RETRACTED ARTICLE: Characteristics and environmental significance of late carboniferous brachiopods within the Qijiagou Area, Southern Margin of the Junggar Basin
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The Qijiagou section on the southern margin of the Junggar Basin preserves thick and extensive Carboniferous carbonate rocks, and is the type section of the Upper Carboniferous in the Junggar Basin. Here, we studied the lithological assemblages and brachiopod fossils of the Upper Carboniferous Qijiagou Formation on the southern margin of the Junggar Basin. Outcrop observations indicate laterally variable thicknesses of this unit, comprising thick-bedded, fossiliferous, channelised, sandy, and calcareous limestone. Petrographic observations revealed four microfacies, indicating deposition of the formation in diverse depositional environments, including lagoonal to delta-dominated middle shelf settings. In addition, we identified 15 species of brachiopod fossils in 11 genera which belong to Spiriferida Waagen, Productida Sarycheva et Sokolskaya, and Strophomenida Öpik. Thus, based on the characteristics of the lithological assemblage and brachiopod fossils, we infer that the Qijiagou area had a relatively warm environment in the middle and late Late Carboniferous. This included a turbulent and shallow water body with two regressions which occurred in a carbonate platform sedimentary environment.Keywords:
Outcrop
Carbonate platform
The slope carbonate facies of the Lower Callovian and Kimmeridgian of the Majorca Island include mounded shape bioclastic and well-bedded laminated units with dominant pelagic components, which growth inthe toe of the carbonate slope. Carbonate slope facies are mainly dominated by carbonate debris sheets with slope and shallow platform components, mainly oolites, in the Lower Callovian and by aggradational mudstones with slumps in the Kimmeridgian.
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Aggradation
Marine transgression
Debris flow
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Outcrop
Characterization
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Understanding the impact of various depositional controls on the growth of carbonate platforms may help in predicting the continuity and spatial distribution of petroleum reservoir fades within ancient platform successions. This paper presents preliminary results from a 3D model for simulating the growth of carbonate platforms, focusing especially on the behavior of platform-margin fades tracts. The 3D model, described here for the first time, was used to investigate the impact of changing sea level on the growth of carbonate platforms, using a range of initial seafloor gradients. The initial depositional surface used in the model features an along-strike transition from gently dipping to more steeply dipping profiles. Simulations were performed to investigate different aspects of platform growth in response to a single cycle of sea-level change (200-meter amplitude, 200,000 yr cycle duration). Using low (∼1°) initial gradients, the model produces a complex depositional unit that consists of several detached platform-margin ‘terraces,’ each of which has a relatively low-relief final profile. Following the full cycle of sea-level change, the final depositional unit contains highly diachronous fades boundaries with significant fades dislocations. In contrast, using the same sea-level oscillation, but a steeper (∼16°) initial depositional gradient, the model creates a narrower platform, with a terminal depositional profile that is steeper overall. Internal chronostratigraphic relationships within the final depositional unit are also complex, although fades dislocations are more areally limited than in the low-gradient example.
Seafloor Spreading
Carbonate platform
Sedimentation
Diachronous
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The well-exposed Djebel Bou Dahar (DBD) carbonate platform (Lower Jurassic, High Atlas, Morocco) demonstrates the role played by different carbonate factories on the growth and architectural evolution of a high-relief, flat-topped carbonate depositional system. It also shows, in contrast with the generally accepted idea that lithiotid bivalve accumulations dominated Lower Jurassic platform margins, that microbial carbonates substantially contributed to the carbonate factory, as in Upper Jurassic reefs.
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In onshore East Sabah, Malaysia, deepwater outcrops can be tied to offshore subsurface deepwater deposits just few tens of miles to the north. This will provide a unique opportunity to obtain a better picture of depositional processes that controlled the distribution of hydrocarbon reservoir sands, as well as seal and source rock distribution, and possible migration fairways in the subsurface. Some previous investigations on the basin-floor fan complex of the Crocker Formation provided insight into the depositional nature and sedimentary characteristics of these deposits. However, the present study is intended to provide an integrated approach of studying the deepwater deposits of Northwest Sabah and tie it to subsurface data in order to get a clearer picture of the depositional systems within a sequence stratigraphic framework and to unravel the paleogeographic and chronostratigraphic history of these deposits. The large areal and vertical extents of these deposits is expected to give the chance to better understand the depositional history and, in turn, help create a relatively accurate lead and prospect inventory in the subsurface by providing an analog from surface data. This would be possible by analyzing and interpreting available well logs, cores, and seismic data and integrating them with detailed outcrop sedimentological and biostratigraphic interpretations using a sequence stratigraphic approach.
