Outer Ramp Settings Research Articles

Depositional and postdepositional processes of an oil-shale analog at the microstructure scale: The Lower Cretaceous Agrio Formation, Neuquén Basin, northern Patagonia

The Lower Cretaceous Agrio Formation is a source rock considered to be a possible unconventional reservoir. However, no detailed microstructure characterization of this hydrocarbon play has been performed. An exceptional oil-prone level is determined through outcrop information, petrography, x-ray diffraction, geochemical analysis, and x-ray micro–computed tomography (μCT). Microscopic studies reveal a lenticular fabric and calcareous nannofossils suggesting high carbonate productivity in the water column and algal-derived organic matter. The high organic matter content points to anoxic–euxinic conditions consistent with pyrite framboids, authigenic fluorapatite, and an elevated concentration of redox-sensitive elements. However, burrows reveal the activity of infaunal organisms, thus rejecting the idea of persistent bottom-water anoxia. The bioturbated, calcareous plankton and carbonate-rich mudstone represents accumulation in a distal outer ramp setting with calcareous plankton blooms and the co-occurrence of benthic microbial mats. Bottom currents, probably as a consequence of storm-related processes, were involved in detrital transport and oxygenation pulses. A three-dimensional (3-D)–microstructure reconstruction reveals three main μCT density-dependent phases: (1) a low-density phase composed of organic matter and calcite, (2) an intermediate-density phase dominated by calcite and minor organic matter, and (3) a high-density phase composed of authigenic minerals (pyrite, marcasite, and fluorapatite) and silt-sized detrital quartz and plagioclase. This analysis showed that burial diagenesis did not create enough pore space to be evaluated through μCT and that besides mineralogy, particle-size distribution, particle arrangement (sorting), and the local compaction degree affect the 3-D reconstructed structure. The analyzed sample is located toward the carbonaceous end member of the shale spectrum and exemplifies new approaches for the characterization of carbonate oil shales in the unit and similar deposits. From the perspective of unconventional exploration, the evaluation of depositional and postdepositional controls on the resultant microstructure might contribute to a better understanding of potential target intervals within the Agrio Formation.

Porosity evolution in sequence stratigraphic framework: a case from Cretaceous carbonate reservoir in the Persian Gulf, southern Iran

Dariyan Formation (Aptian) is one of the important hydrocarbon reservoirs in the Persian Gulf. In this reservoir, porosity is controlled by various depositional and diagenetic factors. This study integrates petrographic, geochemical, petrophysical and reservoir quality data to distinguish various pore types and their evolution in two fields selected from the SE and NW Persian Gulf. Primary (depositional) and secondary (diagenetic) pore types are differentiated using the results of macroscopic (core) and microscopic (thin section and SEM) petrographic studies. Pore size distribution and reservoir importance are evaluated by MICP and porosity–permeability measurements on core-plug samples. Macroscopic pores are formed within the grain-dominated and dissolved mud-dominated facies that are located in the RST of third-order sequences, especially within the lower carbonate unit. Micropores are recorded in mud-dominated facies of middle to outer ramp settings with minor effects of meteoric dissolution. Accordingly, evolution in an open diagenetic system with high water/rock ratio is proposed for grain-supported facies, and a closed diagenetic system for mud-dominated intervals. There are close relationships between micrite micro-textures (MTCs) and reservoir properties of mud-dominated facies. A dynamic pore evolution model is presented for the Dariyan Formation that integrates all depositional and diagenetic features in sequence stratigraphic framework. This model can be used for ongoing reservoir studies of this formation in hydrocarbon fields of the Persian Gulf.

The oldest stage of the Outer Carpathian evolution in the light of Oxfordian\u2013Kimmeridgian exotic clast studies (southern Poland)

