The Upper Triassic Xujiahe Formation in the northeastern Sichuan foreland basin: Sequence-based sedimentary facies and palaeogeography

The Xujiahe Formation sequences consist,in the light of tectonic and astronomic factors controlling high-resolution sequences,of two ultralong-,five long-,eighteen medium-,and many ultra short-period sequence cycles,of which the sedimentary facies and palaeogeography of the ultralong-period sequence cycles are accentuated in this paper.During the deposition of the ultralong-period sequence cycle SLSC1,the steadily uplifting with low amplitudes occurred in the Micangshan-Dabashan tectonic mountain systems,while the thrusting was relatively active in the Longmenshan tectonic mountain systems.Then the northeastern Sichuan foreland basin was on the foreland slope influenced by distal effect of the preceding thrusting event.The braided stream-delta deposits are mostly developed in the Micangshan-Dabashan frontal zone,while the shallow lake deposits appear in the southwestern part of the basin.During the deposition of the ultralong-period sequence cycle SLSC2,the thrusting was further strengthened in the Longmenshan area,whereas the rapid tectonic uplifting occurred in the Micangshan-Dabashan area.The large-scale subsidence took place in the north-eastern Sichuan foreland basin.The alluvial fan depositional systems composed of thick massive conglomerates are well developed along the frontal zones of the Longmenshan and Micangshan-Dabashan thrust zones.