Depositional processes,triggering mechanisms and sediment composition of carbonate gravity flow deposits: examples from the Late Cretaceous of the south-central Pyrenees,Spain

Cenomanian through Coniacian strata near the town of Sopeira in the south-central Pyrenees (northern Spain) are composed of a variety of autochthonous and allochthonous carbonate slope lithologies that are divided into six depositional sequences based on facies distribution patterns and stratal relationships. The sequences record three major phases of platform margin evolution: rifting, burial, and exhumation. During the first phase (sequences UK-1, UK-2, UK-3, UK-4, and lower UK-5), deposition occurred on the edge of a wrench basin, and a normal fault located beneath the platform margin strongly influenced slope evolution. Background hemipelagic sediments on the slope were commonly redeposited by submarine slumps and slides. More intense reworking resulted in matrix-supported, slope-derived megaconglomerates (debrites).During the Cenomanian and Turonian, seismically triggered debris flows originated at the platform margin, bypassed the upper slope, and were deposited on the lower slope as polymictic, clast-supported, matrix-rich megabreccias. The megabreccias form channelized and sheet-like bodies with erosional basal surfaces. Shallow carbonate environments backstepped during the Late Turonian and Coniacian, but displacement along the fault at this time resulted in the development of a steep submarine scarp and the exposure of Cenomanian and Lower Turonian strata to submarine erosion. Matrix-poor, margin-derived megabreccias form a thick talus pile at the base of the scarp. Some of the breccias were transported into the basin as debris falls, forming sheet-like beds.Marl eventually buried the Coniacian scarp in sequence UK-5, resulting in the second major phase of platform slope evolution. The slope profile at this time was relatively gentle, and redeposited material is less common. In the third phase (sequence UK-6), tectonically induced bankward erosion during the Santonian resulted in a high (greater than 800 m) erosional scarp with a regional east–west trend that was subsequently onlapped by siliciclastic turbidites. Rejuvenation of erosion in the same vicinity suggests that long-term tectonism controlled the position of the slope, rates of erosion, and sediment type on the slope.Sediment gravity flow processes are laterally and temporally related. Submarine slide and slump deposits commonly grade laterally downslope into slope-derived megaconglomerates. Debris flows that originated at the platform margin appear to have initiated slumps, slides, and other debris flows on the slope. Debris fall deposits are commonly capped by coarse, graded, lithoclastic packstones that may represent turbidites generated by the debris falls.Sediment fabric exerted a profound impact on depositional processes, distribution of facies, and morphology of the slope. Fine-grained, mud-rich, lower slope deposits were unstable at even moderate slope angles, and have been extensively redeposited. Redeposition of grain-rich, upper slope facies was triggered by syndepositional seismic activity and upslope migration of instability and erosion. In the presence of mud, the transport mechanisms are typically cohesive debris flows, which were able to carry material onto the lower slope and into the basin. When no mud was available, rock falls and debris falls were the dominant sediment gravity flows, and their deposits are restricted to a position on the hanging wall proximal to the fault.     

Sequence architecture and carbonate platform configuration (Late Cenomanian–Santonian), Sinai, Egypt

Abstract Relative sea‐level changes on the mixed carbonate–siliciclastic platform of Sinai are manifested in shifts of distinct facies belts (deep‐water facies, high‐energy subtidal, shallow subtidal, lagoon, shallow shoreface siliciclastics, supratidal) and are interpreted in terms of sequence stratigraphy. Eight sedimentary sequences are recognized for the Upper Cenomanian to Santonian. Their correlation along a north–south transect reveals distinct changes in lithofacies and progradation/retrogradation patterns within the individual systems tracts. The number and stratigraphy of the sequence boundaries of Sinai correlate well with those from adjacent areas. Patterns of increased subsidence are documented for the Central Sinai Basin since the Late Cenomanian by increased thickness of the stratal packages (post‐CeSin 7 HST, post‐TuSin 1 LST and HST, post‐TuSin 2 LST) and are balanced by varying accumulation rates. Based on new sedimentological and biostratigraphic data, large‐scale palaeogeographic maps and cross‐sections show the: (1) temporal and spatial evolution of the Central Sinai Basin, e.g. its latest Cenomanian initial formation, Lower Turonian deep‐water facies, Middle Turonian to Coniacian synsedimentary subsidence; (2) drowning of the Cenomanian platform coinciding with the latest Cenomanian to Early Turonian relative sea‐level rise; (3) re‐establishment of the platform in Middle–Late Turonian times; and (4) a Coniacian basin and swell morphology.     

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.     

