Carbonate Platform Development Research Articles

Development of an equatorial carbonate platform across the Triassic-Jurassic boundary and links to global palaeoenvironmental changes (Musandam Peninsula, UAE/Oman)

The Triassic-Jurassic boundary is marked by one of the ‘big five’ mass extinctions of the Phanerozoic. This boundary event was accompanied by several carbon cycle perturbations, potentially induced by the opening of the Central Atlantic and associated volcanism, and accompanied by an ocean acidification event. Continuous carbonate successions covering this interval of environmental change are however rare. Here data from a shallow-marine equatorial mixed carbonate-siliciclastic succession is presented, that was studied on a regional scale. Four sections that are 48km apart were examined on the Musandam Peninsula (United Arab Emirates and Sultanate of Oman). The system was analysed for its sedimentology, vertical and lateral facies changes, and stable carbon and oxygen isotopes. Strontium isotope analysis was used to determine the position of the Triassic-Jurassic boundary horizon. The studied ramp experienced an episode of demise during the Late Triassic, followed by a restricted microbialite dominated ramp, containing large amounts of siliciclastic facies. During the Latest Triassic the diverse carbonate factory revived and flourished across the Triassic-Jurassic boundary. No clear evidence for a biocalcification crisis or an ocean acidification event across the Triassic-Jurassic boundary is visible. Lateral facies heterogeneities can be observed across the studied interval, attributed to hydrodynamic activity, including tropical storms, crossing the extensive shelf area. Although evidence for synsedimentary tectonic activity is present, the vertical stacking pattern is largely controlled by changes in relative sea level. The refined chronostratigraphy accompanied by the detailed environment of deposition analysis allows for a refinement of the regional palaeogeography. The neritic equatorial carbonate ramp has archived a negative carbon isotope excursion preceding the Triassic-Jurassic boundary that has also been reported from other study sites. The lack of evidence for a biocalcification crisis across the equatorial Triassic-Jurassic boundary indicates that the Tethys did not experience a distinct global acidification event.

Discussion of ‘Oceanographic and eustatic control of carbonate platform evolution and sequence stratigraphy on the Cretaceous (Valanginian–Campanian) passive margin, northern Gulf of Mexico’ by Phelps et al. (2014), Sedimentology, 61, 461–496

AbstractErrors in Phelps et al. () arise from: (i) inclusion of the East Texas Basin and basin‐flank formations in a regional stratigraphic transect purported to represent the San Marcos Arch (axis of the Comanche Platform, central and south Texas); and (ii) revision of Edwards Group strata (Person Formation) that ignores well‐documented studies in the subsurface and adjacent outcrops in the Balcones fault zone and Edwards Plateau. Consequently, the Upper Albian Composite Sequence 104–101 of Phelps et al. () should be revised from one cycle to three and the upper boundary of the Albian Supersequence should be elevated to 99·6 Ma.

Reply to the Discussion by Rose of Phelps et al. (2014) ‘Oceanographic and eustatic control of carbonate platform evolution and sequence stratigraphy on the Cretaceous (Valanginian–Campanian) passive margin, northern Gulf of Mexico’, Sedimentology, 61, 461–496

AbstractDiscussion points raised by Rose () concentrate on late Albian stratigraphic relationships between formations of the East Texas Basin and the San Marcos Arch of the Comanche Platform in the northern Gulf of Mexico. Criticisms of Phelps et al. (2014) regarding stratigraphic nomenclature, palaeogeography and regional lithostratigraphic correlations generally focus on interpretive aspects of the study or do not account for the full scope of published information. Revisions to the top Aptian–Albian Supersequence boundary by Rose are incompatible with the relative location of a subaerial unconformity, as well as deepening lithofacies trends and retrogradational stratigraphic patterns below the interpreted boundary. Rose's placement of the top Aptian–Albian Supersequence boundary precisely at the Albian–Cenomanian stage boundary also implies ca 1·4 Ma of diachroneity in second order sea‐level patterns between the northern Gulf of Mexico and other documented global sedimentary basins.

