A depositional model for offshore deposits of the lower Blue Gate Member, Mancos Shale, Uinta Basin, Utah, USA
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Abstract Depositional models that use heterogeneity in mud‐dominated successions to distinguish and diagnose environments within the offshore realm are still in their infancy, despite significant recent advances in understanding the complex and dynamic processes of mud deposition. Six cored intervals of the main body of the Mancos Shale, the lower Blue Gate Member, Uinta Basin, were examined sedimentologically, stratigraphically and geochemically in order to evaluate facies heterogeneity and depositional mechanisms. Unique sedimentological and geochemical features are used to identify three offshore environments of deposition: the prodelta, the mudbelt and the sediment‐starved shelf. Prodelta deposits consist of interlaminated siltstone and sandstone and exhibit variable and stressed trace fossil assemblages, and indicators of high sedimentation rates. The prodelta was dominated by river‐fed hyperpycnal flow. Mudbelt deposits consist of interlaminated siltstone and sandstone and are characterized by higher bioturbation indices and more diverse trace fossil assemblages. Ripples, scours, truncations and normally graded laminations are abundant in prodelta and mudbelt deposits indicating dynamic current conditions. Mudbelt sediment dispersal was achieved by both combined flow above storm wave base and current‐enhanced and wave‐enhanced sediment gravity flows below storm wave base. Sediment‐starved shelf deposits are dominantly siltstone to claystone with the highest calcite and organic content. Bioturbation is limited to absent. Sediment‐starved shelf deposits were the result of a combination of shelfal currents and hypopycnal settling of sediment. Despite representing the smallest volume, sediment‐starved shelf deposits are the most prospective for shale hydrocarbon resource development, due to elevated organic and carbonate content. Sediment‐starved shelf deposits are found in either retrogradational to aggradational parasequence sets or early distal aggradational to progradational parasequence sets, bounding the maximum flooding surface. An improved framework classification of offshore mudstone depositional processes based on diagnostic sedimentary criteria advances our predictive ability in complex and dynamic mud‐dominated environments and informs resource prospectivity.Keywords:
Siltstone
Bioturbation
Deposition
Paleocurrent
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Comparison of sedimentary structures in certain sedimentary rocks with those that form in the modern environment can provide a powerful interpretive tool. An example can be drawn from a well-exposed middle Miocene marine-nonmarine transition in the southeastern Caliente Range, California. Within this transition, shallow-marine sandstone (part of Branch Canyon Sandstone) intertongues on the northwest with marine siltstone (Saltos Shale Member of the Monterey Formation), and intertongues on the southeast with nonmarine redbeds (part of the Caliente Formation). Flow structure in extrusive basalts in the upper part of the transition and paleocurrent features in overlying Miocene alluvial deposits indicate that the shoreline at the time of deposition trended north-northwest. > The marine-nonmarine transition consists of a succession of individual progradational sequences that extend westward (seaward) into marine strata. The sequences exhibit a fairly consistent internal stratigraphic arrangement. A typical complete sequence has a basal unit of unbedded siltstone (Saltos Shale) lying on an erosion surface. A thin zone of conglomerate commonly occurs within the siltstone directly above the lower contact. The siltstone grades up into bedded or unbedded fine-grained sandstone (Branch Canyon Sandstone). This fine sediment is sharply overlain by a coarse, pebbly, crossbedded facies of the Branch Canyon, which grades upward into finer and predominantly planar-bedded sandstone. The planar-bedded sandstone grades up into muddy structureless sandstone that in turn g ades up into red or green mudstone (Caliente Formation), which caps the sequence. The progradational nature of the sequences implies that the fine-grained sandstone near the base was deposited in the marine environment somewhat shoreward from the gradationally underlying siltstone, which locally contains marine invertebrates. Bedding, where present in the fine sandstone, is defined by concentrations of biotite; the bedding is either planar or shows medium- and small-scale cross-stratification. Bioturbational disruption abounds. Crossbedded lenses of well-sorted granular sand are interbedded with the fine sand in its upper part. Foresets in these lenses dip predominantly toward the southeast. Similar structures form in response to the passage of waves in modern high-energy environments. The foreset orientation thus suggests that during the time of deposition the wav s approached from the northwest. The coarse, pebbly sandstone that sharply overlies the fine-grained sandstone is the thickest and best exposed unit of most sequences. Bedding is generally well developed, and bioturbational structures are rare. Pebbles tend either to occur as lag deposits scattered along extensive erosion surfaces within the sandstone or to be concentrated within conglomeratic beds. Crossbedding is abundant and dips predominantly west-southwest (offshore); a small secondary mode dips south-southeast. This secondary mode may reflect the influence of longshore currents resulting from the oblique approach of the waves with respect to the shoreline. A few large-scale crossbedding units dip east and southeast and suggest the