In the Late Jurassic, the rifting process led to the disintegration of the southern margin of the North European Platform and to the opening of the Outer Carpathian flysch basin sedimentary system. The initial sediments accumulated in the northern part of the basin are related to both the destruction and resedimentation of older platform deposits. Since the sedimentary succession of this pre-flysch phase was consumed by the Miocene subduction, its only traces are represented, nowadays, by clasts preserved as exotics in the succeeding flysch deposits. Our analysis of foraminifers as well as calcareous and organic dinoflagellate cysts found in these exotics confirms the Oxfordian–early Kimmeridgian timing of the platform phase that preceded the opening of the flysch basin. The exotics are represented by three main facies types: sponge–microbial limestones, oncoid–intraclastic–Crescentiella limestones and fine-grained, biodetrital limestones with Saccocoma. These deposits are related to mid-ramp to outer-ramp settings. The land influence was rather weak, and these sedimentary settings were dominated by pelagic/hemipelagic accumulation. The studied facies are similar to facies types widely distributed over the northern shelf area of the Western Tethys (e.g., extra-Carpathian southern Poland, Carpathian Foredeep basement, southern Germany). In turn, coeval strata known from the part of the Magura Basin and of the Penninic–Pieninic Ocean, which were situated in more southern part of the Tethys, yielded different microfacies reflecting significant differences between the sedimentary settings of the study area and its southern extensions.

Regional nutrient decrease drove redox stabilisation and metazoan diversification in the late Ediacaran Nama Group, Namibia

The late Ediacaran witnessed an increase in metazoan diversity and ecological complexity, marking the inception of the Cambrian Explosion. To constrain the drivers of this diversification, we combine redox and nutrient data for two shelf transects, with an inventory of biotic diversity and distribution from the Nama Group, Namibia (~550 to ~538 Million years ago; Ma). Unstable marine redox conditions characterised all water depths in inner to outer ramp settings from ~550 to 547 Ma, when the first skeletal metazoans appeared. However, a marked deepening of the redoxcline and a reduced frequency of anoxic incursions onto the inner to mid-ramp is recorded from ~547 Ma onwards, with full ventilation of the outer ramp by ~542 Ma. Phosphorus speciation data show that, whilst anoxic ferruginous conditions were initially conducive to the drawdown of bioavailable phosphorus, they also permitted a limited degree of phosphorus recycling back to the water column. A long-term decrease in nutrient delivery from continental weathering, coupled with a possible decrease in upwelling, led to the gradual ventilation of the Nama Group basins. This, in turn, further decreased anoxic recycling of bioavailable phosphorus to the water column, promoting the development of stable oxic conditions and the radiation of new mobile taxa.

Sedimentary evolution of the marine Agua de la Mula Member (Agrio Formation, Lower Cretaceous) in the central Neuquén Basin: Source areas and paleogeographic considerations from a distal setting

The Agua de la Mula Member (Upper Hauterivian–Lower Barremian) of the Agrio Formation represents the infill of a mixed siliciclastic–carbonate low-gradient ramp system comprising both hydrocarbon sand reservoirs and a source rock interval considered as potential unconventional shale play. The present study analyzes the unit in offshore settings (Loma La Torre section) where high-quality, laterally continuous exposures allowed the recognition of muddy subaqueous clinoforms prograding northwestward. The muddy-dominated facies throughout the entire section represent an overall evolution from distal outer-ramp to inner-ramp settings associated with tidal flat deposits. Based on previous sedimentological studies and paleogeographic reconstruction, four facies belts are refined for the unit in the central Neuquén Basin: (1) shallow marine and deltaic facies to fluvial deposits documented in the subsurface (mainly siliciclastic); (2) mid-ramp to inner-ramp with subordinated outer-ramp settings (mainly siliciclastic); (3) outer-ramp to distal mid-ramp settings with muddy subaqueous clinoforms (mixed siliciclastic–carbonate); and, (4) basinal to outer-ramp areas (carbonate-dominated). In addition to the main sediment input from the south and southeastern margin of the basin, sedimentological and stratigraphic approaches suggest a sediment supply from the eastern and northeastern regions. Sporadic ash-fall deposits also indicate that the coetaneous magmatic arc was active. In fact, LA-ICP-MS U–Pb zircon ages from two tuff layers recorded in the study section yield an age interval of less than 1 m.yr., which allowed us to estimate the sedimentation and progradation rates of a clinoform set. Finally, four asymmetrical sequences are recognized comprising thin (carbonate–siliciclastic) transgressive systems tracts and thick regressive (highstand) systems tracts with stacked parasequences of shallowing-upward siliciclastic-dominated facies. These sequences are correlated with the four (4th-order) sequences previously defined in proximal settings. The observations suggest that sea-level changes were relatively synchronous events and, according to the present knowledge, controlled not only by orbital mechanisms but also by tectonism.