Coral patch reef system and associated facies from southwestern Gondwana: paleoenvironmental evolution of the Oxfordian shallow-marine carbonate platform at Portada Covunco, Neuquén Basin, Argentina

Fil: Beresi, Matilde Sylvia. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Mendoza. Instituto Argentino de Nivologia, Glaciologia y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivologia, Glaciologia y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivologia, Glaciologia y Ciencias Ambientales; Argentina

Sedimentologic and paleoclimatic reconstructions of carbonate factory evolution in the Alborz Basin (northern Iran) indicate a global response to Early Carboniferous (Tournaisian) glaciations

The Lower Carboniferous Mobarak Formation records the development of a storm-sensitive pervasive carbonate factory on the southern Paleo-Tethyan passive margin following the opening of the Paleo-Tethys Ocean into the Alborz Basin along the northern margin of Gondwana. Its depositional facies encompass inner ramp peritidal environments, peloidal to crinoidal shoals, storm to fair-weather influenced mid-ramps, proximal to distal shell beds and low energy outer ramps. Sedimentological analyses and foraminiferal biostratigraphy reveal four events affecting carbonate platform evolution in the Alborz Basin during the Lower Carboniferous: (1) A transgression following global temperature rise in the Early Tournaisian (middle Hastarian) caused the formation of thick-bedded argillaceous limestones. This interval correlates with Early Tournaisian nodular to argillaceous limestones in the Moravia Basin (Lisen Formation, Czech Republic), the Dinant Basin (Pont d'Arcole Formation, Belgium), and at the Rhenish Slate Mountains (Lower Alum shale, Germany). (2) Late Hastarian–early Ivorian glaciations previously identified in Southern Gondwana but had not yet recognized in Northern Gondwana were recorded through a sequence boundary. (3) During the Late Tournaisian–Early Visean?, a differential block faulting regime along the basin's margin caused uplift of the westernmost parts of the Alborz Basin and resulted in subsidence in the eastern part of the central basin. This tectonically controlled shift in depositional regime caused vast sub-aerial exposure and brecciation preserved in the top of the Mobarak Formation in the western portion of the Central Alborz Basin. (4) Tectonic activity coinciding with a progressive, multiphase sea level drop caused indirectly by the Viséan and Serpukhovian glaciations phases ultimately led to the stagnation of the carbonate factory. Paleothermometry proxies, the presence of foraminiferal taxa with a northern Paleo-Tethyan affinity and evidence for arid conditions in the terrestrial hinterland place the Alborz Basin at lower latitudes than the approximately 45ο–50ο southern paleolatitude reported thus far.

Cephalopod associations and palaeoecology of the Cretaceous (Barremian–Cenomanian) succession of the Alpstein, northeastern Switzerland

The Alpstein (cantons of Appenzell Ausserrhoden, Appenzell Innerrhoden and St. Gallen, northeastern Switzerland) has been of great interest for geologists over the last decades because of its excellent outcrops. However, there was no comprehensive overview over its Cretaceous fossil content. Here, we describe the cephalopod associations, which are moderately to highly diverse in some strata of the Alpstein. Furthermore, we document the regional palaeoecological changes that occurred during the radiation of heteromorph ammonites (ancyloceratids, scaphitids, turrilitids). To examine the palaeoecological changes, we quantitatively determined the macrofossil content of 11 associations of Barremian to early Cenomanian age. Here, we document 6 species (3 genera) of nautilids and 77 species (45 genera) of ammonoids (29 of the species are recorded from Switzerland for the first time). Our palaeoecological analyses revealed the disappearance of nektoplanktonic forms after the late Barremian to the middle early Aptian in the course of the development of a shallow carbonate platform. The upper lower Aptian to middle Albian strata were eroded due to successive emersion phases and condensation processes. In the late Albian, the number of nektoplanktonic species surged again with some benthos, followed by the Cenomanian fauna, which is dominated by nektoplanktonic elements including ammonites, belemnites and nautilids with only very little benthos. These results correlate well with the regional sea level fluctuations.

