Late Ordovician, deep-water gravity-flow deposits, palaeogeography and tectonic setting, Tarim Basin, Northwest China



The Upper Ordovician in the Tarim Basin contains 5000–7000 m of siliciclastic and calciclastic deep-water, gravity-flow deposits. Their depositional architecture and palaeogeographical setting are documented in this investigation based on an integrated analysis of seismic, borehole and outcrop data. Six gravity-flow depositional–palaeogeomorphological elements have been identified as follows: submarine canyon or deeply incised channels, broad and shallow erosional channels, erosional–depositional channel and levee–overbank complexes, frontal splays-lobes and nonchannelized sheets, calciclastic lower slope fans and channel lobes or sheets, and debris-flow complexes. Gravity-flow deposits of the Sangtamu and Tierekeawati formations comprise a regional transgressive-regressive megacycle, which can be further classified into six sequences bounded by unconformities and their correlative conformities. A series of incised valleys or canyons and erosional–depositional channels are identifiable along the major sequence boundaries which might have been formed as the result of global sea-level falls. The depositional architecture of sequences varies from the upper slope to abyssal basin plain. Palaeogeographical patterns and distribution of the gravity-flow deposits in the basin can be related to the change in tectonic setting from a passive continental margin in the Cambrian and Early to Middle Ordovician to a retroarc foreland setting in the Late Ordovician. More than 3000 m of siliciclastic submarine-fan deposits accumulated in south-eastern Tangguzibasi and north-eastern Manjiaer depressions. Sedimentary units thin onto intrabasinal palaeotopographical highs of forebulge origin and thicken into backbulge depocentres. Sediments were sourced predominantly from arc terranes in the south-east and the north-east. Slide and mass-transport complexes and a series of debris-flow and turbidite deposits developed along the toes of unstable slopes on the margins of the deep-water basins. Turbidite sandstones of channel-fill and frontal-splay origin and turbidite lobes comprise potential stratigraphic hydrocarbon reservoirs in the basin.