presence of offshore bars trending oblique to the coast, parallel with the prevai ing wave crests. The dominantly offshore-dipping crossbeds are best explained as the result of rip currents. The erosional contact at the base of the coarse sandstone resembles the contact formed at the base of rip channels in the modern environment. The planar-bedded sandstone in the upper part of the sequence is well sorted and contains planar concentrations of magnetite that resemble those formed on the upper foreshore of modern beaches. Many planar laminations are inversely graded like those that form at present in the upper swash zone. Where attitudes were measured, most of the planar beds dip gently seaward (west), which also supports a beach origin. End_Page 431------------------------------ The overlying structureless muddy sandstone may represent deposition in a vegetated back-beach environment. The redbeds of the Caliente Formation probably formed in coastal swamps, lagoons, or alluvial plains. End_of_Article - Last_Page 432------------
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Based on detailed analysis of cores,logs,and seismic data,ten lithofacies were indentified,including mega-thick and thick massive sandstones,thick normal-graded sandstones,thick inverse-graded sandstones,parallel and tabular cross-stratified sandstones,slumped and deformed sandstones,thin sandstone interlayers,thin mudstone-sandstone interbeds,thick siltstones,thick mudstones,and deep-water limestones.They can be organized into five lithofacies associations.These are sandstone-dominated,mudstone-dominated,mudstone-sandstone interbed,siltstone and deep-water limestone associations.Based on the analyses of lithofacies and their associations,the depositional system of the reservoir in the Zhujiang Formation,Liwan 3-1 field is identified as submarine fan.Four microfacies are further recognized within the fan based on detailed analysis,including channels(including main channels in the inner fan and distributary channels in the middle fan),inter-channel,levee-overbank,outer fan and basin microfacies.Facies analysis allows to reconstruct the depositional model of the deepwater fan.During relative sea-level fall,a basin-floor fan formed as sediments from rivers or shelf margin deltas were further transported down slope and into the basin under gravity flows.During the ensuing relative sea-level rise,a slope fan was deposited as the system stepped back.The lower part of the Zhujiang Formation,mainly sandstone-dominated associations,is inner fan and middle-outer fan deposits of the basin-floor and the slope fan respectively.The upper part,mainly mudstone-dominated associations,siltstone associations,and deepwater associations are outer fan deposits of the slope fan and basin deposits.Along with the transgression,submarine fan was finally replaced by pelagic deposits.
Siltstone
Overbank
Marine transgression
Transgressive
Slumping
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This study focused on the geological mapping of Kg Temiang, Kuala Krai, Kelantan, facies depositional environment and geological history and evolution of that place. Facies analysis shows the finer grained of sedimentary rock with a few of sediment structure such as massive, lamina, and cross-lamination. This place shows a shallow shoreface environment. However, the geomorphology process usually because of faulting, crack and bedding. In addition, it also because of climate change that control weathering and erosional process. Kg Temiang contain one type of rock which is sedimentary rock. The lithology of the area are shale, sandstone, mudstone, limestone and siltstone. They are included in Mesozoic era.
Siltstone
Lithology
Bedding
Lamination
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Jurassic is the main source rocks of Qaidam Basin,distributed mainly in the northern margin of the basin.Based on the comprehensive studies of marker beds,lithological features and sedimentary characteristics,the alluvial fan facies,braided river facies,fan delta facies,braided river delta facies and lacustrine facies are distinguished in Jurassic deposits in the main sections exposed.Both the facies belt distribution and paleogeographic evolution are closely related to the regional tectonic movement.There are two evolution stages that include the faulted lake basin sedimentary stage from Early to Middle Jurassic and the depression lake basin sedimentary stage during Late Jurassic,according to the stratigraphic contact relations and sedimentary evolution.
Alluvial fan
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By using the technique of sedimentary facies analysis and sequence stratigraphy theory,the paper systematically studies the depositional characteristics and sequence surface of Jurassic in Ordos Basin and summarizes that there have seven sequence surface types of material expression.According to the depositional characteristics,assemblage and sequence surface of Jurassic,there exist 2 super-long,9 long and 19 middle base-level cycles of Jurassic in Ordos Basin.Based on the research of the sedimentary sequence characters and depositional system,there develop alluvial fan,river,lake-delta,alluvial plain and drift bed during Jurassic stage.Finally,the paper inquires further out the coupling relationship of sequence-sedimentation-structure in Ordos Basin,and points out that the basin underwent 4 great tectonic fluctuation cycles from Yan'an stage to Fuxian stage,corresponding to 4 long base-level cycle(LSC1~LSC4),and 5 huge cycles during Zhiluo stage to Fenfanghe stage,corresponding to 5 long base-level cycle(LSC5~LSC9).
Sequence (biology)
Sequence Stratigraphy
Alluvial fan
Sedimentation
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