Study of rhythmic bedding sequence in Tung Ping Chau

Abstract

This study has been conducted in Ping Chau, northeast corner of Mirs Bay. The main purpose of this study was to examine different lithologies and structures of the Ping Chau Formation, and to put all the findings in a wider context for the geological evolution and depositional environment of the area. Special attention has been put on the rhythmic bedding of clayey siltstone and fine sandstone. Field description and samples were collected at Cham Keng Chau for thin section, geochemical and magnetic susceptibility analysis. These data, with the help of literature review, were put together to explain the variation of the cyclic deposition of the rhythmic beddings. The main result was that Ping Chau was formed in a shallow lacustrine lake transitioning between marine and terrestrial sedimentary environment. The water was low energy and quiet, with oxygen condition changing between high and low concentration. The rhythmic beddings were caused by seasonal shift between dry and humid periodic cycles, and the input of terrigenous detritus materials. Milankovitch cycles are used to try to explain the periodicities found in the rock record. Some of the cycles matched with the precession cycle of 19,000-23,000 years.