Widespread Carbonate Platform Research Articles

Algal microfacies in the Theniet Et Temar Formation (middle Oxfordian), Central Saharan Atlas, Algeria

During the Oxfordian, sea-level rise created widespread carbonate platform deposition on the margins of Neotethys, although some areas remain poorly described. One such area is the Saharan Atlas of Algeria. Accordingly, the middle Oxfordian Theniet Et-Temar Formation at a new locality, Kef El Meleh, is described for the first time. Formed of marl/limestone cycles, 6 broad microfacies are present within the limestones, representing deposition in lagoon, shoal and nearshore environments. Whilst overall shallowing-up is evident, it is unclear if the smaller scale marl–carbonate cycles represent higher frequency shallowing-up, or allocyclic depositional patterns.The Theniet Et-Temar Formation contains an important archive of macrofossils and microfossils. Of note at Kef El Meleh is the presence of a microfacies rich in dasycladalean algae, notably Salpingoporella annulata Carozzi, 1953. The specimens conform well to the type and subsequent descriptions. The assemblage is effectively monospecific in terms of algae, although the larger foraminifera Alveosepta jaccardi (Schrodt, 1894) is present. This record extends the knowledge of the distribution of S. annulata within Neotethys, especially within the Oxfordian, for which previous records are limited in comparison to those for the later Jurassic and Early Cretaceous.

The Road River Group of northern Yukon, Canada: early Paleozoic deep-water sedimentation within the Great American Carbonate Bank

Cambrian–Devonian sedimentary rocks of the northern Canadian Cordillera record both the establishment and demise of the Great American Carbonate Bank, a widespread carbonate platform system that fringed the ancestral continental margins of North America (Laurentia). Here, we present a new examination of the deep-water Road River Group of the Richardson Mountains, Yukon, Canada, which was deposited in an intra-platformal embayment or seaway within the Great American Carbonate Bank called the Richardson trough. Eleven detailed stratigraphic sections through the Road River Group along the upper canyon of the Peel River are compiled and integrated with geological mapping, facies analysis, carbonate and organic carbon isotope chemostratigraphy, and new biostratigraphic results to formalize four new formations within the type area of the Richardson Mountains (Cronin, Mount Hare, Tetlit, and Vittrekwa). We recognize nine mixed carbonate and siliciclastic deep-water facies associations in the Road River Group and propose these strata were deposited in basin-floor to slope environments. New biostratigraphic data suggest the Road River Group spans the late Cambrian (Furongian) – Middle Devonian (Eifelian), and new chemostratigraphic data record multiple global carbon isotopic events, including the late Cambrian Steptoean positive carbon isotope excursion, the Late Ordovician Guttenberg excursion, the Silurian Aeronian, Valgu, Mulde (mid-Homerian), Ireviken (early Sheinwoodian), and Lau excursions, and the Early Devonian Klonk excursion. Together, these new data not only help clarify nomenclatural debate centered around the Road River Group, but also provide critical new sedimentological, biostratigraphic, and isotopic data for these widely distributed rocks of the northern Canadian Cordillera.

Deciphering the geodynamic evolution of the Dinaric orogen through the study of the ‘overstepping’ Cretaceous successions

AbstractAlong the Dinaric–Hellenic orogen, the Late Jurassic – Early Cretaceous ophiolite obduction over the Adria continental margin was sealed by sedimentation of clastic terrestrial deposits rapidly followed by a widespread carbonate platform system since the Early Cretaceous period. These Cretaceous sediments presently crop out over areas of varying extension, from several hundred kilometre wide undeformed continuous covers to small-scale tectonic slivers involved in the tectonic stack following the latest Cretaceous–Palaeogene collision. These deposits are unconformably sedimented above the units formed by the Late Jurassic to Early Cretaceous nappe stacking above the eastern Adria continental margin. We studied these deposits in a large area between western Serbia and eastern Bosnia. In the studied area, these deposits are divided into three lithostratigraphic groups according to their age, depositional environment and type of underlying basement. The Mokra Gora Group sediments (upper Aptian–Maastrichtian) were deposited on top of previously obducted and weathered ophiolites, the Kosjerić Group (Cenomanian–Campanian) overlies composite tectonic units comprising obducted ophiolites and their underlying continental basement portions, while the Guča Group (Campanian–Maastrichtian) exclusively rests on top of continental basement. The reconstructed sedimentary evolution of these groups, together with the comparison with the syn- and post-obduction deposits at the front of the ophiolitic nappe(s) in a wider area of the internal Dinarides (e.g. Pogari Group and Bosnian flysch), allowed us to clarify the obduction mechanisms, including their tectonic context, the changes in depositional environments and the timing of depositional and tectonic events, and, in a wider view, shed light on the geodynamic evolution of the Dinaric belt.

