Ramp Depositional Model in an Intracratonic Basin: The Cambrian Sedimentary Successions in Yanshan Area, North China

Abstract

The mechanisms for the deposition of the Cambrian carbonate sedimentary successions remain controversial. The carbonate-dominated successions (Stage 3 to Stage 10) are well developed in the North China Craton and they are commonly thought to represent a platform setting because of the flat top. In this study, we propose a new scenarioi, that the Cambrian successions in North China are more likely a carbonate ramp rather than a platform. Detailed sedimentological investigation for the Cambrian rocks in the Yanshan area (Stage 4 to Stage 10) reveals a total of 21 lithofacies which can be grouped into 7 facies associations, including sabkha and mixed tidal flat, shoal, carbonate tidal flat, tidal flat on shoal, protected lagoon, mid-ramp and outer ramp to shelf basin. Sequence stratigraphy analysis of the Cambrian in the Yanshan area reveals nine third-order depositional sequences, which shows an overall second-order transgression. A number of sedimentary characteristics, including the gentle depositional slope, abundant open marine deposits, scattered shoals and microbial mounds, absence of slump or gravity flow sediment, domination of subtidal facies cycles and widespread deposits that formed below the storm wave-base, indicate that the Cambrian sediments in the Yanshan area were deposited in an intracratonic carbonate ramp depositional system. This intracratonic carbonate ramp includes inner ramp, mid-ramp, outer ramp and shelf basin facies belts, with the inner ramp exhibiting sub-facies corresponding to tidal flat, protected lagoon and shoal sub-facies. Sequence stratigraphy analysis indicates that relative sea-level changes, corresponding to these nine third-order sequences, show synchronous with the global sea-level change across the studied sections. This consistency implies that eustatic sea-level changes could have been a major factor for the sequence stacking patterns in the Yanshan Area.