The Cretaceous megabreccias of the Gargano Promontory (Apulia, southern Italy): their stratigraphic and genetic meaning in the evolutionary framework of the Apulia Carbonate Platform

Huge megabreccias occur at the eastern margin of the Cretaceous Apulia Carbonate Platform (Gargano Promontory, southern Italy). Their stratigraphic and genetic meaning are controversial in the debated geological evolution of the Apulia Platform. New stratigraphic analyses have revealed that three distinct megabreccia levels occur within the coarse debrites that were previously interpreted to be the result of repeated collapses of a scalloped platform margin during the late Albian–Cenomanian. Each level has peculiar chronostratigraphic distribution, geometry, composition and genetic features. They are the Posta Manganaro Megabreccias (late early Aptian to late Albian pp. ), Monte S. Angelo Megabreccias (early–middle Cenomanian) and Belvedere di Ruggiano Megabreccias (middle Turonian). These deposits overlie regional, tectonically enhanced unconformities of late early Aptian, late Albian and late Cenomanian age. These megabreccias, which were formed, respectively, during drowning, prograding and exposure events of the Apulia Platform, reflect important turning points in its Cretaceous geodynamic evolution.     

A steep-fronted Carboniferous carbonate platform: clinoformal geometry and lithofacies (Picos de Europa, NW Spain)

A steep‐margined carbonate platform is developed in the Carboniferous synorogenic foreland basin of northern Spain. Dips of 60–90° produced during Late Carboniferous thrusting enable cross‐sections of a 4‐km‐wide portion of the marginal area of this platform (Las Llacerias outcrop) to be studied in aerial photographs at a seismic scale. Three stratal domains are observed: (1) a horizontal‐bedded platform; (2) a clinoformal‐bedded margin with a relief of up to 500 m; and (3) a low‐angle toe‐of‐slope, where slope beds interfinger with basin sediments. The slope shows well‐bedded sigmoidal clinoforms with depositional dips ranging from 15° to 32°. Based on lithology and stratal patterns, four facies groups have been recognized: (1) a flat‐topped platform, in which thick algal boundstone, skeletal packstone–grainstone and peloidal micrite wackestone with a poorly rhythmic character prevail; (2) the platform margin and upper slope, characterized by microbial boundstone spanning a bathymetric range of ≈150 m measured from the break of slope; (3) a slope, predominantly composed of margin‐derived rudstones and breccias; and (4) a toe‐of‐slope to basin zone, where a cyclic alternation of spiculitic siltstones, packstone to grainstone calciturbidites and rudstone/breccia is visible. Five successive stages of platform development are deduced: (1) Bashkirian: flooding of the pre‐existing Serpukhovian platform giving rise to the nucleation of a low‐angle ramp to the south‐east of the study area with microbial mud‐mound accumulations, and breccias and calciturbidites on the margins; (2) Early Moscovian: an influx of siliciclastic sediment buried part of the platform and reduced the area of carbonate sedimentation; (3) Moscovian: aggradation and progradation of the carbonate system produced an extensive steep‐margined and flat‐topped shallow‐water platform (shelf system); (4) Latest Moscovian–earliest Kasimovian: drowning of the platform; and (5) Kasimovian: covering of the platform by marly calcareous ramp sediments.     

Sedimentation in shallow to deep water carbonate environments across a sequence boundary: effects of a fall in sea-level on the evolution of a carbonate system (Ladinian-Carnian, eastern Lombardy, Italy)

In the Concarena‐Pizzo Camino Massif (Lombardy Basin, Southern Alps, Italy) the lateral transition from Ladinian‐Carnian carbonate platforms to coeval intraplatform basins is preserved. The succession records the sedimentological evidence of a sea‐level fall on a flat‐topped platform with a narrow marginal reef rim and its effects in the adjacent deeper‐water basin. Repeated high‐frequency exposures of the platform top are recorded by a peritidal–supratidal succession that overlies subtidal inner platform facies of the former highstand system tract (HST). On the slope and in the basin, the sea‐level fall is recorded by a few metre thick succession of bioclastic packstones. These facies directly lie on coarse clinostratified breccia bodies (slope facies of the former HST) or on resedimented, well‐bedded, dark laminated limestones (basinal facies of the HST). This facies distribution indicates that during the sea‐level fall carbonate production on the platform top decreased rapidly and that sedimentation in the basin was mainly represented by condensed facies. Microfacies record an enrichment, during low stand, in pelagic biota (packstones with radiolarians and spiculae), whereas the occurrence of platform‐derived, shallow‐water materials is limited to thin lenses of reworked and micritized Fe‐rich oolites and bioclasts (mainly pelecypods and echinoderms). The facies association in the Concarena‐Pizzo Camino Massif demonstrates that a highly‐productive carbonate factory was almost completely turned off during the emergence of the platform top at a sequence boundary, leading to low‐stand starvation in the basin. The reconstruction of the stratigraphic evolution of the Concarena‐Pizzo Camino carbonate platform therefore represents a significant case history for the study of the behaviour of ancient carbonate systems during a fall in sea‐level, independent of its origin (eustatic or tectonic).     

