Evolution of depositional environment of the Pliensbachian Stage (Lower Jurassic) coal‐bearing series in the Qaidam Basin, NW China
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The Qaidam Basin is one of the major coal‐bearing basins in northeastern China with the coal seams mainly preserved in the Lower–Middle Jurassic. The Middle Jurassic sedimentary environment, sequence stratigraphy, paleoclimate, and coal accumulation in the Qaidam Basin have been extensively studied, while the sedimentary characteristics and depositional evolution of the Lower Jurassic coal‐bearing series are less well known. We systematically analysed the facies association, depositional system, basin evolution, and coal accumulation of the Early Jurassic Pliensbachian‐aged coal‐bearing series from the Dameigou section in the northern Qaidam Basin. Five lithological types have been identified, ranging from conglomerates, sandstones, siltstones, and mudstones to combustible organic rocks, which were further subdivided into 12 lithofacies. These lithofacies were grouped into four facies associations, representing the braided fluvial, meandering fluvial, lacustrine, and meandering fluvial‐lacustrine transition deposits, respectively. The Pliensbachian Stage mainly developed as a fluvial‐lacustrine depositional system and an evolution history from braided fluvial to meandering fluvial, then to lacustrine, and finally back to meandering fluvial was reconstructed. We suggest that climate change is the major controlling factor for depositional evolution during the Pliensbachian. The transition of the depositional environment from fluvial to lacustrine in the study area can be attributed to the rapid climatic change induced by volcanism at this interval.An integrated study of sediments was conducted to examine the facies architecture and depositional environment of the Cretaceous Pab Formation, Rakhi Gorge, and Suleiman Ranges, Pakistan. This research focused on analyzing architectural elements and facies, which are not commonly studied in sedimentary basins in Pakistan. To identify lithofacies, outcrop analysis and section measurement were performed. The identified lithofacies were then categorized based on their depositional characteristics and facies associations, with a total of nine types identified within a stratigraphic thickness of approximately 480 m. These facies were mainly indicative of high-energy environments, although the specifics varied by location. Sedimentary structures such as planar and trough crossbedding, lamination, nodularity, load-casts, and fossil traces were found within these facies, indicating high-energy environments with a few exceptions in calm environments. The identified facies were grouped into seven architectural elements according to their depositional environments: delta-dominated elements, including laminated shale sheet elements (LS), fine sandstone elements (SF), planar cross-bedded sandstone elements (SCp), trace sandstone elements (ST), and paleosol elements (Pa); and river-dominated elements, including trough cross-bedded sandstone elements (SCt), channel deposit elements (CH), and paleosol elements (Pa). These architectural elements, along with their vertical and lateral relationships, indicate a transitional fluvio-deltaic environment within the Pab Formation. In conclusion, by interpreting facies and architectural elements, it is possible to gain a better understanding of the depositional history of the formation and the distribution of reservoir units.
Outcrop
Trough (economics)
Lamination
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Abstract During the Rupelian–Chattian, the Qom Basin (northern seaway basin) was located between the Paratethys in the north and the southern Tethyan seaway in the south. The Oligocene deposits (Qom Formation) in the Qom Basin have been interpreted for a reconstruction of environmental conditions during deposition, as well as of the influence of local fault activities and global sea level changes expressed within the basin. We have also investigated connections between the Qom Basin and adjacent basins. Seven microfacies types have been distinguished in the former. These microfacies formed within three major depositional environments, i.e., restricted lagoon, open lagoon and open marine. Strata of the Qom Formation are suggested to have been formed in an open-shelf system. In addition, the deepening and shallowing patterns noted within the microfacies suggest the presence of three third-order sequences in the Bijegan area and two third-order depositional sequences and an incomplete depositional sequence in the Naragh area. Our analysis suggests that, during the Rupelian and Chattian stages, the depositional sequences of the Qom Basin were influenced primarily by local tectonics, while global sea level changes had a greater impact on the southern Tethyan seaway and Paratethys basins. The depositional basins of the Tethyan seaway (southern Tethyan seaway, Paratethys Basin and Qom Basin) were probably related during the Burdigalian to Langhian and early Serravallian.