Spatial and temporal evolution of Cenozoic carbonate platforms on the continental margins of the South China Sea: Response to opening of the ocean basin

Widespread and tremendously thick Cenozoic carbonate sequences are present along the margins of the South China Sea (SCS). However, most of the sequences have been drowned since the Late Miocene. The stratigraphic architecture of the carbonate platforms in the SCS can provide information on the tectonic evolution of the ocean basin. Based on 2D/3D seismic, well and regional geologic data, we have interpreted the Cenozoic SCS carbonate platforms along the continental margins. The carbonate platforms developed during rifting and initiated on the fault block of the conjugate rifted margins. Most of the carbonate platforms became drowned after the Middle Miocene. The Malampaya Carbonate Sequences, which have thicknesses of greater than 600 m, developed on a horst of an Oligocene rifted block. Tectonic subsidence provided accommodation for the growth of the carbonate platforms. Tectonic tilting, faulting, and the foreland bulge controlled the distributions, thicknesses, and horizontal seismic reflection variations of the drowned carbonate platforms. The tectonic evolution and relative sea-level fluctuations controlled the depositional cycles of the carbonate platforms. We quantitatively calculated the factors that affected the extension and subsidence rates using balanced cross section and backstepping techniques. Our results have demonstrated that the carbonate platforms flourished during the Middle Miocene due to stable tectonic conditions and shrank during the Late Miocene due to rapid subsidence. The relative sea level exerted a second-order control on the evolutionary trend of the carbonate platforms and a third-order control on the evolutionary periods in each stage.

The youngest stage in the evolution of the Dinaric carbonate platform: the Upper Nummulitic Limestones in the North Dalmatian foreland, Middle Eocene, Croatia

The youngest stage in the evolution of the Dinaric carbonate platform: the Upper Nummulitic Limestones in the North Dalmatian foreland, Middle Eocene, Croatia

Insights into a million-year-scale Rhenohercynian carbonate platform evolution through a multi-disciplinary approach: example of a Givetian carbonate record from Belgium

AbstractIn this paper we formulate answers to three important questions related to Givetian carbonate records and their use for reconstructing million-year-scale past palaeoenvironmental changes. First, we provide detailed illustrations of the fascinating diversity that shaped a significant shallow reefal platform during early to late Givetian time in the Rhenohercynian Ocean; secondly we improve the sedimentological model of the extensive Givetian carbonate platform in the Dinant Basin; and thirdly we evaluate the application of magnetic susceptibility as a tool for long-term trend correlations and palaeoenvironmental reconstructions. These goals are reached by making a sedimentological, geophysical and geochemical study of the La Thure section. Through the early–late Givetian interval we discerned 18 microfacies ranging from a homoclinal ramp to a discontinuously rimmed shelf and then a drowning shelf. The comparison of these sedimentological results with those published for the south of the Dinant Syncline allowed us to provide an up to date model of the vertical and lateral environmental development of one of the largest Givetian carbonate platforms in Europe. This comparison also increased the knowledge on the distribution of facies belts in the Dinant Basin and allowed us to highlight the Taghanic Event. Palaeoredox proxies reveal a substantial change in the oxygenation level, from oxygen-depleted to more oxic conditions, between middle and late Givetian time. We demonstrated the relationship between variation in magnetic susceptibility values and proxies for siliciclastic input (such as Si, Al). The La Thure section is considered a key section for the understanding of internal shelf settings bordering Laurussia's southeastern margin.

Morphologies and depositional/erosional controls on evolution of Pliocene-Pleistocene carbonate platforms: Northern Carnarvon Basin, Northwest Shelf of Australia

The detailed morphologies, evolution and termination of latest Neogene tropical carbonate platforms in the Northern Carnarvon Basin (NCB), on the passive margin of the Northwest Shelf (NWS) of Australia, defined based upon mapping using 3D seismic images, reveal the history of local/regional oceanographic processes, fluctuations in relative sea-level and changing climate. Cool-water carbonate deposition, dominant during the early-middle Miocene, was followed by a siliciclastic influx, which prograded across the NWS beginning in the late-middle Miocene, during a period of long-term global sea-level fall. The resulting prograding clinoform sets, interpreted as delta lobes, created relict topographic highs following Pliocene termination of the siliciclastic influx. These highs constituted multiple favorable shallow-water environments for subsequent photozoan carbonate production. Resultant platform carbonate development, in addition to being a response to cessation of siliciclastic influx and the existence of suitable shallow-water substrate, was also influenced by development of the warm-water Leeuwin Current (LC), flowing southwestward along this margin.Four flat-topped platforms are mapped; each platform top is a sequence boundary defined by reflection onlap above and truncation below. Successive platforms migrated southwestward through time, along margin strike. All platforms exhibit predominantly progradational seismic geometries. Mapped tops are ≥10km wide. Seismic evidence of karst on three of four platform tops, e.g., v-shaped troughs up to 50m deep and ~1km wide, and broader basins with areas up to 20km2, suggests episodic subaerial exposure that may have contributed to platform demise. Platform 4, the most recent, is unique in having interpreted biohermal build-ups superimposed on the progradational platform base. The base of these interpreted patch reefs now lies at a water depth of ~153m; therefore, we suggest that these reefs developed post-LGM (~21Ka); Subsequent reef drowning occurred, possibly as a result of accelerated subsidence and turbidity associated with formation of adjacent sediment drifts and fluctuations in strength of the LC during the late Pleistocene. Progressive drowning and termination of platforms from northeast to southwest along strike may have resulted from unknown combinations off climate factors, supplemented by differential compaction of the deltaic substrate and/or differential tectonic subsidence caused by mantle induced downwelling.

