Jingyan Liu

Depositional architecture and developing settings of large-scale incised valley and submarine gravity flow systems in the Yinggehai and Qiongdongnan basins, South China Sea

In the Ying-Qiong basins in the South China Sea developed a set of giant inner shelf slope and submarine gravity flow deposits in the Ying-Huang formations since the Late Miocene. These deposits can be classified into 9 sequences in which slope slump-debris flow, slope fan and particularly, the large-scale axial basin-floor incised valley fills have been recognized. They were distributed in a distinctive pattern attributed to tectonic control. The development of shelf slopes and abyssal plains in the basins was related to the last episode of rapid subsidence and a large amount of sediment inputs. Large-scale basin-floor incised valleys, formed during the late Miocene and Pliocene, indicate several major sea level falls that occurred in the South China Sea.

Major unconformities, tectonostratigraphic frameword, and evolution of the superimposed Tarim basin, Northwest China

The Tarim basin experienced a complex tectonic evolutionary history from Sinian to Cenozoic. Eight large-scale and more than 20 subordinate unconformities defining tectonosequences of different protobasins formed in various tectonic settings have been identified within the Phanerozoic in the Tarim basin, their distribution determining the general characteristics of sequence stratigraphic framework of the basin. Tectonostratigraphic unit I (magasequence) consists mainly of the Sinian System, which formed in a rift or aulacogen setting and can been subdivided into two subordinate stratigraphic units (supersequences). Unconformity (Tg9) between Sinian and Cambrian with surface karstification is regarded as a post-rift unconformity. Tectonostratigraphic unit II comprises the Cambrian and the Ordovician and can be divided into six subordinate tectonostratigraphic units, recording the tectonogeographic evolution of the prototype basins from Cambrian to Early Ordovician passive carbonate continental margin or cratonic depression and the Late Ordovician submarine to neritic retroarc foreland and cratonic depressions. The tectonic uplift related to the formation of the unconformity Tg5-2 resulted in the remarkable change in basin tectonic setting from a passive divergent to an active convergent, with the development of the Tazhong (塔中) uplift, the Tangguzibasi (塘古孜巴斯), and the northern depression at the end of the Middle to the early Late Ordovician. The widespread angular unconformity Tg5 formed by a relatively strong compressive deformation, which caused an abrupt tectonogeographic change of the basin from abyssal to a neritic setting in response to the collision and associated tectonic deformation of the North Kunlun (昆仑) orogenesis during the Late Ordovician to the Early Silurian. Tectonostratigraphic unit III is composed of the Silurian and the Lower to Middle Devonian and characterized by the development of fluvial or deltaic and clastic littoral and neritic deposits. Large-scale terrigenous clastic depositional wedges progradated from the north to south in the southeastern slope of the basin indicate the continuously shallowing and uplifting along the northern basin margin. Tectonostratigraphic unit IV includes the Upper Devonian, Carboniferous, and Permian and can be classified into two subordinate tectonostratigraphic sequences. The angular unconformity (Tg3) at the base of the unit is the most widespread unconformity and the strong compression and uplift of the basin during this period has been suggested to be related to the collision of the Tianshan (天山) orogenesis and resulted in fundamental change in tectonic geomorphology with higher to the northeast and lower to the southwest. Tectonostratigraphic unit IV records another tectonic cycle from weak extension to compression in basin setting and is composed mainly of nearshore clastic deposits of embayment basin fills. From the Triassic, the Tarim basin evolved into a period characteristic of development of intracontinental depressions and marginal foreland basins and experienced several cycles from rapid subsidence to strong uplift and deformation, resulting in superimposition and reformation of differently orientated protobasins filled with a series of regional depositional cycles bounded by major unconformities and consisting of extremely thick alluvial and lacustrine deposits. The Kuqa foreland depression in the northwestern basin margin developed since the Triassic and deposited a clastic wedge of the Mesozoic to Cenozoic more than 100 000 m in thickness, which progradated and thin towards the southern Tabei (塔北) forebulge. The large-scale sedimentary cycles from alluvial, fluvial to lacustrine, and finally fluvial deposits are attributed to the results of foreland tectonisim from active to relatively quiet stages. The foreland tectonisim was active during the Triassic, relatively quiet during the Jurassic, and active again from the Late Jurassic to the Cretaceous. To the Eogene, the depression subsided again and the compression intermittently increased, resulting in a series of faulted and folded structural belts.