Sequence development of Late Cenomanian–Turonian carbonate ramps, platforms and basins in Israel

The Cenomanian–Turonian carbonate-dominated lithofacies of Israel reflect a complex interplay between tectonics, sea-level change, and palaeoecology. Improved correlation based on revision of the bio- and chronostratigraphic framework has enabled the establishment of a sequence-stratigraphic model comprising five sequences delineated by four sequence boundaries, in the Late Cenomanian–Early Coniacian interval. The Late Cenomanian–Turonian succession begins with prograding, highstand, carbonate-platform deposits of the first sequence. Interruption of progradation and drowning of this platform took place within the Late Cenomanian guerangeri Zone (=the vibrayeanus Zone in Israel), resulting in a drowning unconformity which is regarded as a Type 3 sequence boundary (labelled CeUp). The drowning is attributed in part to extinctions in the rudist-dominated biofacies (e.g., Caprinidae), which led to reduced carbonate production and enhanced the impact of the sea-level rise. Similar drowning of Tethyan platforms around the C/T boundary has been linked to the establishment of coastal upwelling and consequent eutrophication. Outer ramp hemipelagic facies (Derorim and the Lower Ora formations) replaced the platform carbonates, thickening substantially southwards in the Eshet-Zenifim Basin of southern Israel. Along the ancient continental slope (Mediterranean coastal plain) evidence of this drowning is obscured by submarine erosion, while in central and northern Israel the drowned section is represented by condensation or a hiatus, reflecting an elevated, sediment-starved sea-floor. A carbonate platform dominated by rudistid shoals (‘Meleke’ Member; Shivta Formation) was re-established in the Judean hills and northern Negev during the middle part of the Turonian coloradoense Zone (local zone T4). Later, during kallesi Zone times (T7), the platform facies prograded southwards towards the Eshet-Zenifim intra-shelf basin. The drowning succession and overlying resurrected carbonate platform are topped in central and southern Israel by a pronounced Type 1 sequence boundary (Tu1) between the kallesi (T7) and ornatissimum (T8) zones (Middle Turonian). In central Israel and northern Negev the sequence boundary is overlain by lowstand deposits of the ‘Clastic Unit’ and by the transgressive and highstand inner to mid-ramp deposits of the Nezer and Upper Bina formations. In the southern Negev the sequence boundary is overlain by lowstand and transgressive systems tracts of mixed carbonates, siliciclastics, and localized evaporites (Upper Ora Formation), and then by mid to inner ramp carbonates of the Gerofit Formation. The latter represents a very high rate of accumulation, indicating rapid, continued subsidence balanced by platform growth. The Tu2 sequence boundary of the Late Turonian is expressed in the southern Negev by a shift from inner ramp carbonates of the Gerofit Formation to outer ramp chalky limestones of the Zihor Formation, indicating localized drowning. The succeeding Co1 sequence boundary again indicates localized drowning of the prograding highstand deposits of the Zihor Formation (‘Transition Zone’) overlain by Lower Coniacian transgressive deposits of the upper part of the Zihor Formation. All of these third-order sequences are expressed in southern Israel, where the rate of subsidence was in balance with sea-level fluctuations. In contrast, the Judean Hills and eastern Galilee areas have a more incomplete succession, characterized by hiatuses and condensation, because of reduced subsidence. More distal areas of continuous deep-water deposition in western Galilee and the coastal plain failed to record the Middle Turonian lowstand, while a longer term, second-order sequence spanning the entire Late Cenomanian–Early Coniacian interval, is present in the Carmel and Yirka Basin areas.     

Syn-depositional continental rifting of the Southeastern Neo-Tethys margin during the Albian–Cenomanian: evidence from stratigraphic correlation

ABSTRACT

Albian–Cenomanian successions (Kazhdumi and Sarvak formations) represent remarkable variations in thickness, facies, fauna, and environments throughout the Zagros area. In the Coastal Fars (Charmu section), sedimentological and paleontological data evidence an intrashelf, with depths of 10s–100s m, surrounded by a shallow carbonate platform. Due to its depth, deposition of sequences in this setting has been controlled by eustatic sea-level changes rather than eurybathic changes, and several condensation episodes occurred related to marine transgressions. These observations are different from those in the adjacent sections in the Coastal Fars which recorded subaerial exposures instead. Combined with previous studies, this study denotes several intrashelf basins enclosed by a shallow carbonate platform on the southeastern margin of the Neo-Tethys during the Albian–Cenomanian. Development of intrashelf basins corresponds to basement faults in the Fars Salient. Likely, an extensional tectonic regime associated with a rifting event created horst–graben architecture by exerting extension along the basement faults and reactivating salt structures. Deposition on these troughs and highs led to the facies and thickness variations of the concomitant sequences. Development of several intrashelf basins on the southeastern margin of the Neo-Tethys indicates that syn-depositional continental rifting event could occur during the Albian–Cenomanian, prior to the tectonic inversion around the earliest Turonian.     

