Sedimentary characteristics and distribution patterns of grain shoals in the Lower Cambrian Longwangmiao Formation, southern Sichuan Basin, SW China

Abstract

Grain shoals in the Lower Cambrian Longwangmiao Formation of southern Sichuan Basin were analyzed in terms of their characteristics, distribution, and major controlling factors, based on a combination of drilling, testing, logging, core, and thin section data. It is suggested that there should be two main types of rocks in the grain shoals: (1) granular dolomites (limestones) with remarkable granular structures formed mainly by psammitic or oolitic clastic and (2) crystalline dolomites with residual granular structures. Grain shoals in the study area were mainly unexposed shallow shoals with either upward-deepening or upward-shallowing structures vertically. Those grain shoals in the Weiyuan-Ziyang and Gaoshiti-Hechuan areas with the paleo-uplift in the northern part of the study area were featured by good stability and continuity in both scale and horizon, while those outside these areas were mainly patch shoals in small scales. Development characteristics and superimposition patterns of Longwangmiao Formation grain shoals were found to be controlled by changes of sea level and depositional energy, while their differential distribution were thought to be determined by paleogeomorphology during depositional stage.