Depositional evolution, sequence stratigraphic framework and its response to relative sea-level change, the Middle and Lower Ordovician carbonate system on outcrops, north-western margin of Tarim Basin

Abstract

Middle and Lower Ordovician series carbonate platform deposits develop well in the north-western Tarim Basin, Kalpin and Bachu outcrop. An integrated analysis of the outcrop, hand specimen, thin slice and geochemical data allows for interpreting the sequence framework, depositional architecture and the response to sea-level changes. The Middle and Lower Ordovician Yingshan and Yijianfang Formations comprise 2 composite sequences (CS1 and CS2) bounded by regional or major unconformities, 5 third-order sequences (Sq1–Sq5) defined by subaerial exposure unconformities or facies changing and 13–15 fourth-order sequences classified by the outcrop depositional cycle change. Twelve microfacies and seven microfacies associations are identified using hand specimens and thin sections. The primary depositional facies include platform margin reef–bank complexes, open platform intraclast or bioclast shoal, intra-shoal and restricted platforms shoal-dolomitic or intra-clastic tidal flats. The research indicates that the carbonate platform evolved from arid and semiarid ramps, restricted platform and dolomitic tidal flat (CS1) to the open and rimmed platform margins (CS2) and eventually was drowned at the end of the Middle Ordovician. The sea-level changes have been studied using the Fischer plot method, showing two sets of long-term shallow–deep–shallow trends. Large-scale transgressions occurred at the lower parts of Sq4, with a significant negative shift of δ13C and low-energy mudstone or wackestone depositional architectures. The Fischer plot, lithofacies cycle and the geochemical indicators correlate well and are comparable with Haq's global sea-level changes. Sea-level changes dominate the sequence framework and depositional evolution, and dual factors of sea-level change and tectonic subsidence constrain local Sq3-HST. The high-frequent fluctuation of sea-level changes has a noticeable control effect on the development of fourth-order depositional cycles of carbonate deposits.