Stratal architecture of a halokinetically controlled patch reef complex and implications for reservoir quality: A case study from the Aptian James Limestone in the Fairway Field, East Texas Basin

Widespread carbonate platform growth in the Cretaceous Tethys is associated with expansive reef margin and interior patch reef development. These reefs and their associated facies have the potential to be significant hydrocarbon reservoirs, but characterization is challenging because of complex facies distributions, inadequate understanding of biotic communities and processes impacting their development, and difficulties delineating timelines in systems that build their own topography. Other factors that may impact underlying topography, such as tectonics or halokinesis, further complicate the story. This study delves into these complexities in a dataset from the Fairway Field, a large oil and gas field (115 km2, 410 million barrels original oil-in-place) that produces from the Aptian James Limestone in the East Texas Basin, Texas, USA. The field contains a patch reef complex, here defined as an assemblage of patch reefs that grew near each other without necessarily being physically connected, that has significant lateral and vertical facies heterogeneity. Previous attempts at characterization have failed to fully recognize the importance of its proximity to four salt domes, which modified topography and influenced deposition prior to and during reef development. Here, we reassess the stratal architecture of the Fairway Field from a sequence stratigraphic perspective, with the goal of augmenting understanding of timing of reef development and facies distributions and ultimately building a better stratigraphic framework for a halokinetically influenced patch reef complex. Core descriptions and core-calibrated wireline-log correlation across the field yield a model that explains the relationship between halokinetic topography, energy regime, relative water depth, and reef- and reef-associated facies development. Porosity and permeability measurements taken across the field are analyzed to determine best reservoir facies and their distribution. The new stratigraphic model is useful for predicting facies architecture in other analogous reef complexes, such as those elsewhere in the Gulf of Mexico and in the Middle East; its tie to reservoir facies distributions also improves its utility for exploration purposes. Furthermore, this work demonstrates that halokinesis has a major impact on carbonate development and necessitates a new generalized model for halokinetically altered carbonate platforms.

Seismic stratigraphy and tectonic structure from a composite multi-channel seismic profile across the entire Dangerous Grounds, South China Sea

We interpret a more than 500km long composite multi-channel seismic reflection profile across the entire Dangerous Grounds, the South China Sea. Five tectonostratigraphic units are determined, together with seven sequence boundaries. Detailed analysis of extensional features, based on measurements of fault heaves, revealed two major phases of extension, separated by a distinct unconformity, which likely correspond with the beginning of sea-floor spreading in the South China Sea. Early extension occurred during continental rifting (Late Cretaceous–Early Oligocene), and resulted in formation of half-grabens and rotated blocks, controlled by a deeply rooted detachment system. Extension continued in our study area during the drifting phase of the East Subbasin of the South China Sea (Late Oligocene–Early Miocene) until the subsequent opening of the Southwest Subbasin at about 25Ma, but its intensity decreased markedly thereafter. Deeply rooted detachment systems evolved possibly during this second phase of extension at the continent–ocean transition area. From a coincidence of the lowermost reflections and Moho depth as derived from gravity inversion we tentatively interpret the flattened part of this detachment system as the crust–mantle boundary. It is suggested that the Dangerous Grounds reflect a magma-poor rift system at the initial stage of mantle unroofing. We suggest that a widespread carbonate platform developed across the Dangerous Grounds, concurrent with the period of seafloor spreading in the Southwest Subbasin of the South China Sea. Fault-related stretching factors (βf) along the line show discrepancies with the whole crustal stretching factors (βc). The continental crust of the Dangerous Grounds may have experienced nonuniform thinning.