Upper Cenomanian-lower Turonian sea-level rise and its consequences on the Adriatic-Dinaric carbonate platform

The Adriatic carbonate platform existed throughout most of the Mesozoic as an isolated (intraoceanic) carbonate platform. It is characterized by what has been traditionally labelled a continuous shallow marine carbonate sedimentation, without terrigenous influences. However, global sea-level changes left unequivocal imprints in the continuous carbonate record, thus proving that the carbonate platform sensitively responded to sea-level fluctuations. This paper focuses on the relatively short Cenomanian — Turonian drowning event, which temporarily submerged the platform below the euphotic zone, and which is related here to the well known global Cenomanian — Turonian sea-level highstand. This drowning episode is characterized by the abrupt disappearance of numerous shallow water benthic invertebrate taxa and by an abundant occurrence of planktonic organisms, including calcispheres and primitive planktonic foraminifera. Near the middle of the Turonian, the platform regime was re-established, correlating with the global fall in sea level. The situation is different only at the south-west margin of the platform, where pelagic sedimentation, initiated near the end of the Cenomanian, continues into the Santonian and probably marks the Adriatic trough. The drowning began earlier (in the Middle Cenomanian) in those areas nearer to the north-east margin of the platform (the area of what is today central Croatia) and then progressed gradually towards the more interior areas of the platform (the middle Dalmatian islands), which were drowned near the end of the Cenomanian.     

Carbonate platform depositional sequences, Upper Cretaceous, south-central Pyrenees (Spain)

The Upper Cretaceous (Cenomanian-Maastrichtian) of the south-central Pyrenees shows five carbonate platform sequences where the major parameters are tectonism, relative sea-level fluctuations and inherited depositional profile. Depositional geometries and basin analysis permit an understanding of the depositional history.Five depositional sequences have been recognized: (1) The Santa Fe sequence (Middle-Upper Cenomanian), a ramp to a skeletal rimmed shelf with an escarpment bypass margin. The lower boundary is an angular unconformity and the upper one records a sea-level drop. The platform location of the margin was determined by a listric normal fault. (2) An abrupt sea-level rise drowned the former platform. The Congost sequence (Turonian-Lower Coniacian), a distally steepened ramp with erosional distal deep slope. The depositional model was largely controlled by pre-existing basin morphology. Cessation of platform development was due to a relative sea-level drop. (3) The Sant Corneli sequence (Upper Coniacian-Lower Santonian), a mixed terrigenous-skeletal homoclinal ramp with upright margin, deep slope and dysaerobic basin. The slope results from the backstepping by 24 km of the previous margin and gentle basin tilting. The platform margin remained more or less at the same position, and relief between platform and slope increased indicating continuous relative sea-level rise. The upper boundary is an angular unconformity at the platform margin produced by an abrupt sea-level rise and platform drowning, and by listric normal faulting. (4) The Vallcarga sequence (Upper Santonian-Campanian), a distal-steepened skeletal homoclinal ramp, erosional escarpment and turbidite basin, which corresponds to the Mesozoic maximum marine expansion. A listric normal fault created two depositional areas: a more or less flat footwall block with a north-northwest prograding carbonate ramp.     

Facies,geometry and growth phases of the Valdorria carbonate platform (Pennsylvanian,northern Spain)

