Spatial and temporal development of siliceous basin and shallow-water carbonate sedimentation in Oxfordian Northern Calcareous Alps
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Quantitative logs of grain composition for several sections of platform top and flank of the Vercors carbonate platform (Early Cretaceous, SE France) provide platform‐to‐basin correlation with a resolution of a few metres over an area of 70 km 2 . Grain composition was determined by point‐counting thin sections. Point‐count groups that characterize palaeoenvironmental realms (i.e. open sea, platform margin) were defined for the platform–basin trajectory. Grain‐composition logs revealed marked peaks in the number of open‐sea biota and peaks in ooid abundance. The peaks in open‐sea biota correspond to back‐stepping intervals and deepening upward facies successions at the platform margin. These peaks probably relate to incipient drowning of the platform and may be used to delineate marine‐flooding surface‐bounded sequences. Peaks in ooid occurrence show no relationship with the progradation, aggradation or retreat of the platform. Apparently, the oolitic sands were not part of a facies tract that shifted up and down the platform. Instead, they represent a depositional mode that was either on or off. Times of prolific ooid production and shedding probably occurred during wide but shallow submergence of the platform, accompanied by suitable water chemistry. Peaks in both ooids and open‐sea biota are excellent markers for platform‐to‐basin correlation, as they are recorded in successions on the platform top as well as on the flank. Altogether, the grain‐composition logs show that each of the lithologically rather similar platform tongues of the Vercors has a unique signature or compositional fingerprint. These compositional fingerprints are most helpful in evaluating the lateral extent of different stratigraphic units. In outcrops of the Vercors platform, the physical tracing of bedding surfaces delineate wedges of toe‐of‐slope sediments that show a conspicuous thinning towards the platform. However, our correlation shows that these sediment bodies are not truly basin‐restricted wedges but have a platform top equivalent. This implies that these units were, at least partly, deposited during high stands of sea level that flooded the platform.
Ooid
Carbonate platform
Aggradation
Progradation
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The Silurian Shiniulan Formation in the south of the Shichuan Basin was deposited on the carbonate platform developed based on a flat shelf.Four lithologic segments and two sedimentary cycles can be identified in the formation.The depositional system tracts formed in the environments including,from south to north,clastic seashore,localizing platform,open platform,biogenic reef or beach,slope of platform edge,shallow shelf beach,argillaceous shallow shelf and deep-water shelf.Temporally,the whole sedimentary environment changed from argillaceous or calcite deep-water to shallow shelf and to carbonate platform.The most important factor of controlling the spatial and temporal evolution of depositional system is sea level fluctuation.
Carbonate platform
Palaeogeography
Lithology
Marine transgression
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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
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Diachronous
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Carbonate platform
Subaerial
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This work focuses on well‐exposed Lower Triassic sedimentary rocks in the area of Torrey (south‐central Utah, USA). The studied Smithian deposits record a large‐scale third‐order sea‐level cycle, which permits a detailed reconstruction of the evolution of depositional settings. During the middle Smithian, peritidal microbial limestones associated with a rather low‐diversity benthic fauna were deposited seaward of the tidal flat siliciclastic red beds. Associated with siliceous sponges, microbial limestones formed small m‐scale patch reefs. During the late middle to late Smithian interval, the sedimentary system is characterized by tidal flat dolostones of an interior platform, ooid‐bioclastic deposits of a tide‐dominated shoal complex, and mid‐shelf bioclastic limestones. Microbial deposits, corresponding to sparse stromatolites formed in the interior platform, are contemporaneous with a well‐diversified marine fauna living in a seaward shoal complex and mid‐shelf area. The nature and distribution of these Smithian microbial deposits are not related to any particular deleterious environmental condition, highlighting that observed patterns of biotic recovery after the end‐Permian mass extinction were directly influenced by depositional settings. Facies evolution and stratal stacking patterns allow us to identify large, medium and small‐scale, as well as elementary depositional sequences. Large‐ and medium‐scale sequences are consistent with sea‐level changes, whereas small‐scale and elementary sequences are better explained by autocyclic processes. Copyright © 2015 John Wiley & Sons, Ltd.
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Ooid
Siliciclastic
Early Triassic
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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
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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
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Sedimentation
Diachronous
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Microbial carbonate platforms are common in the Pre-Cambrian and most of the Phanerozoic, often hosting hydrocarbons, but since they are distinct from modern coral reefs or skeletal carbonate ramps, a standardised facies model for such microbial-dominated carbonate bodies is still missing. The reconstruction of the stratigraphic architecture of these types of depositional systems must often rely on investigation of outcrop analogues. The authors propose herein a 2 day field itinerary in which depositional features, depositional geometries and facies architecture of two Triassic microbial carbonate platforms of the Dolomites (Southern Alps, NE Italy) will be presented at key outcrops and discussed. The Dolomites host a variety of Triassic carbonate platforms of this type. They are characterised by high relief and steep slopes and have been studied as analogues to Palaeozoic and Mesozoic hydrocarbon reservoirs such as those of Central Asia, Central America, northern Russia or North America, with which they share significant similarities. In this guide, two excursions are described and focus on the upper Anisian (Middle Triassic) Latemar and the lower Carnian (Upper Triassic) Sella; two examples that are particularly suitable for directly appreciating and understanding architecture and facies characteristics of such microbial sedimentary units because of their limited size and excellent exposure. Easy accessibility makes it possible to examine several key features through a short visit, with only minor safety issues. The Latemar platform has escaped pervasive dolomitization, so that sedimentary facies are very well preserved. Furthermore, the Sella and the Latemar crop out perfectly, exposing the depositional geometries and their lateral facies relationships with coeval deep-water basins at seismic scale. The proposed field itinerary is aimed at geologists, either from industry or from academia, who have interest in microbial carbonate platforms, wish to learn sequence stratigraphy in the field and to look at outcrop analogues of carbonate depositional systems as seen in seismic surveys.
