i.e. 100 and 400 ka, respectively), suggesting that sedimentation on the platform and in the basin was at least partly controlled by cyclic environmental changes induced by insolation variations in the Milankovitch frequency band. The comparison of the high-resolution temporal framework defined in the Swiss Jura and Vocontian Basin with the sequence-stratigraphic interpretation realized in other Western European basins shows that the large-scale sequence boundaries defined in the Kimmeridgian of the Swiss Jura appear in comparable biostratigraphic positions in most Western European basins. Discrepancies that occur are probably because of local or regional tectonics.     

The infill of a supradetachment(?) basin: the continental to shallow-marine Upper Permian succession in the Dolomites and Carnia (Italy)

In the Dolomites and Carnia (eastern Southern Alps), the Upper Permian succession is represented by red beds of Val Gardena Sandstone, grading upwards and eastwards into the evaporitic and carbonate deposits of the Bellerophon Formation.

An overall transgressive pattern is shown by the general trend of the depositional setting, which evolved from flashy alluvial fans, through multiple-channel bedload rivers, mixed-load sinuous rivers, terminal fans, coastal sabkha and evaporitic lagoon, to a shallow, low-gradient marine ramp. The inferred fluvial regime was subject to rapid and erratic fluctuations in discharge. Palaeosols are represented by calcic soils, and suggest a warm to hot, semi-arid or dry subhumid climate with strongly seasonal rainfall. Vertic features are associated with more inland alluvial complexes: they are missing in the terminal-fan deposits, suggesting greater aridity in lowland (coastal) areas.

The Bellerophon Fm. consists of two units: a lower evaportte-bearing unit, deposited in a barred basin, and an upper shallow-marine carbonate unit, laid down on a very low-energy, low-gradient ramp.

Five third-order sequences and the lower part of a sixth sequence, collectively showing a backstepping pattern, have been identified in the succession. Due to the presence of a very low-gradient ramp margin, and the consequent ineffectiveness of relative sea-level falls in producing large-scale erosion in coastal areas, it is suggested that, rather than eustatic changes, degradational episodes marking sequence boundaries in the red-bed succession reflect changes in the climate-modulated intrinsic variables of discharge and sediment supply, and/or tectonic uplift.

Base level rises in, red-bed sequences are recorded by upward change from thick channel-belt sandstone bodies with an often high connectedness ratio, to progressively thinner channel deposits, ranging from isolated ribbons to channel-belt sandstone bodies potentially reaching high width/thickness ratios, encased in a comparably greater volume of overbank fines. This trend is thought to reflect the change from a confined geomorphic setting, with a limited area of potential avulsion, to an unrestricted setting with rivers free to move extensively; in addition, it documents the transition from an inland fluvial system with laterally migrating perennial or semi-perennial channels, to an ephemeral network of randomly migrating and frequently avulsing small terminal-fan distributaries, through a drastic downstream decrease in channel depth and discharge.

Identification of key sequence-stratigraphic relationships within the red-bed succession was mostly aided by the presence of easily recognizable and regionally correlated marine and marginal marine bed packages, traceable landwards into alluvial deposits showing faint traces of tidal activity, interpreted as the equivalent of marine maximum-flooding sediments. They may grade upwards into progradational fluvial packages showing basinward increase in thickness.

The Upper Permian deposits of the Southern Alps are considered part of an Upper Permian-Lower Triassic, second-order, structurally controlled sequence. The location of the basin on a thickened, previously active crust, affected by thermal perturbance after the last stages of the Variscan orogeny, the relatively reduced thickness of the basin fill, and predominantly long, transverse drainage networks, mostly derived from the denuded Insubric footwall, all suggest that sedimentation took place in a supradetachment basin, with a major detachment fault located in the palaeo-Insubric belt.     

The aftermath of the Carnian carbonate platform demise: a basinal perspective (Dolomites, Southern Alps)

In the Dolomites of northernmost Italy the carbonate‐platform growth came to a standstill late in the Early Carnian (Late Triassic). The response to this shutdown of shallow‐water carbonate production in the interplatform basins is largely unknown because erosion has removed most of the soft basinal sediments, giving rise to today's scenic landscape of the Dolomites. Mapping in the central part of the Dolomites and newly available core material has recently revealed a well‐preserved succession of basinal rocks within the Heiligkreuz Hospiz Basin (ital. Ospizio di Santa Croce Basin). In this paper, the regional depositional nature of arrested carbonate platform production is reconstructed by tracing its sedimentological record across the slope and into the basin. The uppermost St. Cassian Formation, the time‐equivalent basinal rocks to the prograding carbonate platforms, is overlain by the Heiligkreuz Formation, whose basal succession was deposited in a restricted and oxygen‐depleted environment immediately post‐dating the platform demise. The succession consists mainly of mudrocks, marlstones, and peloidal packstones, with abundant low‐diversity ostracod and pelecypod fauna and early diagenetic dolomite. C and O isotope values of the basal Heiligkreuz Formation, post‐dating platform demise, average + 2·4 and ? 2·4‰, respectively, and largely overlap the isotopic composition of St. Cassian carbonates. A shift toward slightly lower δ¹³C values in the Heiligkreuz Formation may reflect incorporation of isotopically depleted C released during bacterial sulphate reduction in the Heiligkreuz sediments. Sedimentological, palaeobiological and geochemical indices suggest that near‐normal marine conditions persisted long after the shutdown of shallow water carbonate‐platform growth, although there are clear indications of severely reduced oxygen levels in the restricted Heiligkreuz Hospiz interplatform basin. The Early Carnian platform demise induced a distinct switch in the locus of carbonate production from the shallow‐water platform and slope to the basin floor and a decrease in the availability of dissolved oxygen in the basinal waters. It is inferred that anoxia extended at least temporarily to the top of the carbonate slope, as indicated by the onlap of normal‐marine mounds by dark marlstones of the basal Heiligkreuz Formation.     

Cyclic palaeokarst surfaces in Aptian peritidal carbonate successions (Taurides, southwest Turkey): internal structure and response to mid-Aptian sea-level fall

The sedimentology and cyclic stratigraphy of palaeokarst structures in Aptian peritidal carbonate successions are interpreted using field and laboratory microfacies analyses of closely spaced samples from measured outcrop stratigraphic sections in southwest Turkey. Cycles displaying shallowing-upward metre-scale cyclicity are generally composed of lime mudstones/wackestones/packestones at the bottom and stromatolites or lime mudstones with charophytes and ostracods at the top. Subaerial exposure structures such as in situ karst breccias, dissolution vugs/pipes, mud cracks and sheet cracks are encountered at the top of the cycles. The presence of limestone layers between the successive karst breccia levels indicates that they are in situ palaeokarst structures, not recent karstifications or deep penetration from the upper palaeokarst surface down to the older strata. Palaeokarst breccia deposits are interpreted as mantling breccia formed as a result of epikarstification. Three main palaeokarst levels are recorded in nearly all sections. The sedimentology of the palaeokarst breccias, their position in cyclic peritidal carbonates and the biostratigraphic framework are used to trace the record of the global mid-Aptian sea-level fall in the southwest Taurides. The successive occurrences of three karst breccia levels close to the mid-Aptian sea-level fall correspond to falling periods of high-amplitude sea-level fluctuation within a late high-stand or early fall condition of a third-order sea level.