The 14 km wide Valdorria outcrop (Pennsylvanian, northern Spain) is one of the few examples of entirely exposed flat‐topped and high‐relief carbonate platforms reported in the fossil rock record. Laterally and vertically traceable stratal patterns expose three phases of growth. Phase I is a 430 m thick platform to slope succession that prograded over 6 km, and is dated as Early Bashkirian (Akavasian–Askynbashian). Phase II aggraded and prograded, exhibiting 180 m thickness of cyclical platform top deposits, dated as Late Bashkirian (Asatauian). Phase III is a mound‐shape structure that developed over the platform top of Phase II as a new phase of platform nucleation. It is 535 m thick and 2 km wide, and dated as Late Bashkirian (Asatauian–Transition interval). The observed changes of growth styles during platform evolution, from a prograding to an aggrading–prograding system, and a rapid aggradational phase, are inferred to be controlled by flexural subsidence in the active Cantabrian foreland basin, at the Variscan orogenic front. The metre‐scale shallowing‐upward cycles of the platform top are most probably due to glacioeustasy, as evidenced by well‐recorded subaerial exposure surfaces superimposed on subtidal deposits, and by a stratal pattern recurrent in a short interval of about 160 kyr. Observations of outcropping Bashkirian cyclothems in an isolated carbonate system, devoid of siliciclastic input, are relevant for a better understanding of the impact of high‐frequency sea‐level fluctuations on the carbonate factory. Moreover, progradation of the platform margin during Phase I reaches a rate of 2500 m/Myr, and 1810 m/Myr during Phase II; rates that are high when compared to other Pennsylvanian examples. The aggradation rate of 447 m/Myr calculated for the Late Bashkirian–Transition interval (Phases II and III; uncorrected for compaction, missing beats and erosion) is uncommonly high in comparison to coeval Pennsylvanian examples. The platform exhibits a self‐nourishing prograding microbial boundstone‐dominated slope. Thus, the slope‐shedding model applies well to Valdorria. However, Phase II recorded eustatic variations able to inhibit the slope microbial boundstone factory during low sea‐level stands; this is marked by common slope red‐stained breccias synchronous to platform top subaerial exposure phases. Contrarily, periods of relative high sea‐level and rapid subsidence in Phase III registered a greater development of cemented microbial boundstone. These observed, partly opposing relationships of sea‐level stands, shedding modes and slope architecture provide an improvement of the currently used slope‐shedding model. The overall architecture of the Valdorria outcrop compares well with that of other contemporaneous platforms, such as Sierra del Cuera and Bolshoi Karatau. Valdorria shares the high‐relief and flat‐topped, steep slopes, cyclothemic patterns and occurrence of karst features with the Pricaspian Basin platforms (Tengiz, Karachaganak and Kashagan), with minor variations in facies distribution of the internal platform. Furthermore, the continuous seismic‐scale outcrop of Valdorria, together with its isolated setting and asymmetrical growth, makes it a very good candidate for potential subsurface analogues of hydrocarbon‐bearing systems.     

Cenomanian–Turonian facies succession in the Guerrero–Morelos Basin, Southern Mexico

The Cenomanian–Turonian succession of southern Mexico is characterized by an abrupt change from shallow marine to pelagic facies. The drowning of the platform coincides with the widely documented Cenomanian–Turonian Oceanic Anoxic Event (CTOAE). A proper understanding of the drowning event and the effects of the OAE requires, as an essential first step, the construction of a detailed stratigraphic framework. This has been achieved and utilizes sedimentological data as well as a combination of benthic and planktonic biostratigraphic schemes.

Deposition of the Cenomanian–Turonian sedimentary rocks of the Guerrero–Morelos basin was controlled by tectonic and oceanographic factors resulting in depositional environments ranging from a semi-restricted shelf, ramp, pelagic and prodelta deposits. Facies analysis indicates that shallow marine limestones of the Morelos Formation (lower-upper Cenomanian) were deposited in intertidal–shallow supratidal and subtidal environments in a semi-restricted shelf. Peloidal-bioclastic packstone–wackestones with minor grainstones are the predominant texture of these rocks. Abundant large benthic foraminifers, calcareous algae (dasycladacean) and mollusks (gastropods and rudists) characterize the fossil assemblage.

The Cuautla Formation (uppermost Cenomanian–Turonian) represents sedimentation on a low-energy, wave-dominated, carbonate ramp. The inner ramp accumulated bioclastic banks and shoals composed of peloidal-benthic foraminifer-grainstone, calcareous red and green algae, rudists and minor solitary corals. The middle ramp is represented by nodular packstones with a diverse assemblage of echinoderms, green and red algae, bryozoan, rudists, solitary corals, roveacrinids, calcisphaerulids, and non-keeled planktonic foraminifers. The outer ramp is dominated by argillaceous wackestone–packstone characterized by calcisphaerulids, roveacrinids, and non-keeled planktonic foraminifers. An increase in terrigenous-clastic material towards the eastern part of the area indicates progradation of a deltaic system while the Mexcala Formation (uppermost Cenomanian–Turonian) was deposited in a pelagic setting.

The drowning of the platform is at the contact between the Morelos and Cuautla or Mexcala formations and is dated as latest Cenomanian. The drowning is a hiatus in most sections and it began before the end of the Cenomanian by a minimum of 150 ky if the top of the Morelos is not eroded.     