Facies and evolution of the carbonate factory during the Permian–Triassic crisis in South Tibet, China

AbstractThe nature of Phanerozoic carbonate factories is strongly controlled by the composition of carbonate‐producing faunas. During the Permian–Triassic mass extinction interval there was a major change in tropical shallow platform facies: Upper Permian bioclastic limestones are characterized by benthic communities with significant richness, for example, calcareous algae, fusulinids, brachiopods, corals, molluscs and sponges, while lowermost Triassic carbonates shift to dolomicrite‐dominated and bacteria‐dominated microbialites in the immediate aftermath of the Permian–Triassic mass extinction. However, the spatial–temporal pattern of carbonates distribution in high latitude regions in response to the Permian–Triassic mass extinction has received little attention. Facies and evolutionary patterns of a carbonate factory from the northern margin of peri‐Gondwana (palaeolatitude ca 40°S) are presented here based on four Permian–Triassic boundary sections that span proximal, inner to distal, and outer ramp settings from South Tibet. The results show that a cool‐water bryozoan‐dominated and echinoderm‐dominated carbonate ramp developed in the Late Permian in South Tibet. This was replaced abruptly, immediately after the Permian–Triassic mass extinction, by a benthic automicrite factory with minor amounts of calcifying metazoans developed in an inner/middle ramp setting, accompanied by transient subaerial exposure. Subsequently, an extensive homoclinal carbonate ramp developed in South Tibet in the Early Triassic, which mainly consists of homogenous dolomitic lime mudstone/wackestone that lacks evidence of metazoan frame‐builders. The sudden transition from a cool‐water, heterozoan dominated carbonate ramp to a warm‐water, metazoan‐free, homoclinal carbonate ramp following the Permian–Triassic mass extinction was the result of the combination of the loss of metazoan reef/mound builders, rapid sea‐level changes across Permian–Triassic mass extinction and profound global warming during the Early Triassic.

Upper Ordovician bryozoans of Morocco

Abstract The majority of Upper Ordovician bryozoan occurrences in Morocco are known from the Anti-Atlas. These faunas are abundant and diverse; however, many of them still remain unstudied. The few explored bryozoan faunas of the Katian age exhibit high abundance and diversity of erect branched and massive trepostomes accompanied by few esthonioporines, cryptostomes and fenestrates, whereas cystoporate bryozoans are not known here. They occupied various biotopes within inner- and outer-ramp settings. Bryozoan faunas from the Upper Ordovician of Morocco are rather related to faunas from other parts of the Mediterranean Province (France, Carnic Alps, Sardinia) containing few species of Laurentian and Baltic origin. Trepostome bryozoans from Moroccan occurrences often display signs of gigantism explained by undisturbed growth in cold and quiet environmental conditions.

Contribution to the stratigraphy and sedimentology of the Upper Jurassic – lower Eocene succession of the Mitla–El Giddi stretches, west Central Sinai, Egypt