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Hydrocarbon exploration
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Sequence (biology)
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A Jurassic marginal depositional system of the Adriatic carbonate platform was analyzed in order to determine its depositional architecture and major depositional controls. Based on their facies characteristics, seven lithofacies units have been distinguished, which constitute four paleoenvironmental associations: top of the platform (shallow subtidal below and above the fair-weather wave-base), upper foreslope, toe-of-slope and basin. The environmental changes are interpreted to be related to tectonic activity as a consequence of regional extensional movements, connected with the opening of the Dinaridic branch of the Neo-Tethys. These extensional movements resulted in multi-stage drowning of the northeastern part of the Adriatic carbonate platform, leading to its gradual back-stepping and accordingly the expansion of the pelagic basin. The interpretation presented here can serve as a useful model for re-evaluating previously analyzed sections of the Adriatic Carbonate Platform margin.
Extensional tectonics
Carbonate platform
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A 3D Model for Carbonate Platform Sedimentation: Response to Sea-Level Change and Seafloor Gradients
Abstract Understanding the impact of various depositional controls on the growth of carbonate platforms may help in predicting the continuity and spatial distribution of petroleum reservoir facies within ancient platform successions. This paper presents preliminary results from a 3D model for simulating the growth of carbonate platforms, focusing especially on the behavior of platform-margin facies tracts. The 3D model, described here for the first time, was used to investigate the impact of changing sea level on the growth of carbonate platforms, using a range of initial seafloor gradients. The initial depositional surface used in the model features an along-strike transition from gently dipping to more steeply dipping profiles. Simulations were performed to investigate different aspects of platform growth in response to a single cycle of sea-level change (200-meter amplitude, 200,000 yr cycle duration). Using low (?1°) initial gradients, the model produces a complex depositional unit that consists of several detached platform-margin 'terraces,' each of which has a relatively lowrelief final profile. Following the full cycle of sea-level change, the final depositional unit contains highly diachronous facies boundaries with significant facies dislocations. In contrast, using the same sea-level oscillation, but a steeper (?16°) initial depositional gradient, the model creates a narrower platform, with a terminal depositional profile that is steeper overall. Internal chronostratigraphic relationships within the final depositional unit are also complex, although facies dislocations are more areally limited than in the lowgradient example. Introduction Many of the world's major oil and gas fields are found in carbonate platform strata (e.g., the Middle East and the Permian Basin of West Texas). Within these carbonate platform successions, platform-margin facies commonly constitute some of the best reservoir facies so it is important to understand the factors that control the stratigraphic evolution of these facies tracts. Platform-margin facies develop within a relatively narrow range of water depths so the combined effects of variable seafloor gradients and changes in sea level are critical for understanding the depositional history of these facies. Although investigation and characterization of natural carbonate platform systems is important for understanding the distribution of lithofacies and the internal stratal architecture within each platform, computer models can provide insight into the relative influence of various depositional processes and environmental conditions on carbonate sedimentation. The main objective of this study was to test a preliminary 3D depositional model for carbonate platforms. Here we report preliminary model results that focused on the relative influence of variable seafloor gradients on platform development. Fig. 1 General flow diagram of the 3D carbonate depositional model showing the complex interrelationship of direct and indirect processes controlling accumulation of carbonate sediments. (Available in full paper) General description of the model Our 3D model is based on a geometrical representation where a reference Cartesian coordinate system is set up such that both the most transgressive strandline position and the most basinward extent of carbonate facies lie within the initial model limits, over the entire duration of the model run.
Seafloor Spreading
Carbonate platform
Sedimentation
Diachronous
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