Evolution of rimmed carbonate platform architecture controlled by sea-level change: Insights from the Lower-Middle Cambrian of the Northern Tarim Basin, China

The sedimentary architecture of carbonate platforms is determined by distinct paleobiological, tectonics, climatic, oceanic and environmental conditions. During the Cambrian period, a rimmed carbonate platform system developed over an area of ∼28 × 10⁴ km² in the Tarim Basin. However, the evolution mechanism and its predominant controlling factors remain poorly understood. The investigation utilized integrated analysis of cores, thin sections, 3-D seismic, well logging and geochemical data to explain the sequence stratigraphic framework, depositional architecture and main controlling factors. Sixteen distinct microfacies (MF1–MF16) are identified based on thin-section analysis of the five lithofacies, which could be further grouped into fourteen microfacies associations (MA1-MA14). These fourteen microfacies associations respectively represent fourteen facies belts of the five facies of the shelf, outer ramp, middle ramp, platform margin and restricted platform. Microfacies and seismic characteristics have provided the basis for establishing a sequence-stratigraphic framework. Two regionally extensive second-order sequences (CS1-CS2) are developed in the Lower-Middle Cambrian succession; CS1 incorporates five third-order sequences (Sq1-Sq5); CS2 incorporates three third-order sequences (Sq6-Sq8). The lateral migration and vertical arrangement of depositional facies resulted in a unique depositional architecture. The overall carbonate platform architecture is interpreted from 3D seismic data, integrated with the depositional facies interpretations to document the evolution of the depositional setting over time from a broad shelf (Sq1-Sq2), to a distally steepening ramp (Sq3-Sq4), to a weekly rimmed platform (Sq5), and finally to a strongly rimmed platform (Sq6-Sq8). Relative sea-level (RSL) curves were reconstructed through an integrated analysis of Fischer plots from three wells. The determined RSL curve matches closely with reconstructed paleo-water depths indicated by the distribution of microfacies types. The inferred paleo-water depths changes comprise two long-term shallow–deep–shallow trends, upon which eight intermediate-term cycles are superimposed. Microfacies also have been applied to explain the evolution of the platform in response to RSL change and other environmental factors. A major transgression occurred in the lower parts of CS1. Results demonstrate that RSL and paleo-water depth changes in the Lower-Middle Cambrian are consistent with known global sea-level changes indicated by geochemical elements. The architectural and sequence-stratigraphic evolution of the progradational rimmed carbonate platform was controlled principally by eustasy. This study is important due to the limited information on Cambrian rimmed platforms.