Seismic Stratigraphy, Tectonic Structure and Extension Factors Across the Dangerous Grounds: Evidence from Two Regional Multi‐Channel Seismic Profiles

AbstractTwo recently acquired regional multi‐channel seismic profiles across the Dangerous Grounds, the South China Sea (SCS), are interpreted. One line crosses the entire Dangerous Grounds from the deep oceanic Southwest Subbasin of the South China Sea, along the Spratly Islands to close to Borneo, and the other one runs east of the Reed Bank. Based on the interpretations of these profiles, we worked out the stratigraphic sequences, tectonic structures and extension factors. Five tectonic‐stratigraphic units are determined, together with 7 sequence boundaries. Detailed analyses on the extension factors based on the measurement of fault heaves reveal two episodes of continent extension separated by a distinct unconformity. The main extension occurred during the continental rifting (Early Tertiary), and resulted in the formation of half‐grabens and rotated blocks, controlled by deeply rooted detachment system. During the second extension phase (drifting period, Late Oligocene‐Early Miocene) intensity reduced sharply and focused on the continental‐oceanic transition area. A widespread carbonate platform developed across the Dangerous Grounds concurrent with the spreading period of the SCS. Fault‐related stretching factors (βf ) to the west and east of Reed Bank are found to be comparable and show discrepancy with the whole crust stretching factors (βc) in the study area as derived from gravity modeling. Thus we conclude that the continental crust of the Dangerous Grounds experienced depth‐dependent extension.

Stratigraphy and Larger Foraminifera of the Eocene Shallow-marine and Olistostromal Units of the Southern Part of the Thrace Basin, NW Turkey

The Eocene marine sequence in the southern part of the Thrace Basin (NW Turkey) involves a variety of platform and deep-marine olistostromal units, the stratigraphy of which have been vigorously debated in the past. A detailed analysis of larger foraminifera in these either foraminifera or foraminifera-coral-coralline algae-dominated platform and associated comparatively deeper-marine units permits us to establish a high-resolution biostratigraphy in the context of shallow benthic zonation (with SBZ zones) of Tethyan Paleogene. The oldest zone (SBZ 5, corresponding to the basal Ypresian) was observed only in olistoliths. An old erosional remnant of a transgressive shallow-marine to basinal sequence (Dişbudak series; late Ypresian−? middle Eocene) was recognized below the regionally most widespread carbonate platform unit, the Soğucak Formation. The Dişbudak sequence, previously considered to belong to the Soğucak Formation and formally introduced recently, contains larger foraminifera, such as orthophragmines, nummulitids and alveolinids in its shallow-marine package referred to SBZ 10 (late Ypresian). The Soğucak Formation, which often exhibits patch reef developments, contains a rich and diverse assemblage of orthophragmines (Discocyclina, Orbitoclypeus and Asterocyclina), nummulitids (reticulate and other Nummulites, Assilina, Operculina, Heterostegina and Spiroclypeus), and other benthic taxa (Silvestriella, Pellatispira, Chapmanina, Orbitolina, Linderina, Gyroidinella, Fabiania, Halkyardia, Eoannularia, Sphaerogypsina, Asterigerina, Planorbulina and Peneroplis). Their assemblages, referred to SBZ 15/16, 17, 18, 19 and 20 Zones, provide a precise tool for recording the history of marine events which resulted in the deposition of the Soğucak Formation during four main periods. Their spatial distribution, recorded as late Lutetian, early Bartonian, late Bartonian and Priabonian, suggests a marine transgression from WSW to ENE. The Çengelli flysch sequence overlying the Soğucak Formation in a limited area to the east of the Gelibolu Peninsula, contains benthic foraminifera, mainly from limestone olistoliths mostly derived from the Soğucak Formation, and also in the turbiditic strata. The assemblages in the olistoliths reveal the existence of various shallow marine limestone sequences ranging in age from late Bartonian to early Priabonian.

Tying platform drowning to perturbations of the global carbon cycle with a δ13COrg‐curve from the Valanginian of DSDP Site 416

The carbonate carbon isotope record of the Phanerozoic is marked by repeated high‐amplitude excursions towards more positive values. Although the interpretation of C‐isotope events remains controversial, they are regarded as a proxy of the global C‐cycle. Using δ13COrg‐measurements of samples from DSDP Site 416, it is shown that a classic example of platform drowning coincided with the onset of the mid‐Valanginian carbon‐cycle excursion. Widespread carbonate platform drowning would have contributed to the observed positive shift in the C‐isotope record. As choking of carbonate production was compensated by increased organic production, both processes favoured a shift in marine carbon partitioning from the oxidized to the reduced carbon reservoir. This would have resulted in an increased flux of CO2 from the atmosphere into the marine and sedimentary carbon reservoir.

