Sequence architecture and depositional evolution of the Ordovician carbonate platform margins in the Tarim Basin and its response to tectonism and sea-level change

The sequence architecture and depositional evolution of the Ordovician carbonate platform margins in the Tarim Basin, China, were formed in response to the interplay of tectonism and sea-level change, their history being documented by the integrated analysis of many seismic lines, drilling and outcrop data. The Ordovician carbonate system in the basin is divided into four composite sequences defined by major unconformities. Each sequence consists of a regional depositional cycle from transgression with an onlapping transgressive systems tract (TST) to regression with a prograding highstand systems tract (HST), and can be further subdivided into 10 third-order sequences based on subordinate discontinuous boundaries at the carbonate platform marginal zones. Constrained by the marginal slope of the early-rifted Manjiaer aulacogen, the carbonate platform margins of the Lower and Middle Ordovician that prograded eastward in an arcuate belt extending generally north-south across the northern part of the basin. The development of the Tazhong uplift due to compression resulted in an extensive paleokarst hiatus between the Middle and the Upper Ordovician in the south-central basin, and subsequently constrained the formation of a peninsula-shaped carbonate platform whose margins were controlled by marginal thrust-fault belts of the paleo-uplift during the Late Ordovician. In the northern basin, the Late Ordovician carbonate platform margin developed around the marginal slope of the Tabei paleouplift. The transgressiveregressive cycles of the carbonate system are comparable and seem to have occurred simultaneously across the entire basin, suggesting that the cyclic sequence architecture was fundamentally controlled by eustatic fluctuations. Stacking patterns of the composite sequences varied due to the interplay between the accommodation produced by tectonism and sea-level change, and the carbonate production rate. The reefshoal facies complexes that developed along the platform margins, with paleokarst development at unconformities, constitute the major reservoir of large petroleum reserves in the basin.