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This data report presents sedimentological data obtained from Site 1130 in the Great Australian Bight (southern Australia) during Leg 182, a setting that is dominated today by strong ocean currents, downwelling, and water temperatures rarely above 20°C. The purpose is to characterize lithofacies and cyclicity. The different lithofacies reflect different texture, grain composition, grain size, and sorting as seen in thin section. Cyclicity is shown by repetition of coarsening-upward wackestone to packstone packages with an upward increase in neritic components. The cyclicity is corroborated by grain counts, point counts, and X-ray diffraction mineralogy. The cyclicity is interrupted by the deposition of nannofossil-rich wackestones. These data can be used to more effectively interpret processes affecting cool-water carbonate margins. INTRODUCTION Studies in the Great Australian Bight (GAB) are revealing intriguing insights into the understanding of the sedimentology, paleoceanography, and paleoecology of cold-water carbonate environments (James, 1997; Li et al., 1996; Boreen and James, 1993; James and von der Borch, 1991; James and Bone, 1994; Feary, Hine, Malone, et al., 2000). The GAB forms a prominent reentrant in the southern margin of the Austra1Simo, J.A., and Slatter, N.M., 2002. Data report: Sedimentology of a Pleistocene middle slope cool-water carbonate platform, Great Australian Bight, ODP Leg 182. In Hine, A.C., Feary, D.A., and Malone, M.J. (Eds.), Proc. ODP, Sci. Results, 182, 1–15 [Online]. Available from World Wide Web: . [Cited YYYYMM-DD] 2Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison WI 53706, USA. Correspondence author: simo@Geology.wisc.edu Initial receipt: 2 November 2001 Acceptance: 2 July 2002 Web publication: 6 September 2002 Ms 182SR-016 J.A. SIMO AND N.M. SLATTER DATA REPORT: SEDIMENTOLOGY OF A COOL-WATER CARBONATE PLATFORM 2 lian continent and is located between 123° and 134°E longitude and 32° and 37°S latitude (Feary and James, 1998). Modern sediments on the shelf are a mixture of relict calcareous grains and Holocene skeletal grains and are affected by seasonal downwelling, upwelling, and longperiod waves and swells (James et al., 2001). During Ocean Drilling Program (ODP) Leg 182 (Feary, Hine, Malone, et al., 2000), we drilled a thick (~550 m) Pleistocene slope succession in the GAB (Fig. F1). The drilling has extended previous shelf observations onto the slope and basin as well as provided a temporal framework to understand the margin evolution and processes. This study focuses on the thick middle–upper Pleistocene slope sequence recovered from Site 1130 drilled at a water depths of 488 m, an upper bathyal setting (Fig. F1). The site is located on the Eyre Terrace, a region dominated by coastal downwelling during most of the year and oligotrophic waters (James et al., 2001). Landward of Site 1130 are bryozoan mounds (Site 1132) and seaward are pelagic oozes (Sites 1126 and 1134) (Feary, Hine, Malone, et al., 2000). Thus, sediments at Site 1130 reflect the mixing of upper-slope and shelf-derived sediments and those derived from the water column and midslope. The goal of this report is to investigate the interaction between shelf and slope processes based on a high-resolution study of the sedimentological and faunal trends at Site 1130. PLEISTOCENE SEDIMENTS AND SITE 1130 Pleistocene sediments recovered during Leg 182 correspond to spiculitic skeletal packstone and fine-grained spiculitic foraminifer wackestone. The section is punctuated by thin intervals of nannofossil ooze. One of these intervals is the objective of this report. The color of the sediments ranges from a buff light gray to a pale olive-green. They represent continuous sedimentation at rates that sometimes exceeded 40 cm/k.y., which is equivalent to many shallow-water tropical carbonates (Eberli, Swart, Malone, et al., 1997; Feary, Hine, Malone, et al., 2000). Site 1130 intersected an almost complete Pleistocene succession with some exceptions (Feary, Hine, Malone, et al., 2000). Sedimentation rates were as high as 24–26 cm/k.y. in the middle and late Pleistocene, but much slower, 1.5–2 cm/k.y., in the early Pleistocene. Physical properties suggest strong cyclicity on a 100-k.y. frequency from 43 to 175 meters below seafloor (mbsf) and a higher 41-k.y. frequency between 175 and 254 mbsf (Feary, Hine, Malone, et al., 2000). An interval containing a cyclic succession of packstone and finegrained packstone-wackestone interrupted by a thin nannofossil ooze was selected for this study. The hypothesis is that the cyclicity resulted from changes in shelf processes and the deposition of nannofossil ooze is the outcome of a short-lived major reorganization of the slope and shelf processes. The studied interval (~123–151 mbsf) corresponds to the transition between Subunits IA and IB described on board (Feary, Hine, Malone, et al., 2000). These two subunits contain cyclic bioclastic packstones and wackestones and are separated by the white nannofossil ooze with bioclasts (~133.6 mbsf), which is the object of this report. The interval is part of the expanded middle–upper Pleistocene succession showing cyclicity in the color reflectance (700–400 range) and in the natural gamma ray (Feary, Hine, Malone, et al., 2000). The age of the studied interval is around the boundaries between the NN19 and NN21–NN20 nannofossil zones and the PT1b and PT1a planktonic fora4000 300
Ooid
Micropaleontology
paleoceanography
Wackestone
Carbonate platform
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