Sequence stratigraphy, paleoclimate patterns, and vertebrate fossil preservation in Jurassic-Cretaceous strata of the Junggar Basin, Xinjiang Autonomous Region, People's Republic of China

Abstract

Multi-kilometre-thick Jurassic-Cretaceous-age sedimentary successions exposed in the southern, northwestern, and northeastern regions of the Junggar Basin display a consistent and correlative stratigraphy comprising four, stacked second-order megasequences: Badaowan, Sangonghe, Shishugou, and Kalaza. Each consists of a basal erosional unconformity or discontinuity surface and lower, middle, and upper units that are interpreted as forestepping, backstepping, and aggradational systems tracts, respectively. Each megasequence is interpreted as recording an upsection shift from active tectonism and uplift to tectonic quiescence with associated changes in crustal response and sediment supply. Basin-wide analysis of megasequences indicates that tectonism was intermittent and regionally variable. A maximum phase of subsidence and sediment accommodation is recorded in the middle unit of each megasequence and correlates with a notable abundance of fossil vertebrates suggesting a primary tectonic and basin-response control on fossil preservation. Seasonally dry climatic conditions were developed first in the northeastern region of the basin during the Pliensbachian, followed by basin-wide seasonal dryness during the Bajocian. Seasonally dry climatic conditions were permanently established across the basin by the Oxfordian and intensified during the Early Cretaceous. A seasonally dry climate from Oxfordian through the Early Cretaceous correlates positively with the widespread presence of fossil vertebrates and suggests an additional climatic control on fossil preservation.