The present study deals with the stratigraphy, petrography and facies analysis of the stratigraphic succession of Mitla-El Giddi stretches (west Central Sinai, Egypt). Lithostratigraphically, the study area is covered by a thick succession extending from Late Jurassic to early Eocene time. This succession is subdivided into the Masajid Formation (Upper Jurassic (Oxfordian) at the base followed upwardly by the Risan Aneiza Formation (Lower Cretaceous (? Aptian- Albian), the Galala Fm. (Cenomanian), the Abu Qada Formation (lower Turonian), the Buttum Formation (middle Turonian), the Wata Formation (middle–upper Turonian), the Themed Formation (Coniacian–Santonian), the Sudr Chalk (Campanian–Maastrichtian), the Mitla Formation (= Esna Shale) (Paleocene–lower Eocene), the Thebes Formation (lower Ypresian) and the Minia Formation (upper Ypresian) at top. The Aptian – Albian Risan Aneiza Formation is first recorded at G. El Hamra in the study area. As well, a new formational name (the Mitla Formation) is used to describe the Paleocene – lower Eocene unit equivalent to the Esna Shale due to variation in lithology mainly represented by chalky limestone and argillaceous limestone. Many unconformities, represented by variable sedimentologic features, i.e., red paleosols, conglomerates, crustified laminated limestone and glauconitic facies were investigated and mostly attributed to the Syrian Arc System.The petrographic investigations especially for the lithified rocks revealed eighteen limestone microfacies, three dolostone microfacies and four sandstone microfacies, besides the non-lithified shale, sandy shale, clay and marl lithofacies. The recognized facies and their related paleoenvironments document lateral and common vertical transition between inner, middle and outer ramp setting. These facies could be subdivided into eight associations that are related to six depositional environments: peritidal flat/beach clastics, peritidal flat carbonates, lagoonal clastics, lagoonal carbonates, high energy shoal of ooids and patch reefs (oolitic shoal), intertidal-subtidal open marine, storm influenced subtidal open marine (mid-ramp) and hemipelagic outer ramp facies.The study indicates that, the area was controlled by a long-term transgressive phase and several higher order sea level fluctuations during the deposition of the studied succession. The main factors controlling the ramp deposition and the described events are; structure control mainly Syrian Arc deformation, eustatic sea level fluctuations combined with environmental influences such as autochthonous carbonate productivity-siliciclastic supply and paleorelief conditions.

Sedimentology of Marl and Marly Limestone Sequence of Upper Cretaceous Kawagarh Formation from Northern Kalachitta Range, Attock Hazara Fold and Thrust Belt, Pakistan

Upper Cretaceous Kawagarh Formation is well exposed in the Attock Hazara Fold and Thrust Belt (AHFTB) and shows significant lateral and vertical variations in lithology. The present work deals with the sedimentological studies of marl and marly limestone sequence of Kawagarh Formation exposed at the Bagh Neelab, Ghariala north and Sojhanda villages in Northern Kalachitta Range. Detailed petrographic studies of marly limestone and hard marl substrate show that planktons and oysters are the main skeletal constituents of studied samples and clay and detrital quartz mainly composed the non skeletal fraction. X-Ray diffraction analyses of selected marl samples confirm the petrographic data. On the basis of skeletal and non skeletal content, two microfacies—marl microfacies and Planktonic microfacies are constructed. The faunal content, their paleoecology and detrital content of microfacies suggest that marl and marly limestone sequence of Kawagarh Formation was deposited over the mid and outer ramp settings.

Benthic foraminiferal communities of the Eocene platform, north Eastern Desert, Egypt

The Eocene successions on Beni Suef – El Zaafarana road at north Eastern Desert of Egypt have been examined for their benthic foraminiferal contents from three rock units: El Fashn, Beni Suef and Maadi formations. The studied material of this work allowed us to identify 153 benthic foraminiferal species. The Eocene age of the studied successions is defined by the co-occurrence of the planktonic and benthic foraminiferal species. The studied succession could be subdivided into five benthic foraminifera biozones from the late Middle to Late Eocene age. These zones are Palmula ansaryi Range Zone (late Middle Eocene), Bulimina jacksonensis - Uvigerina mediterranea Concurrent-Range Zone, Cancris auricula Lowest Occurrence Zone, Pararotalia audouini Concurrent-Range Zone and Pyrgo elongata Concurrent-Range Zone (Late Eocene). The identified benthic foraminiferal species were analyzed using R- and Q-mode cluster analysis in order to better understand the linkage between the benthic foraminiferal assemblages and paleoenvironmental conditions. The cluster analysis revealed that the benthic foraminifera could be grouped into 7 assemblages and 7 biofacies in the measured sections. The recorded benthic foraminiferal biofacies suggest that the environments are deep outer ramp setting with meso-to moderately eutrophic conditions, and minimum to moderate oxygenation level on El Fashn Formation. The environment of Beni Suef and Maadi formations is inner to middle ramp with high oxygen level and oligotrophic conditions.

Early Silurian (Telychian) bryozoan reefs in the epeiric sea of South China: Are heterotrophic metazoan buildups promoted by internal waves?

