Architecture and evolution of the carbonate platform margin of the Lower Cambrian Xiaoerbrak Formation, northwestern Tarim Basin, China

The objective of this study is to systematically document the depositional architecture and evolution of the carbonate platform margin in the Lower Cambrian Xiaoerblak Formation of the northwestern Tarim Basin. This study uses the following approaches: (1) seismic reflector identification; (2) lithofacies and paleoenvironmental interpretation based on Paleozoic outcrops; and (3) thin section examination. Identification of seismic reflectors and determination of lithofacies associations in Paleozoic outcrops reveal two phases of architectural evolution of the Xiaoerblak Formation platform margin. Phase 1 corresponds to the Lower Xiaoerblak Formation, characterized by discontinuous to semicontinuous moderate- to high-amplitude reflectors, revealing a uniform, gentle ramp platform margin thinning toward the basin. The dolomudstone and laminate lithofacies associations in the outcrops show a middle-ramp low-energy depositional environment. Phase 2 corresponds to the Upper Xiaoerblak Formation. The seismic stratigraphic units display upwardly convex irregular reflectors, indicating the development of a rimmed carbonate platform margin system. The lithofacies associations reveal reef-shoal interbedding, suggesting a high-energy marginal marine environment. The tectonic and paleomorphic evolution of the Tarim Basin, along with Paleozoic outcrop features, suggest that paleomorphic inheritance from the Neoproterozoic created a homogeneous, broad, low-angle shelf. This, combined with the continuous sea-level fall in the early Cambrian and the tropical environment, provided an ideal depositional environment for carbonate platform development in the Xiaoerblak Formation.