Depositional evolution and sedimentary response to sea level changes of the lower Cambrian carbonate platform of the Wusonggeer Formation, Tarim Basin, northwest China

The Wusonggeer Formation of the Cambrian Series 2 is one of the important carbonate oil/gas bearing intervals in the Tarim Basin. It recorded significant paleoclimatic conditions and history of sea-level variation and has long been a popular research issue in the region. Based on drilling data and thin sections from core wells, 18 microfacies (Mf1-Mf18) and 11 microfacies associations (MA1-MA11) of the Formation are identified, and the main depositional facies include mixed tidal flat, evaporated platform, restricted platform, semi-restricted platform, platform margin, and slope-shelf deposits. According to unconformity contacts such as toplap and onlap, lithofacies transition and karst interfaces, the Wusonggeer Formation comprises a composite sequence (WCS1) and three shallowing-upward third-order sequences (WSQ1-WSQ3). The highstand systems tracts (HST) dominate each sequence and consist mainly of evaporated lagoon deposits (MA3), reef-shoal complexes and shoal deposits (MA8), while the transgressive systems tracts (TST) are predominately composed of intershoal deposits (MA9) and restricted tidal flat deposits (MA7). The depositional architecture from WSQ1 to WSQ3 evolved from restricted to evaporated carbonate platforms, revealing three depositional stages of carbonate platform in gradual arid and warm climatic background, including semi-restricted rimless platform in a relatively humid climate (WSQ1), restricted weak-rimmed platform in a semi-arid climate (WSQ2), and evaporated weakrimmed platform in an arid climate (WSQ3). The salinity changes of the seawater seem to be consistent with the relative sea-level change indicated by the stacking pattern of facies associations and this may suggest that the depositional cycles of the platform were mainly constrained by the synergistic processed of climate and sea level change. WSQ2 marks a key turning period from a relatively humid climatic condition to a dry and hot climate is suggested to be the result of the global climate change during the late Cambrian Series 2.