Sedimentary Environments and Coal Accumulation of the Middle Xishanyao Formation, Jurassic, in the Western Dananhu Coalfield, Turpan-Hami Basin

The Dananhu coalfield, at the southern margin of Turpan-Hami Basin, Xinjiang, has good coal-forming geological conditions. Due to the low research level on the depositional environment and coal accumulation, the comprehensive coal mining is restricted. Based on drilling and sectioning outcrop data, the depositional characteristics, lithofacies, paleogeography, and coal accumulation of the coal-bearing formations in western Dananhu coalfield are analyzed. The middle member of the Xishanyao Formation is the major coal-bearing strata of the Dananhu coalfield, composed of conglomerate, sandstone, siltstone, mudstone, and combustible organic rock, which can be further subdivided into 14 lithofacies according to substance composition, sediment texture, and structure. During the early Middle Jurassic coal-accumulating period, alluvial fan, fluvial plain, fluvial delta, and shallow lacustrine deposits were developed from northeast to southwest in the western Dananhu mining area. Coal accumulation was most developed in the center regions of the study area extending from northeast to southwest, which were primarily concentrated in margin fans, floodplains, interdeltaic bays, and lakeshore environments, especially the transitional zone between the fluvial plain and the delta plain in the west of No. 1 mining area. This coal accumulation was mainly controlled by synsedimentary tectonic subsidence and clastic sediments supply or sedimentary environment, where tectonic subsidence was the primary controlling factor and the source supply was the secondary controlling factor for coal accumulation, which provided theoretical support for the coal prediction and geological exploration in western Dananhu coalfield.

Ombilin Basin as Inverted Oblique Rift in Barisan Mountains, Sumatra: Considerations on Subsidence Mechanisms and Fault Development

Ombilin Basin is a NW-SE inverted oblique rift which is currently being part of Barisan Mountains in western Central Sumatra. Regarding its current position, Ombilin Basin can be one of the windows to see the evolution of Barisan Mountains since Paleogene. Two schools of thought, namely rift basin and pull-apart basin, have been established to explain the evolution of Ombilin Basin. This paper aims to present another perspective on the evolution of Ombilin Basin based on subsidence mechanisms and fault development. This study integrated remote sensing and subsurface interpretations. Remote sensing interpretation took the role to delineate surface fault lineaments using digital elevation model, while subsurface interpretation dealt with log and seismic interpretations, subsidence analysis, and palinspatic reconstruction. Fault lineaments derived both from remote sensing and seismic interpretations were combined to construct structural framework of the basin. Subsidence analysis generated geohistory and backstripped tectonic subsidence charts. Palinspatic reconstruction illustrated structural configurations through time. This study figured out that Ombilin Basin went through fault-controlled subsidence in Middle Eocene – Late Oligocene and thermal subsidence in Early Miocene – Late Pliocene. Each subsidence mechanism was terminated by an uplift. Subsidence mechanisms in Ombilin Basin represented the criteria of rift basin in terms of amount and rate of tectonic subsidence, duration of subsidence, and contribution of thermal subsidence. On the other hand, fault development captures extensional and strike-slip components during rifting and development of flower structures during inversion of the basin. Oblique rifting operates when dominant extensional component works together with strike-slip component. Therefore, subsidence mechanisms and fault development are in agreement to regard Ombilin Basin as inverted oblique rift.

Mid-Cretaceous thick carbonate accumulation in Northern Lhasa (Tibet): eustatic vs. tectonic control?

Widespread accumulation of thick carbonates is not typical of orogenic settings. During the mid-Cretaceous, near the Bangong suture in the northern Lhasa terrane, the shallow-marine carbonates of the Langshan Formation, reaching a thickness up to ∼1 km, accumulated in an epicontinental seaway over a modern area of 132 × 103 km2, about half of the Arabian/Persian Gulf. The origin of basin-wide carbonate deposits located close to a newly formed orogenic belt is not well understood, partly because of the scarcity of paleogeographic studies on the evolution of the northern Lhasa. Based on a detailed sedimentological and stratigraphic investigation, three stages in the mid-Cretaceous paleogeographic evolution of northern Lhasa were defined: (1) remnant clastic sea with deposition of Duoni/Duba formations (Early to early Late Aptian, ca. 125−116 Ma); (2) expanding carbonate seaway of Langshan Formation (latest Aptian−earliest Cenomanian, ca. 116−99 Ma); and (3) closure of the carbonate seaway represented by the Daxiong/Jingzhushan formations (Early Cenomanian to Turonian, ca. 99−92 Ma). Combined with data on tectonic subsidence and eustatic curves, we emphasized the largely eustatic control on the paleogeographic evolution of the northern Lhasa during the latest Aptian−earliest Cenomanian when the Langshan carbonates accumulated, modulated by long-term slow tectonic subsidence and high carbonate productivity.

Supplemental Material: Mid-Cretaceous thick carbonate accumulation in Northern Lhasa (Tibet): eustatic vs. tectonic control?

Biozones of the Baoji, Xiongmei, Xiagezi and other central sections, and stratigraphic data used to reconstruct tectonic subsidence of the north Lhasa.<br>

Supplemental Material: Mid-Cretaceous thick carbonate accumulation in Northern Lhasa (Tibet): eustatic vs. tectonic control?

Biozones of the Baoji, Xiongmei, Xiagezi and other central sections, and stratigraphic data used to reconstruct tectonic subsidence of the north Lhasa.<br>

Cenozoic subsidence characteristics and evolution at a hyper-extended continental margin: Revealed by 3D high-resolution seismic data from the Baiyun Sag, northern South China Sea

&lt;p&gt;The hyper-extended continental crust in the passive margins, which recording the extensional processes in relation with the breakup of continental crust and lithosphere as well as the onset of seafloor spreading, have been widely recognized and studied at present-day rifted margins. The Baiyun Sag (BS) represents one of the hyper-extended continental marginal basins with a sharply thinned continental crust from 25 km to 7 km over a ~ 50 km distance along the Northern South China Sea, which experienced syn-rift to post-rift during the Cenozoic. Although the Cenozoic infill of the BS has been extensively described, newly acquired 3D seismic profiles revealed a thick succession (up to 10 km) with thicken syn-rift but relatively thin post-rift strata particularly well imaged in the central part. The imaged succession is controlled by the interaction between well-developed detachment systems and depth-dependent stretching, resulting in different and complex subsidence architecture. Attempts had been made to quantify the subsidence in the BS, while most studies were only carried out in a limit set with one or few 2D seismic sections and generally focused on the post-rift subsidence but ignoring that in the syn-rift stage. As result, we investigate the interaction between spatial-temporal distributions of tectonic subsidence from continent break-up to post-rift and the evolution of hyper-extended rift systems along the relatively young age passive margins.&lt;/p&gt;&lt;p&gt;In this presentation we analyze the vertical and horizontal motions of tectonic subsidence and sedimentary processes with integrated high-quality multi-channel seismic profile grid data (~30 seismic sections). This study enables us to 1) interpret the main unconformities and analyze the depth conversion of the BS, 2) reconstruct the tectonic subsidence from syn-rift to post-rift, 3) provide a 3D subsidence analysis unravelling the temporal and spatial architecture of Cenozoic infill of the BS. The main objectives of this contribution is to discuss the possible mechanisms accounting for the origin and subsidence at the BS, reveal its interrelationships with magmatic activities, and explore the style of rift to post-rift subsidence pattern at a hyper-extended continental margin.&lt;/p&gt;