Spit-platform temperate carbonates: the origin of landward-downlapping beds along a basin margin (Lower Pliocene, Carboneras Basin, SE Spain)

ABSTRACT Lower Pliocene temperate carbonates exhibit landward‐downlapping beds at the southern margin of the Carboneras Basin in south‐eastern Spain. This rarely documented stratal geometry resulted from the accumulation of bedded bioclastic carbonate sand and gravel by longshore currents along a spit platform located a few hundred metres from the palaeoshoreline. The top of the spit platform was covered by shoals that extended over a gently dipping ramp inclined to the north. On the landward slope of the spit, sediments washed over from the shoal area were deposited in parallel‐laminated beds with a southward dip of 8–11°. These beds aggraded and retrograded after an increase in accommodation space, probably related to an Early Pliocene eustatic sea‐level rise. As a result, the beds downlap onto the underlying unconformity surface in a shoreward direction. Eventually, the depression between the shoreline and the spit platform was filled, and a gentle ramp became established. These Pliocene exposures in the Carboneras Basin and a similar Upper Miocene example in southern Spain suggest that landward‐downlapping stratal geometries can be expected in nearshore temperate carbonates along basin margins, and demonstrate a similarity in sedimentary dynamics to siliciclastic sands and gravels.     

Sequence stratigraphy of a Mesozoic carbonate platform-to-basin system in western Sicily

Sequence stratigraphic studies of the Triassic through Paleogene carbonate successions of platform, slope and basin in western Sicily (Palermo and Termini Imerese Mountains) have identified a sedimentary cyclicity mostly caused by relative oscillations of sea level. The stratigraphic successions of the Imerese and Panormide palaeogeographic domains of the southern Tethyan continental margin were studied with physical-stratigraphy and facies analysis to reconstruct the sedimentary evolution of this platform-to-basin system. The Imerese Basin is characterized by a carbonate and siliceous-calcareous succession, 1200–1400m thick, Late Triassic to Eocene in age. The strata display a typical example of a carbonate platform margin, characterized by resedimented facies with progradational stacking patterns. The Panormide Carbonate Platform is characterized by a carbonate succession, 1000–1200 m thick, Late Triassic to Late Eocene, mostly consisting of shallow-water facies with periodic subaerial exposure. The cyclic arrangement has been obtained by the study of the stratigraphic signatures (unconformities, facies sequences, erosional surfaces and stratal geometries) found in the slope successions. The recognized pattern has been compared with coeval facies of the shelf. This correlation provided evidence of sedimentary evolution, influenced by progradation and backstepping of the shelf deposits. The stratigraphic architecture of the platform-to-basin system is characterized by four major transgressive/regressive cycles during the late Triassic to late Eocene. These cycles, framed in a chronostratigraphic chart, allows the correlation of the investigated shelf-to-basin system with the geological evolution of the African continental margin during the Mesozoic, showing tectono-eustatic cycles. The first cycle, encompassing the late Triassic to early Jurassic, appears to be related to the late syn-rift stage of the continental margin evolution. The following three cycles, spanning from the Jurassic to Eocene, can be related to the post-rift evolution and to thermal subsidence changes.     

A Middle Ordovician Drowning Unconformity on the Northeastern Flank of the Okcheon (Ogcheon) Belt, South Korea

The Maggol Limestone of Ordovician age was deposited in the Taebaeksan (Taebacksan) Basin which occupies the northeastern flank of the Okcheon (Ogcheon) Belt of South Korea. Carbonate facies analysis in conjunction with conodont biostratigraphy suggests that an overall regression toward the top of the Maggol Limestone probably culminated in subaerial exposure of platform carbonates in the early Middle Ordovician (earliest Darriwilian). Elsewhere this subaerial exposure event is manifested as a major paleokarst unconformity at the Sauk-Tippecanoe sequence boundary beneath the Middle Ordovician succession and its equivalents, most in notably North America and North China. Due to its global extent, this paleokarst unconformity has been viewed as a product of second- or third-order eustatic sea level fall during the early Middle Ordovician. The Sauk-Tippecanoe sequence boundary in South Korea, however, appears to be a discrete marine-flooding surface in the upper Maggol Limestone. Strata beneath this surface represent by a thinning-upward stack of exposure-capped tidal flat-dominated cycles that are closely associated with multiple occurrences of paleokarst-related solution-collapse breccias. This marine-flooding surface is onlapped by a thick succession of thin-bedded micritic limestone that is eventually overlain by a Middle Ordovician condensed section. This physical stratigraphic relationship suggest that second- and third-order eustatic sea level fall may have been significantly tempered by regional tectonic subsidence near the end of Maggol deposition. The tectonic subsidence is also evidenced by the occurrence of coeval off-platform lowstand siliciclastic quartzite lenses as well as debris flow carbonate breccias (i.e., the Yemi Breccia) in the basin. With continued tectonic subsidence, a subsequent rise in the eustatic cycle caused drowning and deep flooding of the carbonate platform, forming a discrete marine-flooding surface that may be referred to as a drowning unconformity. This tectonic interpretation contrasts notably with the slowly subsiding carbonate platform model for the basin as has been previously suggested. Thus, it is proposed that the Taebaeksan Basin in the northeastern flank on the Okcheon Belt evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early Middle Ordovician.     

