Climate and tectonic-driven deposition of sandwiched continental shale units: New insights from petrology, geochemistry, and integrated provenance analyses (the western Sichuan subsiding Basin, Southwest China)

How the depositional setting efficiently governs the characteristic sandwiched continental shale units remains uncertain, which restricts an integrated assessment of organic-rich fluvio-lacustrine shale reservoirs, and accurate estimation of potential natural gas resources. Here, we present new results from petrological observations, element geochemical fingerprinting, and integrated analyses of heavy mineral, basinal subsidence history, and sandstone / stratum ratio on typical and terrigenous sandwiched-like depositional systems of the targeted Upper Triassic Xujiahe Formation in the western Sichuan subsiding Bain, Southwest China. In view of the representative and environment-sensitive indices, we suggest that passive continental margin and continental island arc dominated by granite to granodiorite source are the major tectonic settings of the provenance, and both a more warm-humid climate characterized by intensified chemical weathering conditions and a calm tectonically quiescent setting are identified as two major drivers forcing the accumulation and preservation of organic matter in organic-rich continental shale units. Finally, a comprehensive depositional model is established for providing new insights into the linkage between palaeoclimatic conditions, tectonic pulses and terrigenous clastic sedimentation. Both the cyclic palaeoclimate fluctuations and episodic tectonic activities are believed to have exerted a very considerable force on development of the unique sandwiched-like stratigraphic framework, and the coupling interactions between the tectono-climatic evolution and fine-grained sedimentation are thus also stressed.