LATE QUATERNARY ENVIRONMENTAL-CHANGES IN HONG-KONG

The offshore Quaternary deposits of Hong Kong include a wide range of elastic sediments that accumulated in a variety of settings including lacustrine. channel, overbank, coastal and marine environments. Detail ed studies of the sedimentology and diatoms of several long, continuou sly-sampled cores, combined with seismic reflection profiles, has enab led the reconstruction of Late Pleistocene and Holocene palaeoenvironm ents 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 bed rock and weathered bedrock, which is separated from a complex assembla ge of Pleistocene channel. floodplain, backswamp and lacustrine deposi ts (seismic Unit 2) by a major erosion surface (U1). Sediments that re present the basal part of Unit 2 have been dated at about 248.000 yr B .P. Well preserved laminations at several horizons suggest periodic de position and probably seasonal flooding. The presence of kaolin indica tes strong chemical weathering and warm, humid conditions at the time of deposition. Seismic Unit 3 consists of marine mud that unconformabl y overlies a fluvial erosion surface (FE1) cut into deposits represent ed by seismic Unit 2. The sediments comprising Unit 3 are tentatively correlated with oxygen isotope stage 5e and high sea levels at about 1 28,000-105,000 yr B.P. A marine planation surface (R1) terminates Unit 3 and is overlain by coastal and nearshore deposits comprising seismi c Unit 4 at about 95,000 to 80,000 yr B.P. This phase of marine deposi tion was terminated by a major fall in sea level that caused widesprea d fluvial incision across Hong Kong and produced surface FE2. A subseq uent rise in sea level after about 18,000 yr B.P. led to deposition of a complex succession of marine sediments and erosion surfaces. Initia lly, homogeneous mud (seismic Unit 5A) was laid down. Tidal currents, perhaps related to base level changes, then caused incision and the fo rmation of surface TS1 at several localities. Further shallow marine m ud, comprising Unit 5B, was laid down between about 9500 and 8000 yr B .P. A planar erosion surface (R2), with local palaeosols and minor inc ision, cuts across earlier deposits, The overlying seismic Unit 5C rep resents diatomaceous marina mud and a basal sand. At one coring sire, these sediments have been divided into five major diatom zones that re flect 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 o ne of two periods of marine flooding separated by a major regressive e pisode, with no evidence of older Quaternary marine sediments being pr esent. (C) 1998 Elsevier Science B.V.