After the mass extinction at the end of the Ordovician period, metazoan reefs recovered and became widespread on the Yangtze carbonate platform by the late Aeronian, as recorded by a diverse and complex reef system in the Shihniulan Formation. In the overlying siliciclastic-dominated Hanchiatien Formation, the reefs declined in size and biodiversity along with a sudden facies change during the lower Telychian. Without any phototrophic organisms, small reefs were primarily built by abundant bryozoans, which crop out in the lower part of this formation at the southern tip of Chongqing, Southwest China. These reefs occur as massive argillaceous limestones, varying in size from 1.6 to 2.8 m in thickness and 6 to 16 m in diameter. The reefs are surrounded by gray-green shales and silty shales. Distinct vertical ecozonation is present, and four growth stages have been recognized, represented by four facies types from bottom to top: (1) auloporid coral-echinoderm-bryozoan packstone/floatstone, (2) fenestrate bryozoan-echinoderm bafflestone, (3) Fistulipora-Asperopora framestone, and (4) Fistulipora framestone. In tropical mid- to outer-ramp settings, these bryozoan reefs showcase an unusual reef community in heterozoan carbonates, which were probably influenced by internal waves.

Microfacies and depositional environment of Late Cretaceous to Early Paleocene oil shales from Jordan

The purpose of this study is to better understand the depositional setting of Late Cretaceous to Early Paleocene oil shales from southeast and central-east Jordan. One core from both the Jafr and the Azraq-Hamza Basins was logged, and their lithology, texture, and ichnofabrics were recorded. A total of 79 thin sections were analyzed petrographically, and eight microfacies types recognized. Both cores show lithologic and petrographic similarities. The oil shales are Maastrichtian to Danian in age and can be described as organic matter and calcite-rich mudrocks. The most abundant granular components are foraminifera and various types of phosphatic bio- and lithoclasts. Macrofossils (bivalves, ostracods, echinoderms) were recorded in some intervals. The current results were compared with data from a previous publication on Maastrichtian oil shales from the Jafr Basin. A new model explaining the deposition of the oil shales of the Jafr and Azraq-Hamza Basins is proposed. The onset of the Maastrichtian oil shale deposition in both basins coincides with the early Maastrichtian transgression in this region. The organic matter-rich sediments were deposited in a mid to outer ramp setting below the storm wave base. Younger oil shales of Late Maastrichtian to Danian age were deposited in a shallower environment, below the fair weather wave base. The Cretaceous/Paleogene boundary is marked by a hiatus in both cores. The Danian oil shales show relatively lower “total organic carbon” content than the Maastrichtian ones. The former are believed to have been deposited in more oxygenated bottom waters of a mid-ramp zone.

Brachiopods and rugose corals in an upper Serpukhovian (Mississippian) biostrome: preliminary results from the Djebel Arhlal (Béchar Basin, Algeria)

The Djebel Arhlal is the southernmost outcrop of upper Serpukhovian strata in the Béchar Basin. Here the Djenien member of the Djenien Formation is three-folded and it contains a 10 m-thick coral biostrome in its middle unit formed by Siphonodendron, Diphyphyllum and Lithostrotion. Distances between in situ colonies are in the order of several decimetres and the space between them is filled with bioclastic limestone containing coral rubble. Few brachiopods and solitary corals are found as dwellers within the biostrome, but these groups are much more common in the strata below and above the coral biostrome. This is especially true for the brachiopods, which reach diversities of more than a dozen species in specific horizons. They mainly belong to the orders Productida, Spiriferida and Athyridida. Agitated open-marine platform interior or platform margin settings are the general facies encountered in the Djenien member at the Djebel Arhlal. The field data, including brachiopod coquinas and many fragmented brachiopods and corals, are confirmed by microfacies analysis. However, the coral biostrome records a quieter setting at the interface of middle and outer ramp settings, as seen in the co-existence of in situ coral growth, input of reworked material, deposition of carbonate mud, and sparitic textures. This autoparabiostrome at Djebel Arhlal is compared to other rather thick and of exceptional horizontal extension upper Serpukhovian biostromes (few kilometres).

