Late Quaternary environmental changes in Hong Kong

Abstract

Abstract The offshore Quaternary deposits of Hong Kong include a wide range of clastic sediments that accumulated in a variety of settings including lacustrine, channel, overbank, coastal and marine environments. Detailed studies of the sedimentology and diatoms of several long, continuously-sampled cores, combined with seismic reflection profiles, has enabled the reconstruction of Late Pleistocene and Holocene palaeoenvironments in Hong Kong. The Quaternary sediments comprise several informal seismic units that are separated by marine planation, tidal scour and fluvial erosion surfaces. Seismic Unit 1 represents pre-Quaternary bedrock and weathered bedrock, which is separated from a complex assemblage of Pleistocene channel, floodplain, backswamp and lacustrine deposits (seismic Unit 2) by a major erosion surface (U1). Sediments that represent the basal part of Unit 2 have been dated at about 248,000 yr B.P. Well preserved laminations at several horizons suggest periodic deposition and probably seasonal flooding. The presence of kaolin indicates strong chemical weathering and warm, humid conditions at the time of deposition. Seismic Unit 3 consists of marine mud that unconformably overlies a fluvial erosion surface (FE1) cut into deposits represented by seismic Unit 2. The sediments comprising Unit 3 are tentatively correlated with oxygen isotope stage 5e and high sea levels at about 128,000–105,000 yr B.P. A marine planation surface (R1) terminates Unit 3 and is overlain by coastal and nearshore deposits comprising seismic Unit 4 at about 95,000 to 80,000 yr B.P. This phase of marine deposition was terminated by a major fall in sea level that caused widespread fluvial incision across Hong Kong and produced surface FE2. A subsequent rise in sea level after about 18,000 yr B.P. led to deposition of a complex succession of marine sediments and erosion surfaces. Initially, homogeneous mud (seismic Unit 5A) was laid down. Tidal currents, perhaps related to base level changes, then caused incision and the formation of surface TS1 at several localities. Further shallow marine mud, comprising Unit 5B, was laid down between about 9500 and 8000 yr B.P. A planar erosion surface (R2), with local palaeosols and minor incision, cuts across earlier deposits. The overlying seismic Unit 5C represents diatomaceous marine mud and a basal sand. At one coring site, these sediments have been divided into five major diatom zones that reflect changes in salinity and sea level. Modern sea level was attained by about 6000 yr B.P. In broad terms, the late Quaternary record is one of two periods of marine flooding separated by a major regressive episode, with no evidence of older Quaternary marine sediments being present.