Influence of bathymetry and oceanic currents on the development of carbonate platforms: Northern Carnarvon Basin, Northwest Shelf of Australia

Cenozoic continental-shelf deposition in the Northern Carnarvon Basin was dominated by heterozoan carbonates until siliciclastic sediments prograded across the shelf in the late-middle Miocene to Pliocene. These siliciclastic sediments were deposited as outer-shelf and shelf-edge deltas, which created subtle bathymetric highs. Interpretation of extensive, high-quality multichannel seismic data shows broad, flat-topped carbonate platforms overlying the remaining bathymetric highs following the retreat of deltaic sedimentation. Platform geometries suggest that a combination of relative sea-level changes and ocean currents controlled their internal architectures. Clinoforms on the northeastern flanks of the platforms are more steeply dipping than those on the southwestern flanks and this platform architecture may reflect strengthening, or initiation, of a proto-Leeuwin Current. The earliest platform developed in the northeast and successive platforms migrated southwest (along strike), a process that could have resulted either from differential compaction of the underlying clastic depocenters, or variable rates of tectonic subsidence along depositional strike, probably in combination with environmental factors.

Cambrian Series 3 carbonate platform of Korea dominated by microbial-sponge reefs

Metazoans have been considered as negligible components of Cambrian Series 3 and Furongian microbial-dominated reefs, in contrast to their presence in earlier Terreneuvian–Cambrian Series 2 microbial–archaeocyath reefs. However, recent discoveries of sponges in Cambrian Series 3–Furongian reefs of Australia, China, Iran, USA, and Korea have raised question regarding their contribution in terms of carbonate platform development, which have never been assessed. This study examines Cambrian Series 3 deposits of the Daegi Formation, Korea to elucidate this question. The 100-m-thick middle part of the Daegi Formation is dominated by boundstone facies, which occupies 45% of the study interval, as well as bioclastic wackestone to packstone, bioclastic grainstone, and ooid packstone to grainstone facies. The Daegi reefs are primarily thrombolitic in composition, with 90% (n=26/29) of the reefs containing an average of 9% sponges in aerial percentage calculated from thin sections. Lithistid sponges composed of peloidal fabrics, some desma spicules, and spicule networks commonly occupy the interstitial space in microbial clusters, are encrusted by mesoclots and Epiphyton, and are surrounded by micrite. Subordinate non-lithistid demosponges occur within clusters of microbial elements. The middle Daegi Formation can be largely subdivided into shoal environment dominated by grainstone to packstone facies and shallow subtidal platform interior environment located behind shoal with wackestone to packstone facies. The microbial-sponge reefs mainly developed around platform interior as patch reefs. The current study indicates that metazoans in the form of lithistid and non-lithistid demosponges are nearly ubiquitously incorporated in Daegi reefs and contributed greatly to the formation of microbial-sponge reefs as well as carbonate platform during the time. Study of these microbial-sponge reefs and their distribution within the carbonate platform may help us to understand how carbonate sedimentary environments responded to the extinction of archaeocyaths.

Geology of Monte Gallo (Palermo Mts, NW Sicily)

ABSTRACTThe promontory of Monte Gallo (Palermo, NW Sicily) is a spectacular site where Upper Triassic-Eocene carbonate platform rocks and Quaternary continental to marine deposits are well exposed. A Mesozoic-Paleogene rock succession allows the potential visitor to easily detect the features and the evolution of the Panormide carbonate platform, a shallow-water paleogeographic domain of the Southern Tethyan margin. Quaternary deposits, as well many landforms, enable the visitor to directly identify the interplay between climate changes, tectonics and fluctuations of marine level that occurred during the Quaternary Period. A detailed geological map (1:15,000 mapping scale) is presented, accompanied by a stratigraphic correlation of logged sections and morpho-stratigraphic and tectonic schemes of the area, aimed at highlighting the geological heritage of Monte Gallo in support of the establishment of a Geosite. The Geological Map and relevant explanatory notes should be used as cartographic support and as a field trip guide for possible geological itineraries.

