Jurassic continental coal accumulation linked to changes in palaeoclimate and tectonics in a fault-depression superimposed basin, Qaidam Basin, NW China

Abstract

The Middle Jurassic Aalenian–Bajocian stages represent the tectonic transition period for the group of Jurassic sedimentary basins in northwestern China and include accumulation of the most important recoverable coal seams. Geological data from borehole cores, well loggings and rock-mineral determination in the Aalenian to Bajocian-aged Dameigou Formation in the Yuqia coalfield, Northern Qaidam Basin were employed to unveil the relationship between coal accumulation and changes in palaeoclimates and provenance tectonic activity. The results identify six kinds of lithofacies from fluvial and lacustrine sedimentary systems, and allow the Dameigou Formation to be divided into two third-order sequences (SIII1–SIII2) and their corresponding system tracts by regional unconformity surfaces, forced regression surfaces and their correlative conformity surfaces, and the abrupt shift surfaces of depositional facies. The main coals, the #7 and #6 seams, were deposited in a lake and swamp environments during lake transgression system tracts of SIII1 and SIII2, respectively. The varying trend of the Tectonic Index values indicate that the source area experienced strong tectonic activity from the SIII1 to the lowstand system tract of SIII2, and then gradually stabilized. The Mineral Alteration Index reveals that the climate during SIII1 was from dry-hot to humid-warm, and there is a climate cycle from dry-hot to humid-warm then a return to dry-hot in SIII2. During the faulting stage, the occurrence and termination of coal accumulation was in response to weakening and strengthening periods of tectonic activity respectively, while during the depression stage it was in response to the humid-warm and dry-hot climate respectively.