Evolution of an early Proterozoic foreland basin carbonate platform, lower Pethei Group, Great Slave Lake, north-west Canada

The Taltheilei, Utsingi, McLean and Blanchet formations form a 175–390 m thick carbonate platform-to-basin succession in the lower part of the PaleoProterozoic Pethei Group, preserved in the eastern arm of Great Slave Lake. Carbonates accumulated along the south-east margin of the Slave Craton within a foredeep formed during the collision of the Slave and Churchill Cratons. The rocks include eight, predominantly microbial, carbonate facies that comprise five facies associations representing (1) shallow-water rimmed shelf, (2) shallow-water open shelf, (3) shallow-water ramp, (4) upper slope and deep ramp, and (5) lower slope and basin plain environments. Microbialite facies grew by organically mediated precipitation of spar and micritic cement and trapping and binding of lime mud. These wholly subtidal facies typically reflect progressive shallowing and changing geometry of the lower Pethei sea floor, from ramp, to open shelf, to shallow rimmed shelf, with associated slope and basin plain deposition. Repeated relative sea-level changes influenced platform growth. This resulted in five shallowing upward packages; each separated by an incipient drowning event of varying magnitude. Antecedent topography and the size of the preceding drowning event strongly influenced the initial growth of each interval. This repeated pattern is attributed to interaction between (a) the inherent tendency of microbial carbonates to aggrade vertically, (b) changing sedimentation rates and (c) readjustments of relative base level. The lower Pethei succession is one of few PaleoProterozoic examples of carbonate platform growth within a foreland basin. It has (1) a low gradient profile, (2) extensive slope and basin plain carbonate production and sedimentation, (3) no ooids, (4) minor terrigenous clastic sediments, and (4) a mobile, submergent shelf rim lacking substantial carbonate sand shoals.     

A Middle Ordovician Drowning Unconformity on the Northeastern Flank of the Okcheon (Ogcheon) Belt,South Korea

The Maggol Limestone of Ordovician age was deposited in the Taebaeksan (Taebacksan) Basin which occupies the northeastern flank of the Okcheon (Ogcheon) Belt of South Korea. Carbonate facies analysis in conjunction with conodont biostratigraphy suggests that an overall regression toward the top of the Maggol Limestone probably culminated in subaerial exposure of platform carbonates in the early Middle Ordovician (earliest Darriwilian). Elsewhere this subaerial exposure event is manifested as a major paleokarst unconformity at the Sauk-Tippecanoe sequence boundary beneath the Middle Ordovician succession and its equivalents, most in notably North America and North China. Due to its global extent, this paleokarst unconformity has been viewed as a product of second- or third-order eustatic sea level fall during the early Middle Ordovician. The Sauk-Tippecanoe sequence boundary in South Korea, however, appears to be a discrete marine-flooding surface in the upper Maggol Limestone. Strata beneath this surface represent by a thinning-upward stack of exposure-capped tidal flat-dominated cycles that are closely associated with multiple occurrences of paleokarst-related solution-collapse breccias. This marine-flooding surface is onlapped by a thick succession of thin-bedded micritic limestone that is eventually overlain by a Middle Ordovician condensed section. This physical stratigraphic relationship suggest that second- and third-order eustatic sea level fall may have been significantly tempered by regional tectonic subsidence near the end of Maggol deposition. The tectonic subsidence is also evidenced by the occurrence of coeval off-platform lowstand siliciclastic quartzite lenses as well as debris flow carbonate breccias (i.e., the Yemi Breccia) in the basin. With continued tectonic subsidence, a subsequent rise in the eustatic cycle caused drowning and deep flooding of the carbonate platform, forming a discrete marine-flooding surface that may be referred to as a drowning unconformity. This tectonic interpretation contrasts notably with the slowly subsiding carbonate platform model for the basin as has been previously suggested. Thus, it is proposed that the Taebaeksan Basin in the northeastern flank on the Okcheon Belt evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early Middle Ordovician.     