The Racławka Valley – an example of an educational geosite related to the development of a Paleozoic carbonate platform

The location and most interesting geological features of the Upper Devonian through Lower Carboniferous limestones in the Raclawka Valley were presented. The limestones outcropped almost continuously along the natural valley cutting. They comprise over 60 million years of continuous growth of the carbonate platform, which developed on the ancient terrane – the Upper Silesian Block. Geosites described herein may constitute a destination for geological trips for undergraduate students of Environmental Sciences, where they can actively discover the development of an ancient carbonate platform and accompanied post depositional structures. // W dolinie Raclawki znajdują sie wazne geologicznie naturalne odsloniecia wapieni (od poźnego dewonu do wczesnego karbonu). Wapienie te obejmują okres ponad 60 milionow lat ciąglej sedymentacji weglanowej, ktora wystepowala na obszarze paleozoicznego kratonu – bloku gornośląskiego. Opisane stanowiska mogą byc celem wycieczek geologicznych studentow pierwszego stopnia studiow przyrodniczych, w czasie ktorych mogą oni w aktywny sposob śledzic rozwoj kopalnej platformy weglanowej i struktur postdepozycyjnych.

Type and evolution of carbonate platforms in Jixian Period Mesoproterozoic: southwestern margin of Ordos Basin

The Ordos block experienced extension in the Mesoproterozoic, during which the Longshan–Kuanping and Haiyuan rifts developed on its southwest margin. The Jixian Group (1.6–1.4 Ga) is mainly exposed on the rimmed and ramp-type platforms at the Qishan County, southwest of the Ordos block. The deposition associated with the rimmed platform margin consists of platform facies, platform edge facies, and slope facies, which are characterized by small tidal creeks, mud cracks and scour troughs, rockfalls, respectively. The ramp-type platform is dominated by platform edge facies, shelf facies and slope facies and featured by intraclastic dolostones, tempestites and debris flow sediments respectively. The syndepositional normal faults were well developed at the early stage of the rifting. Fault-related cracks and folds present in the adjacent strata. The uplift of the footwall rimmed the platform. The drag structures determine the local sedimentary environment. In the final stage of the rifting, the fault movement stopped. The surrounding rocks were undeformed. The slope became less steep, resulting in the formation of ramp-type platform. The study area in the Jixian period, Mesoproterozoic is located at a palaeo-continental margin, which has already shown good petroleum prospectivity. A clear study on the type of carbonate platforms and the evolution of the platforms can help reduce exploration risks by improve the understanding of the presence of the source rocks.

Variations in architecture and cyclicity in fault-bounded carbonate platforms: Early Miocene Red Sea Rift, NW Saudi Arabia

The Early Miocene was a period of active rifting and carbonate platform development in the Midyan Peninsula, NW Saudi Arabia. However, there is no published literature available dealing with the detailed characterization of the different carbonate platforms in this study area. Therefore, this study aims to present new stratigraphic architectural models that illustrate the formation of different carbonate platforms in the region and the forcing mechanisms that likely drove their formation. This study identified the following features formed during active rifting: a) a Late Aquitanian (N4) fault-block hangingwall dipslope carbonate ramp, b) a Late Burdigalian (N7-N8) isolated normal fault-controlled carbonate platform with associated slope deposits, and c) a Late Burdigalian (N7-N8) attached fault-bounded platform with reef buildups, rimmed shelf developed on a footwall fault-tip within a basin margin structural relay zone that formed coinciding with the second stage of rifting. Variations in cyclicity have been observed within the internal stratigraphic architecture of each platform and also between platforms. High-resolution sequence stratigraphic analysis shows to be parasequences the smallest depositional packages (metre-scale cycles) within the platforms. The hangingwall dipslope carbonate ramp and the attached platform demonstrate aggradational-progradational parasequence stacking patterns. These locations appear to have been more sensitive to eustatic cyclicities, despite the active tectonic setting. The isolated, fault-controlled carbonate platform reveals disorganized stratal geometries in both platform-top and slope facies, suggesting a more complex interplay of rates of tectonic uplift and subsidence, variation in carbonate productivity, and resedimentation of carbonates, such that any sea-level cyclicity is obscure. This study explores the interplay between different forcing mechanisms in the evolution of carbonate platforms in active extensional tectonic regions. Characterization of detailed parasequence-scale internal architecture allows the spatial variation in syn-depositional relative base-level changes to be inferred and is critical for understanding the development of rift basin carbonate platforms. Such concepts may be useful for the prediction of subsurface facies relationships beyond interwell areas in hydrocarbon exploration and reservoir modeling activities.