Sedimentology, rhythmicity and basin-fill architecture of a carbonate ramp depositional system with intermittent terrigenous influx: The Albian Kharfot Formation of the Jeza-Qamar Basin, Dhofar, Southern Oman

The Albian Kharfot Formation is preserved in the eastern margin of the Jeza-Qamar Basin which straddles across the Oman–Yemen border. This study addresses the sedimentological attributes of the formation and deduces its depositional setting, cyclicity and relative sea level changes in local (within the basin) and regional (Arabian) contexts. The interaction among siliciclastic influxes, in-situ carbonate production and tectono-climatic controls on the stacking nature of the various lithofacies that build-up the formation is discussed. In the study area, the formation lies unconformably over Barremian–Aptian Qishn Formation and conformably under late Albian–?Turonian Dhalqut Formation. The Kharfot Formation thickens from ~140m in the eastern side of the study area to ~300m at the Oman–Yemen border. It consists of eight lithofacies: Orbitolina-rich marls, peloidal bioclastic packstone, bioclastic mudstone to wackestone, argillaceous, bioclastic floatstone to rudstone, bioclastic rudstone, sandy, peloidal, bioclastic mudstone to packstone, peloidal, bioclastic grainstone and dolostone. The vertical arrangement of these lithofacies defines recurring meter- to decameter-scale, shallowing-upward units deposited on a westward-deepening inner- to outer-ramp setting. Tectonic rejuvenation of the siliciclastic source area was accompanied by warm, humid climatic conditions as suggested by high kaolinitic marls of the Kharfot Fm. and coeval quartz-rich sandstone units (Harshiyat Fm.). The shallowing-upward rhythmic sedimentation of the formation has close resemblance with cycles of the Nahr Umr Formation in northern Oman and partially comparable with the global sea level changes. The Kharfot basin was an intrashelf depression that was part of the much larger Arabian epeiric platform. The latter is defined by a rimmed margin in northern Oman where Al-Hassanat Formation represents platform margin deposits and Nahr Umr Formation representing back-rim intrashelf depression which received fine-grained siliciclastic influx from the land (westward). In southern Oman where Kharfot Formation accumulated, the platform was unrimmed ramp type basin with high fine clastic influx along with elevated carbonate production.

Sequence architecture of a Jurassic ramp succession from Gebel Maghara (North Sinai, Egypt): Implications for eustasy

The Jurassic succession at Gebel Maghara, North Sinai, Egypt, represents a mixed carbonate–siliciclastic sequence. Combining information from both fossils and rocks allowed a plausible reconstruction of the depositional environments and of the basin evolution. The Jurassic succession of Gebel Maghara was deposited on a ramp, and the architecture of the ramp facies was strongly controlled not only by sea–level changes but also by extensional tectonics in connection with rifting of the Tethys, North Gondwana. Seven tectonically modified thirdorder sequences (DS 1–DS 7) have been recognized. The first three sequences (DS 1–DS 3), ranging from the Toarcian to the Bajocian, record sea invasion (intertidal to shallow subtidal conditions) across an intracratonic area as a result of eustatic sea–level changes during a quiescent rift stage. The remaining sequences (DS 4–DS 7) reflect open marine mid to outer ramp settings. Non–marine conditions around the Bajocian–Bathonian boundary, documented by caliche, represent the maximum regression of the sea. During an active extensional stage, horsts, which formerly acted as barriers separating the Maghara sub–basin from the main ocean, subsided. Subsequent rejuvenation and reactivation of faults shifted the homoclinal physiography of the ramp to a distally steepened ramp during the early Bathonian, creating a 200–m–thick deltaic wedge. Similar processes during the early Kimmeridgian created a calcirudite–calcarenite succession of slope origin. The diversity and the epifaunal/infaunal percentage of the macrofauna display a cyclic pattern which coincides more or less with the sequence stratigraphic architecture.

The upper Cenomanian–lower Turonian of the Preafrican Trough (Morocco): Platform configuration and palaeoenvironmental conditions