Lithostratigraphy and depositional environments of the upper cretaceous deposits in the central Taurides (S Turkey)

The study area is located in the Central Taurides (southern Turkey), which is bounded by the K?rkkavak fault to the west and Ecemi? fault to the east. The sequences are studied in detail based on measured sections composed of the rocks deposited during the Cenomanian–Maastrichtian and located within different tectonic units previously described in the Taurides. The study materials include 217 thin section data from seven Cenomanian–Maastrichtian sequences of outcropping in different parts of the Central Taurides. The sediments deposited during the Cenomanian–Maastrichtian period in the Central Taurides are subdivided into eight units based on their lithological, paleontological, and textural properties. The lower boundaries of the upper Santonian and Campanian are unconformable contacts. The Upper Cretaceous sequence starts with the middle Cenomanian and represents a continuation of the Lower Cretaceous tidal flat and shelf lagoon sequence. Upper Turonian–Coniacian sediments are not observed due to the eustatic sea level drop. The second main transgression period of the Upper Cretaceous platform took place in the Santonian. This unit is represented by limestones composed of wackestones/packstones containing benthic foraminifera and rudist fragments, which are deposited in tidal flats and subtidal environments. The late Campanian starts with a transgression, and the environment transformed transitions into slope facies from inner platform facies, as a result of the thrust of ophiolitic rocks. In the following period, slope front and basin plain environments were dominant due to the increasing slope. Slumped pelagic limestones were deposited on the slope. Planktonic foraminiferal pelagic limestones were unconformably deposited on plaque limestone in the slope front environment depending on the increase in slope gradient and local faulting. As a result of decreasing tectonic activity, the sediments were deposited onto a stable basin plain. They were initially fed from the nearby carbonate platform and then by siliciclastic turbidites derived from the thrusted ophiolitic rocks. In this study, the lithostratigraphic properties of the Cenomanian–Maastrichtian units outcropping in various parts of the Central Taurides are described. The sedimentary deposits described here suggest different basinal conditions in the region.     

海西—印支期上扬子南缘碳酸盐岩台地边缘沉积特征、演化及其控制因素

海西—印支期上扬子地块发育稳定型的碳酸盐岩大台地,在碳酸盐岩台地南缘台地边缘沉积体系发育,主要包括4种沉积类型:台地边缘滩,台地边缘礁、台地前缘斜坡、滩(礁)间海。据海西—印支期台缘沉积发育及分布特征,可分为4个沉积演化阶段:中泥盆世—晚泥盆世(SS1),台缘沿弥勒—师宗—晴隆—南丹—河池一线呈带状分布,台缘礁滩发育;早石炭世—早二叠世早期(SS2-SS3),台缘沿晴隆—六盘水—安顺呈带状分布,台缘生屑滩发育;早二叠世晚期—晚二叠世(SS4),沿贞丰—紫云—罗店—河池—柳州—娄底呈带状分布,台缘滩、台缘礁交互发育;早三叠世—中三叠世(SS5),沿贞丰—镇宁—贵阳—福泉一线呈带状分布,早三叠世主要发育台缘鲕粒滩,中三叠世台缘礁占优势。这4个阶段的台缘沉积特征表明:台地边缘礁滩沉积体系垂向演化受海平面变化控制,横向迁移主要受同沉积断裂控制,不同类型生物的出现和绝灭控制台缘生物礁滩的类型及特征。     

Stratal architecture and platform evolution of an early Frasnian syn‐tectonic carbonate platform,Canning Basin,Australia

Facies architecture and platform evolution of an early Frasnian reef complex in the northern Canning Basin of north‐western Australia were strongly controlled by syn‐depositional faulting during a phase of basin extension. The margin‐attached Hull platform developed on a fault block of Precambrian basement with accommodation largely generated by movement along the Mount Elma Fault Zone. Recognition of major subaerial exposure and flooding surfaces in the Hull platform (from outcrop and drillcore) has enabled comparison of facies associations within a temporal framework and led to identification of three stages of platform evolution. Stage 1 records initial ramp development on the hangingwall dip slope with predominantly deep subtidal conditions that prevented any cyclic facies arrangements. This stage is characterised by basal siliciclastic deposits and a major deepening‐upward facies pattern that is capped by a sequence boundary towards the footwall (north‐west) and a major flooding surface towards the hangingwall. Stage 2 reflects the bulk of platform aggradation, significant platform growth towards the hangingwall and the development of reef margins and cyclic facies arrangements. Thickening of this stage towards the hangingwall indicates that accommodation was generated by rotation of the fault block and overlying platform. Stage 3 records a major flooding and backstep of the platform margin. The Hull platform illustrates important elements of margin‐attached carbonate platforms in a half‐graben setting, including: (i) prominent, but limited, coarse siliciclastic input that does not have a major detrimental effect on carbonate production near the rift margin in arid to semi‐arid settings; (ii) wedge‐shaped accommodation created by syn‐depositional rotation of fault blocks and tilting of the hangingwall dip slope, resulting in shallow‐water facies and subaerial exposure up‐dip of the rotational axis and deeper water facies down‐dip; and (iii) evolution of a ramp to rimmed shelf, coincident with a sequence boundary–flooding surface, that is accelerated by tilting of the hangingwall dip slope during fault‐block rotation.