Improving shallow‐water carbonate chemostratigraphy by means of rudist bivalve sclerochemistry

AbstractDeep‐time shallow‐marine carbonate platforms record distinct biotic responses to climatic and environmental stressors. Unfortunately, precise temporal assignment of these biotic responses is often problematical due to poor biostratigraphic control and/or a significant diagenetic overprint of the neritic bulk carbonate chemostratigraphic inventory. An accurate stratigraphic framework is essential to better understand the causal relation between biotic events recorded by carbonate platforms and environmental changes that, for instance, culminated in mass extinction events or prolonged episodes of oceanic anoxia. Here we provide an integrated carbon and strontium‐isotope stratigraphy of the Early Cretaceous subtropical Provence carbonate platform in SE France that is based solely on pristine low‐Mg calcite from rudist bivalves. Carbon‐isotope data of geochemically screened rudist fragments enabled reconstruction of a characteristic Barremian pattern including the Mid‐Barremian Event (MBE) that allowed for a precise correlation with stratigraphically well‐constrained Tethyan shallow‐water and hemipelagic reference sections. In order to evaluate ontogenetic carbon‐isotope changes and the overall variability of the shell‐derived carbon‐isotope data, numerous sclerochronological carbon‐isotope profiles of individual large rudist shells are presented. Strontium‐isotope stratigraphy supports the carbon‐isotope‐based age of the studied sections, but also provides unequivocal evidence for a major hiatus in the depositional record covering large parts of the Late Barremian. In contrast to biostratigraphic and bulk carbonate chemostratigraphic archives, the here established chronostratigraphy of carbonate platform evolution in the southern Provence region demonstrates a twofold resurgence of rudist‐rich carbonate platform production during the Early Aptian and arguably the latest Early Aptian.

Diagenesis and fracturing of a large-scale, syntectonic carbonate platform

The influence of coeval tectonics on carbonate platform development is widely documented, yet the diagenesis of such syntectonic platforms is barely evaluated. An outcrop, petrographic and geochemical study details here for the first time the diagenesis of the Tonasa Limestone Formation developed in an extensional regime in central Indonesia. This equatorial carbonate system was affected by block faulting, tilt-block rotation, differential uplift and subsidence throughout its Eocene to Early Miocene history (Wilson, 1999; Wilson et al., 2000). The Tonasa carbonate platform is dominated by alteration in shallow to deeper burial depths by fluids with predominantly marine precursor origins. Mechanical and chemical compaction features are common, as are a range of mainly burial-related granular mosaic, blocky and equant calcite cements. Earlier marine cements and meteoric influences are rare, being highly localised to block faulted highs and/or bathymetrically upstanding platform margin areas. Early marine micritisation of allochems was common on the platform top. Tectonic uplift together with a major oceanic throughflow current are thought to be key influences on localised karstification, meteoric diagenesis and marine cementation. The distribution and orientation of faults, fractures and calcite veins together with evidence for their relative timing are the strongest manifestation of tectonism coeval with diagenesis. There is concordance in the orientation and timing of structures affecting the Tonasa Platform with those basin-wide, with the potential for reactivation of pre-existing basement fabrics. Tectonic subsidence, including fault-associated differential subsidence, controlled the degree of burial diagenesis impacting different areas of the platform. A predominance of burial diagenetic features and dearth of earlier marine or meteoric cementation is seen in other Tertiary equatorial platforms and is partly attributed to: (1) predominance of non-framework building larger benthic foraminifera and/or algae that are prone to remobilisation, have low production rates and limited potential to build to sea level, and (2) high runoff due to the equatorial humid climate contributing to lowered marine salinities in SE Asia. Underlying tectonic reasons for the preponderance of a “regional” diagenetic signature over a “syntectonic” one, fracturing excepted, are: (1) development on the flanks of a backarc basin not on typical continental crust, (2) key platform influencing structures are oblique to the main extensional direction in the basin, and (3) development in an overall subsiding tectonic regime, post-dating basin initiation. The aim here is that this study will contribute to understanding diagenetic alteration of syntectonic carbonate platforms, and those from equatorial regions.