A synthetic study was carried out based on sedimentological, palaeontological, geochemical and mineralogical data of the upper Cenomanian–lower Turonian carbonate platform of the Preafrican Trough (eastern Morocco) in order to (1) propose a 3D representation of the platform and constrain the temporal framework of the dysoxic/anoxic episodes recorded during the OAE2, (2) define and discuss the prevailing climate on the platform during this period, and (3) make comparisons with other Cenomanian–Turonian platforms.During the late Cenomanian, both before and during the CCIE (Cenomanian Carbon Isotope Excursion), the platform displayed an east–west polarity. Three third-order sequences of transgression–regression can be defined. Dysoxic conditions were developed in the sediments and the bottom waters of the deepest environment (mid- to outer-ramp setting), in the western part of the platform. Well-oxygenated waters were present in the eastern part of the platform (peritidal zone to mid-ramp environment). The climate was arid before the CCIE, becoming warm with contrasted seasons during the CCIE. This climate is associated with a low palaeoproductivity over the entire platform, along with the presence of photozoan followed by heterozoan carbonate-producers, as found also in other parts of the Saharan platform. However, such conditions are not in accordance with many studies which suggest a wet climate during the CCIE, leading to intense chemical weathering of the continent favouring the appearance of high palaeoproductivity at a global scale and the establishment of dysoxic/anoxic conditions. In the Preafrican Trough, poorly-oxygenated waters spread outwards from the deep basins and covered the platform in response to sea-level rise.Many disturbances are recorded in the platform succession during the early Turonian, after the CCIE. Indeed, just after the C/T boundary, the development of an outer-ramp environment over the entire Preafrican Trough reflects flooding of the platform, linked to the end of the major Cenomanian transgression and the presence of eutrophic conditions which disrupted carbonate-producing organisms. This high palaeoproductivity, due to considerable nutrient input, led to the establishment of highly dysoxic conditions in the bottom and intermediate waters, causing the disappearance of the majority of the Cenomanian palaeontological groups, except for the opportunist benthic and planktonic foraminifera which proliferated. Part of the nutrient input could be due to the presence of a hot and wet climate that may have led a slight increase in the degree of chemical weathering of the continent. Nevertheless, many lines of evidence, such as the decrease of the detrital influx during the early Turonian and the reduction of the weatherable continental areas after the Cenomanian transgression, suggest the existence of another source of nutrient inputs. Combined with the presence of dysoxic/anoxic conditions during the lower Turonian in several regions of the tropical Central Atlantic, the occurrence of these nutrient inputs may be linked to the volcanism/hydrothermalism of the Caribbean LIP and the mid-ocean ridges.

Upper Cretaceous sequence stratigraphy of the Galala Plateaux, western side of the Gulf of Suez, Egypt

Based on the study of lithology, biostratigraphy and analysis of facies, this study deduces the occurrence of nine third-order Upper Cretaceous (Cenomanian–Maastrichtian) depositional sequences, which are bounded by eleven sequence boundaries of geographic extent in the Galala Plateaux, situated along the western side of the Gulf of Suez, Egypt. Some of the recorded sequence boundaries (e.g., Cenomanian/Turonian, Turonian/Coniacian, intra Santonian, and Campanian/Maastrichtian sequence boundaries) are not yet confirmed to eustatic sea-level changes. Fourteen main mixed carbonate/siliciclastic facies associations were recognized on a lateral transition between inner to outer ramp settings, with apparent subsidence towards the south. The tectonic position of the Galala Plateaux played an important role in the profound complexity of facies changes and the configuration of depositional sequences representing variable time gaps.

Middle Eocene Nummulites and their offshore re-deposition: A case study from the Middle Eocene of the Venetian area, northeastern Italy

The Middle Eocene Calcari nummulitici formation from northeastern Italy, Venetian area, represents a shallow-marine carbonate ramp developed on the northern Tethyan margin. In the Monti Berici area, its main components are larger foraminifera and coralline red algal communities that constitute thick carbonate sedimentary successions. Middle ramp and proximal outer ramp environments are recognized using component relationships, biofacies and sedimentary features. The middle-ramp is characterized by larger flattened-lenticular Nummulites on palaeohighs between which rhodoliths formed. Larger Nummulites palaeohighs containing Nummulites millecaput, Nummulites crassus, Nummulites discorbinus and Nummulites cf. gizehensis developed more basin-wards. The following relatively quiet environments of basin-wards of the palaeohighs represent areas of maximum carbonate production. The transition between the distal middle- and the proximal outer-ramp settings is marked in the study area by a large erosional surface which is interpreted to have been formed as a result of an erosive channel body filled in by deposits re-sedimented from shallower depths. These off-shore re-sedimented channelized deposits, ascribed to the Shallow Benthic Zone SBZ 15, lying on hemipelagic marls (planktonic foraminiferal zone E9 (P11)) allow for a biostratigraphic correlation to the Late Lutetian.The studied deposits, represented by packstone to rudstones, were displaced whilst still unlithified. The Lutetian–Bartonian regression along with the local tectonic activity promoted the production of a high amount of biogenic shallow-water carbonates mainly produced in the Mossano middle-ramp settings. These prograded towards the basinal areas with high-sedimentation rate of carbonate deposits characterized by the larger Nummulites rudstones. Such high amounts of sediment led to sediment instability which potentially could be mobilized either by return currents due to occasional major storms or by earthquakes induced by tectonic activity. These will have led to the offshore re-deposition of the Nummulites sediments into deeper water setting via the observed channels. Since potential migration pathways are short, such distal re-sedimented channel-filled material surrounded by hemipelagic marls is optimally placed for the formation of potential subsurface oil reservoirs.

