Sequence-palaeogeography And Coal Accumulation Of The Late Triassic Xujiahe Formation In The Sichuan Basin

Thirteen facies and five facies associations are recognized within the Xujiahe Formationand these are interpreted, respectively, as alluvial fan, transverse and longitudinal braided river,meandering river, overbank, shallow lacustrine, and deltaic deposits. The Xujiahe Formation issubdivided into four sequences, and sequence boundaries are recognized on the basis oftectonic unconformities, facies-tract dislocations and associated fluvial rejuvenation andincision. Thickness patterns demonstrated by the Xujiahe Formation as a whole and byindividual members are consistent with a foreland-basin setting for the Sichuan Basin,including foredeep, forebulge and backbulge, in which foredeep is the main depocenter.Sequence development in the Xujiahe Formation is closely related to the tectonic evolution ofthe basin. The Sichuan Basin experienced three periods of thrust loading and lithosphereflexure, two periods of stress relaxation and basin widening and one phase of isostatic rebound.Paleogeographic reconstruction of the Sichuan Basin in the Late Triassic indicates that theLongmen Mountains to the west and Jiangnan Mountains to the east were the major sources ofsediment, and subordinate sediment sources were the Micang-Daba Mountains to the north.Coal mainly developed in deltaic and lacustrine swamps, with the best accumulation withinSQ3, followed by SQ2. Coal development is controlled by depositional evironment, tectonicactivities, sequence framework and paleoclimate.