Factors controlling the evolution of a wedge-top temperate-type carbonate platform in the Miocene of the northern Apennines (Italy)

An integrated study including stratigraphy, composition and biostratigraphy was performed on temperate-type shallow-water carbonates of early middle Miocene age deposited in a wedge-top basin within the Apenninic chain (Romagna–Tuscan area). The succession rests unconformably on the deep-water sediments of allochthonous Ligurian units and is represented by up to 350m thick carbonates and mixed siliciclastic-carbonate rocks. Two main units are recognized, each characterized by a distinct facies association. The basal unit (upper Burdigalian) consists of rhodalgal rudstones and packstones of shelf environment, dominated by echinoids, bryozoans, coralline algae and benthic foraminifera, deposited in temperate conditions and showing a transgressive pattern. It gradually passes into mixed carbonate-siliciclastic shallow water-facies, (Langhian in age) characterized by an increase in terrigenous components and planktonic and benthic taxa. Sediment starvation during the Langhian is suggested by the occurrence of glaucony-rich packstones with abundant planktonic foraminifera. The succession grades upwards into upper Langhian-lower Serravallian planktonic foraminifera-rich marls. Increasing detrital discharge and water depth up section results in drowning of the carbonate shelf. Debris-flow deposits including material of extraformational origin are intercalated at different levels, in particular from the uplifting south-western margins, close to the Ligurian thrusts. The inception and demise of these temperate rhodalgal-foramol carbonate sediments located in wedge-top Epiligurian basins are primarily controlled by synsedimentary tectonics, which in turn affects relative sea-level rise, and nutrient and detrital input. In particular, tectonics controlled the initial stage of the transgression, determining the configuration of the basin, subdivided in narrow submerged palaeotopographic lows and relatively elevated areas. Synsedimentary tectonics related to the thrust migration was also the main controlling factor in the demise of carbonate sedimentation, through the combined effect of two processes: increasing the subsidence of the basin, and triggering the terrigenous discharge from the erosion of the uplifted Apenninic relieves.

NEW EVIDENCE ON THE ROLE OF SILICEOUS SPONGES IN ECOLOGY AND SEDIMENTARY FACIES DEVELOPMENT IN EASTERN PANTHALASSA FOLLOWING THE TRIASSIC-JURASSIC MASS EXTINCTION

Paleoecological consequences of the global Triassic–Jurassic mass extinction (201.3 Ma) are poorly understood. Fossiliferous marine boundary records are rare, commonly condensed, and typically reveal facies changes previously attributed to eustacy. Sedimentology and biofacies analyses from stratigraphically expanded successions of the lowest Jurassic strata, New York Canyon, Nevada, were investigated with high-resolution paleoenvironmental observations, fossil surveys, and microfacies analysis. Following the collapse of the uppermost Triassic carbonate ramp, the lowest Jurassic Ferguson Hill Member of the Sunrise Formation records a midshelf habitat dominated by previously unrecognized siliceous sponges for approximately two million years. In addition, the earliest Jurassic strata from the Pucara Group, central Peruvian Andes, were examined and record a more greatly expanded stratigraphic succession of facies across the inner to middle shelf. Like Nevada, the lowest Jurassic Aramachay Formation is replete with intense concentrations of siliceous sponges. The revelation of widespread, ecologically dominant siliceous sponges has been overlooked despite detailed biofacies studies in both depositional systems. Sponges expanded across shallow environments with sparse benthic biocalcifier populations, and were likely aided by increased ocean silica concentrations from the weathering of the Central Atlantic Magmatic Province. Facies changes previously attributed to sea-level change are thus interpreted to result from the collapse of the carbonate factory concomitant with the mass extinction, with transition to an alternate state dominated by siliceous sponges before a return to carbonate platform development in the Sinemurian. Our study highlights the need to separate biofacies from paleoenvironmental analysis during mass extinction times when nonactualistic assemblages may dominate and deviate from expected environments (e.g., siliceous sponges as indicators of deep paleoenvironments).