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Deposition
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Paleogene
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Stratum
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Lateral profiling techniques have been utilised to define the three-dimensional fluvial architecture of the Fell Sandstone Group (Arundian-Holkerian) of the Northumberland Basin, UK; the Lee-type sandstones (Morrowan-Atokan) of the central Appalachian Basin, USA; the Mansfield and Brazil Formations (Morrowan-Atokan) of the Illinois Basin, USA; and the Anisian Hawkesbury Sandstone of the Sydney Basin, Australia. These strata are characterised by sandstones of braided fluvial origin. Individual fluvial channels are dominated by downstream accreting mesoforms and macroforms, interpreted to represent mid-channel and bank attached bars and dunes. Palaeocurrents are unimodal and of low variance. Evidence of low stage reworking is rare, indicating that the fluvial systems were perennial. Cross-stratified sandstones are interbedded with structureless sand bodies, which display three distinct geometric forms: Sms, Smc and Sme. The texture and composition of facies Sms, Smc and Sme are distinct from associated structured facies. Facies Sms forms erosively based sandsheets 250 m parallel and transverse to the flow. The upper surface is planar. Facies Smc forms elongate channels trending at high angles to the palaeoflow of fluvial channels. The sandbodies preserve a symmetrical cross-section with margins dipping 6 m thick, and may be traced >200 m parallel and transverse to flow direction. Amalgamation of the facies results in sandsheets >20m thick. Scours, elongate both parallel and oblique to fluvial flow are preserved along the basal surface. A classification scheme of massive sandstone facies has been developed. The facies are interpreted in terms of deposition from highly concentrated, laminar sediment/water flows. Sediment-laden currents were generated through primary and secondary mechanisms related to flooding and mass flow.
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A study of sediment dynamics has been conducted on the Tarakan sub-basin, North Kalimantan, Indonesia using multivariate analysis. Multivariate statistical techniques can be used to determine sediment with similar characteristics and be a good proxy to recognize sedimentary facies and depositional environment. These methods have been applied to characterize the sedimentary facies at Tarakan sub-basin. A total of 23 samples were taken from several locations on the different depths of 56–2554 m that represent varieties of environment. The study is interpreted using compositional data analysis associated with a grain size trend analysis (GSTA), principal component analysis (PCA), and cluster analysis (CA). The GSTA value showed the dominance of poorly sorted silt indicating that the sediment is mostly deposited in low-energy depositional. The CA and the PCA determined three distinct sedimentary facies: shelf facies, upper bathyal facies, and lower bathyal facies. The facies were grouped as similar sediment and depositional environment. Sedimentological variables applied in the characterization were described to be an important tool for the interpretation of depositional environments – indirectly showing hydrodynamic energy.
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On the basis of predecessors' work and having done a lot of field and laboratory work, the author will deal with the features of lithofacies palaeogeography of Tongzi Age in this paper. According to the features of vertical and horizontal distributions of biological associations and depositional rock associations, the lithofacies palaeogeography pattern of Tongzi Age in Guizhou and its adjacent areas can be divided into following depositional systems and facies:
Ⅰ. The depositional system of epicontinental clastic littoral: littoral fades and offshore facies.
Ⅱ. The depositional system of carbonate platforms: restricted marine platform facies, opening marine platform fades, semi-restricted marine platform facies and platform marginal interclastic beach facies.
Ⅲ. The depositional system of semi-deep sea and deep sea basins: semi-deep sea basin facies and deep sea basin facies.
This special pattern of lithofacies palaeogeography was gradually formed from early to late and from east to west under the sedimentary condition that marine space was expanded and old land was reduced.
Palaeogeography
Carbonate platform
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Primary depositional structures are important parameters that can provide the means of unraveling the processes and process responses in sedimentary depositional settings. This paper is aimed at determining the paleoenvironment of the Upper Bima Member in Fufore and environs using the primary depositional structures. This research was carried out in two phases which involved fieldwork that dealt with logging the outcrops sections and desk work whereby the logged sections and other field data are digitized, remodeled and analyzed using relevant computer programs. The result of the study shows that the Upper Bima Member is composed of five lithofacies succession; trough cross-bedded sandstone facies (St), planar cross-bedded sandstone facies (Sp), horizontal bedded sandstone facies (Sh), ripple cross-laminated sandstone facies (Sr), and mudstone facies (Fm). These lithofacies gave rise to two facies associations; sand dominated and fine-grained facies dominated facies association. The cross-stratified facies show an overall paleocurrent trend in the NW direction. These facies succession represent both shallow perennial and ephemeral sand-bed in a braided river depositional environment.
Paleocurrent
Trough (economics)
Outcrop
Lithology
Ephemeral key
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