Neoproterozoic carbonate sequences of the Una Group, Irecê Basin, Brazil: chemostratigraphy, age and correlations

The Una Group, Irecê Basin (Bahia state) is correlative with the better known Bambuí Group of the São Francisco Basin (Minas Gerais state); these basins are remnants of widespread carbonate platform that covers > 300 000 km 2 of the São Francisco Craton in Brazil. The Irecê area contains a well-controlled stratigraphic succession hosting economic phosphate deposits and sulfide mineralization. Una Group strata comprise a basal siliciclastic sequence, mostly composed of glacial diamictites (Bebedouro Formation), followed by a carbonate succession that is several hundred meters thick, the Salitre Formation. An angular unconformity separates the two units. The carbonates of the Salitre Formation are characterized by 87Sr/ 86Sr ratios ranging between 0.70745 and 0.71776, and δ 13C values ranging between −4.4 and +9.4‰ PDB. The carbonates also contain beds and nodules of barite and gypsum with highly positive δ 34S values (+25.2 to +32.8‰ CDT). The ‘best preserved’ carbonate samples selected on the basis of their Mn and Sr concentrations as well as their ratios, yield 87Sr/ 86Sr values of 0.70745–0.70765. Comparison of these values with the secular strontium isotopic curve for the Neoproterozoic seawater suggests a 600 (to 670) Ma age for the Salitre Formation and for its correlative units in the São Francisco Basin.

Correlation of Early Cretaceous carbon isotope stratigraphy and platform drowning events: a possible link?

The Early Cretaceous carbonate carbon isotope record is marked by three positive high-amplitude (> 1.5‰) excursions each covering time spans of more than 106 years. They are of late Valanginian-Hauterivian and early and late Aptian age. In a case study along a transect across the western Tethys Ocean we identified a coincidence between δ13C excursions, black shale formation, and widespread carbonate platform drowning events. We conclude that the δ13C excursions reflect a change in partitioning of carbon between the organic and carbonate carbon sinks which was triggered by climate induced ecological changes in Cretaceous pelagic and neritic environments. Episodes of intensified greenhouse climate conditions led to an increase in weathering, erosion and runoff rates and to elevated nutrient transfer rates from continents into oceans. The resulting increase in oceanic nutrient levels favoured marine phytoplankton production and black shale deposition while conditions for carbonate producing biotas became unfavourable. Partial choking of carbonate production along river influenced coasts resulted in widespread carbonate platform drowning during times of sea-level rise in the Valanginian and Aptian. Widespread contemporaneous black shale deposits and drowned carbonate platforms therefore reflect the contrasting response of the marine organic and carbonate carbon pumps to nutrient-enhanced phytoplankton productivity. The change in marine carbon partitioning is mirrored in a shift of the δ13C record towards more positive values. The transition to the δ13C peak values lasted up to several hundred thousand years. The peaks of the excursions, also covering a time span of up to several hundred thousand years, reflect a new stabilisation of the carbon partitioning between carbonate and organic carbon sinks. A renewed intensification of carbonate sedimentation under mesotrophic conditions was facilitated by stabilisation of the sea-level rise at a high level. Decreasing δ13C values record increasing carbonate carbon burial rates at constant or decreasing organic carbon accumulation rates. These changes contributed to the stabilisation of the marine carbon budget and the global carbon cycle up to millions of years after its initial perturbation.

Rift-related Devonian sedimentation and basin development in South China

During Devonian times South China lay to the north of the Palaeo-Tethyan ocean, the boundary being a passive continental margin. A shallow sea covered the southern parts of the continent while northern areas, forming the Huanan Landmass, were emergent. At the beginning of the Devonian most of South China was above sea level. Subsequent transgression from the south gave rise to an irregular coastline with the development of many fault-controlled gulfs. Further transgression led to the development of an epicontinental sea with reefs forming along the margins of the submerged gulfs and black shales deposited within them. By Emsian time a widespread carbonate platform was established, while anoxic deposition continued in the troughs. The marine transgression peaked in the Frasnian Stage. During Famennian time widespread regression occurred and much of South China became once more emergent. Peneplanation of the Huanan Landmass led to the partial infilling of many of the older fault-bounded depressions. Throughout the Devonian the local distribution of sediments was strongly controlled by NE-SW trending transtensional faults that bounded NW-SE trending normal faults. These structures continued to influence sedimentation in the Late Palaeozoic, the Mesozoic and possibly the Tertiary in the offshore Beibu Gulf Basin.