Reconstruction of the paleo-depositional environment at Ap Lei Chau, Hong Kong

Abstract

A high-level coastal deposition feature at the southern tip of Ap Lei Chau, Hong Kong Island, is studied in this report to understand its paleo-depositional environment. Previous systematic surveys for similar deposits in Hong Kong were conducted between 1986 and 1996 during the production of fifteen 1:20,000 scale geological maps, and subsequently reviewed wholistically by Wong & Shaw in 2009. Studies of specific sites were conducted by different parties as well such as at Po Chue Tam, Lantau Island in 2000 and 2003. Possible formation processes for these features such as eustatic changes in sea level, local variations in sea level, extreme wave events, high waves and wind action, winnowing, and neotectonic activity were proposed by the Wong & Shaw (2009) study. In this study, literature reviews are carried out to understand the existing models for Quaternary sea level changes in Hong Kong, methods for quantifying clast size, sphericity and roundness, findings from previous studies on similar site in Hong Kong, and the ground model of the site. Field studies comprising joint set survey, vertical photographic scan line assessment of 198 clasts and general observations such as grain sizes and shape distribution, clasts preferred orientations, are conducted and discussed to deduce the possible mode of deposition, potential sources, and likely age of the site. It is concluded in this study that the site consists of five layers exhibiting fluvial depositional processes at the lower three layers and colluvial processes for the upper two layers. A joint set bounded wave-cut notch was formed after the uplifting of the area and favors the deposition of coarse grain material on a beachor tombolo-like environment. Subsequent debris slide and collapse of the wavecut notch formed the colluvial layers of the site.