The U-Pb chronologic evidence and sedimentary responses of Silurian tectonic activities at northeastern margin of Tarim Basin

U-Pb ages of Devonian detrital zircons from Tabei Uplift have been determined through LA-ICP-MS test technology. The results revealed that most zircon ages concentrate on 460–414 Ma, especially around 436–423 Ma, indicating possible occurrences of strong tectonic events at the northern margin of Tarim Basin during that period. Combined with previous researches on the basin marginal orogenic belts, intense tectonic activities developed at the northeast margin of Tarim Basin and its obvious sedimentary responses in basin during the end of Ordovician to Early-Middle Silurian are discussed. These include (1) several unconformities within the Late Ordovician-Silurian, showing truncation, erosion, and onlap characteristics, which reflected the local uplift formed during the surrounding extrusion process; (2) the Silurian fluvial delta system from northeast to southwest in Keping, Yingmaili, Hade, and Caohu areas, which reflected the partial uplift at the northeast margin and provided clastic supply to basin; and (3) as indicated by heavy mineral analysis, the Silurian sediments came mainly from the recycles of orogenic belts provenance, which indicated the compress tectonic setting. In addition, a wide range of red mudstone layer distribution in the upper part of the Silurian may be closely related to the surrounding tectonic uplift and the rapid decline of sea levels.

Detrital zircon U-Pb geochronology and its provenance implications on Silurian Tarim basin

As one of the major exploration objects of marine deposit in Tarim basin, Silurian has been paid more attention from oil/gas exploration and geologists. However, due to the widely deposit and later erosion, it is difficult to restore the original basin. The surrounding tectonic activity and provenance systems of Silurian Tarim basin have a lot of controversy. Aid of detrital zircons U-Pb dating data obtained from well drilling of Tabei (塔北) and Tazhong (塔中) areas and Sishichang (四十场) and Xiangyangcun (向阳村) outcrop profiles, integrated with other geological and geophysical data, the tectothermal evolution and provenance nature of Silurian deposit have been revealed. Zircons U-Pb dating shows Tarim basin has experienced 5–6 significant tectothermal events: 3 500–3 000 Ma Paleo-Mesoarchean, around 2 500 and 1 800 Ma Paleoproterozoic, around 1 000 and 800 Ma Neoproterozoic, and 500–400 Ma Eopaleozoic tectothermal events. These tectothermal events reflected the evolution of Tarim microplates and Tarim basin, respectively, corresponded to the forming and spilitting process of Ur supercontinent, Kenorland, Columbia and Rodinia supercontinent. Difference between the samples of Tazhong and Tabei areas indicated that North and South Tarim microplates were different in Paleo-Mesoarchean, and later evolutions were more synchronous after Paleoproterozoic. Integrated with seismic data and outcrop interpretation, the U-Pb dating results also revealed that the surrounding tectonic activities were still very active during Silurian, and indicated different regions had different source systems. At Tadong (塔东) and Manjiaer (满加尔) depressions, major source systems came from Ordovician Altyn orogenic belts. At Tabei area and northwest of Tarim basin, major source systems came from recycling orogenic zone (the activity of South Tianshan (天山) Mountain) and Precambrian stable basement (local paleo-uplifts at north of Tabei). The Ordovician uplift and orogenic zone at the south of Tarim basin and Precambrian granite basement provided lots of source systems to Tazhong area.

Sedimentary facies and sequence stratigraphy of the Silurian at Tabei uplift, Tarim Basin, China

Sequence stratigraphy division and comparison of the Silurian in Tarim Basin were a hot research field in oil industry and academia. However, basic geological problems limited the exploration needed for further research. In this paper, 21 lithofacies and 5 facies associations were identified based on the grain size of sediments, sedimentary characteristics, and bioturbation conditions: (1) fluvial-dominate delta front facies association; (2) tidal flat facies association; (3) tidal channel facies association; (4) offshore-transition facies association; (5) shoreface facies association. The seismic, outcrops, and logging data were involved to divide the Silurian (including upper Ordovician Tierekeawati Fm.) at Tabei uplift into five sedimentary sequences. SQ1 (Tierekeawati Fm.) is mainly characterized by tidal flat facies association, while delta front facies association locally develops; SQ2 (the lower Kepingtage Fm.) generally consists of offshore-transition facies association; SQ3 (the upper Kepingtage Fm.) is mainly characterized by shoreface and delta front facies association. For SQ4 (Tataaiertage Fm.), the transgressive system tract (TST) is dominated by shoreface facies association, while the fluvial-dominate delta facies association widely develops in highstand system tract (HST). SQ5 (Yimugantawu Fm.) is mainly characterized by tidal flat facies association. From SQ1 to SQ2, an overall sea level transgressive process is shown, while an overall sea level regressive process is found from SQ2 to SQ5. The results are consistent with the progradation and regression trends of large regions reflected by sequence framework pattern. As to SQ3 sequence, TST and HST sandstones are the main reservoir intervals in the Silurian. Hercynian movement led to the strong uplift and extensive erosion in the Silurian at Tabei and Tazhong uplift, and is favorable to the formation of strata erosion unconformable traps.