Metre-scale cyclicity in Permian ramp carbonates of equatorial Pangea (Venezuelan Andes): Implications for sedimentation under tropical Pangea conditions

Palmarito strata in the mid-Permian of the Venezuelan Andes show three different types of metre-scale cycle: Type A cycles are mixed clastic-carbonate, shallowing-upward peritidal cycles. The upper boundaries of the cycles are exposure surfaces with calcrete. This kind of cycle shows a significant degree of randomness in its thickness pattern and this is attributed to an autocyclic origin through tidal-flat progradation, and variations in carbonate productivity and clastic input to the depositional area. Type B cycles consist entirely of shallow subtidal facies with a shallowing-upward arrangement and an alternation between heterozoan and photozoan assemblages. Type C cycles were deposited in a middle to outer ramp setting and are characterized by a heterozoan assemblage and shallowing-upward trends that includes spiculitic wackestone–packstone passing up into neomorphic bioclastic wackestone or bioclastic crinoidal–bryozoan wackestone–packstone. All cycles are interpreted as 4th–5th order (104 to 105years duration) and some can be linked to Milankovitch orbital rhythms, notably short eccentricity (~100,000years). Overall, the origin of the Palmarito cycles was a complex combination of autocyclic and allocyclic controls; however, the evidence suggests that cycles in the lower part of the succession were dominated more by autocyclic processes whereas those in the upper part were more the result of allocyclic controls. Climate-driven processes, such as migration of the intertropical convergence one and the waxing and waning of polar ice-caps, had significant effects on deposition of the Palmarito succession and the cycles therein, controlling the clastic sediment supply, nutrient availability and thus carbonate production, and sea-level change, and these factors determined the vertical stacking pattern of the cycles.

Sedimentologic and diagenetic controls on pore-network characteristics of Oligocene–Miocene ramp carbonates (Majella Mountain, central Italy)

This article addresses the controls exerted by sedimentologic and diagenetic factors on the preservation and modification of pore-network characteristics (porosity, pore types, sizes, shapes, and distribution) of carbonates belonging to the Bolognano Formation. This formation, exposed at the Majella Mountain, Italy, is composed of Oligocene–Miocene carbonates deposited in middle- to outer-ramp settings. The carbonates consist of (1) grainstones predominantly composed of either larger benthic foraminifera, especially Lepidocyclina, or bryozoans; (2) grainstones to packstones with abundant echinoid plates and spines; and (3) marly wackestones to mudstones with planktonic foraminifera. The results of this field- and laboratory-based study are consistent with skeletal grain assemblages, grain sizes, sorting, and shapes, all representing the sedimentologic factors responsible for high values of connected primary macroporosity in grainstones deposited on the high-energy, middle to proximal outer ramp. Cementation, responsible for porosity reduction and overall macropore shape and distribution in grainstones to packstones deposited on the intermediate outer ramp, was mainly dependent on the following factors: (1) amount of echinoid plates and spines, (2) grain size, (3) grain sorting and shapes, and (4) clay amount. Differently, in the wackestones to mudstones, laid down on the low-energy, distal outer ramp, matrix is the key sedimentologic factor responsible for low values of scattered macroporosity and dominance of microporosity. The aforementioned results may be useful to improve the prediction of reservoir quality by means of mapping, simulating, and assessing individual carbonate facies with peculiar pore-network characteristics.