Search

The Paleogene Shahejie and Dongying formations of the eastern Bohai Bay Basin contain >500 m fluvial-deltaic deposits and >600 m deltaic-lacustrine deposits, both of which are important for oil and gas exploration. This study aims to figure out the deposition system distribution within the sequence stratigraphy and predict the exploration target. Sequence boundaries are recognized by unconformities in the seismic profiles and abrupt changes in the well-logging curves (acoustic, spontaneous potential, natural gamma-ray and resistivity), and two second-order and five third-order sequences (SQs1, SQs3 in the Shahejie Formation and SQd1-SQd3 in Dongying Formation) are identified. Four types of depositional system and distribution are determined by well-logging profiles, cores, relative sand content and root-mean-square seismic attributes. The SQs3 sequence is predominantly fan deltas and lacustrine deposits, the SQs1 sequence is predominantly shallow lacustrine deposits, the SQd3 sequence is predominantly braided-river delta, semi-deep lacustrine, and turbidite fan, and the SQd2 sequence is predominantly braided-river delta deposits, whereas the SQd1 sequence is predominantly braided-river deposits. The controlling factors are determined by basin subsidence simulation and fossil data of biological palynology, indicating that tectonic activity is the main controlling factor in SQs3 and SQd3 while climate is the main controlling factor in the SQs1, SQd2 and SQd1 sequences. Combined with the physical properties of sandstone, it is concluded that the beach bar in the SQs1 sequence, turbidite fan in the SQs3 and SQd3 sequences, and thick sandstone in the SQd2 sequence are the important exploration targets. [ABSTRACT FROM AUTHOR]

Hyperpycnites are extrabasinal turbidites related to hyperpycnal flows which form during river floods. Compared to intrabasinal turbidite systems in deep marine or lacustrine settings, hyperpycnites in shallow lakes settings are not understood. In this work, we studied the typical sedimentary characteristics of shallow lacustrine hyperpycnal flow deposits in outcrops of the Luanping Basin, Northeast China. Based on the sedimentary architecture and analysis of the facies associations in these outcrops, the hyperpycnites can be classified into the following three types: (a) bedload‐dominated erosional channel deposits, (b) bedload‐ and suspended‐load‐dominated channel‐to‐lobe transitional deposits, and (c) suspended‐load‐dominated lobe deposits. The erosional channels in proximal locations are filled by mud clasts, parallel‐laminated sandstones, massive sandstones, cross‐bedded sandstones, massive siltstones, and mudstones. The channel‐to‐lobe transitional deposits in a middle location are composed of parallel‐laminated sandstones, cross‐bedded sandstones, mud clasts near internal erosional surfaces, ripple cross‐bedded sandstones, massive siltstones, and mudstones. The lobe deposits in distal locations are composed of ripple cross‐bedded sandstones, massive siltstones, and mudstones. The sedimentary process of the shallow lacustrine hyperpycnal flows shows obvious response to periodic flood energy fluctuations. Gentle slope, small capacity of flood discharges and semiarid paleoclimate may be the possible reason for the observed sedimentary architecture of the shallow lacustrine hyperpycnal flow deposits. Therefore, this study offers new insights into the sedimentology of shallow lacustrine deposits in rift basins. [ABSTRACT FROM AUTHOR]

Shallow-marine fans have been analyzed for sedimentary characteristics and genetic mechanisms far less commonly than deep-marine fans. Based on outcrop observations, this study reveals that large-scale submarine fans in the Middle Triassic Baise area, south China, consisted of a braided channel-levee facies complex in the inner fans; sinuous channel-levee facies complex, overbank facies and crevasse splay facies in the medial fans; and sand sheet facies in the outer fans, which present different lithology assemblages and depositional successions. The lithofacies of dominant massive sandstone, secondary graded sandstone and laminated sandstone from bottom to top is similar to the Bouma sequence, indicative of gravity flow processes. However, the laminated sandstones mostly bear sedimentary structures of gutter casts, wave-generated ripple marks, hummocky cross-bedding, wave-generated cross-bedding and ripple bedding, and were thus attributed to frequent storm reworking processes. Moreover, discontinuous deposition recorded by abrupt sediment grain-size changes, distinct interfaces and divergent paleocurrent orientations occurred between the laminated sandstones and the underlying massive (or graded) sandstones. Accordingly, storm reworking processes together with abundant fossils of Daonella , Ammonite and Crinoidea indicate a shallow-marine paleo-environment. As a whole, this study provides a good example of large-scale shallow-marine fans and laminated sandstones resulted from storm reworking. [ABSTRACT FROM AUTHOR]

Field investigation and laboratory research on flysch of the Liufengguan Group in Qinling indicate the following: (1) Sandstone of the Liufengguan Group is categorized as feldspathic lithic graywacke with a minor amount of lithic graywacke in the QFR triangular diagram. Grain size ≤0.3 mm. Bedding plane structures such as groove casts and suspected flute casts can be found at the bottom of the sandstone. It is inferred that currents may have come from the southeast during deposition. Bedding structures such as ripple marks, graded bedding, parallel bedding, small-scale cross bedding, climbing bedding, suspected convolute bedding, microlamination and sliding structures have also been observed, which are of indicative significance. It is thought that the Liufengguan Group has the sedimentary characteristics of bedding, bedding plane structures and lithological assemblages of deep-sea low-density turbidity current deposits. The vertical succession of the Bouma sequence in the inner fan subfacies zone is generally incomplete: the assemblage of Ta and Tabc is commonly seen; the succession of the middle fan subfacies zone is relatively complete; and divisions Te and Tb are common in the outer fan subfacies zone. (2) The flysh of the Liufengguan Group is a sequence of deep-sea argillaceous-arenaceous submarine fan deposits, in which the authors recognize the inner, middle and outer fan subfacies and also nine types of lithofacies: normal graded sandstone (A1), medium- to thick-bedded, fine-grained sandstone (A2), medium- to thick-bedded and massive siltstone (A3), thin-bedded, fine-grained sandstone and mudstone (B1), irregular interbeds of thin-bedded, fine-grained sandstone and siltstone (B2), thin-bedded, fine-grained sandstone (C1), very thin-bedded, fine-grained sandstone (D1), olistostromes (E1) and deep-sea mudstone (F). The inner fan consists of four microfacies: natural levee (A1), water channel (A2, A3) and olistostromc (E1); in the middle fan there also occur four microfacies, i.e., branch channel (B1), branch channel (B2), interdistributary bay (D1) and olistostrome. The outer fan is made up of the branch channel (C1) and sheet sand (D1) microfacies, which alternate vertically with sediments of deep-sea plain subfacies (F). There occur fining- and thin ning-upward channel deposits in the outer-fan subfacies zone of the submarine fan of the Liufengguan Group observed in this study. The quartz content of the graywacke of the deposits is all higher than 40% and may reach as high as 60%. Therefore, on the basis of the aforementioned features, this flysh should be formed in a passive continental-margin tectonic environment. [ABSTRACT FROM AUTHOR]