香港東灣的沙堤堆積物及其地史學意義

Abstract

The sand levee with a maximum elevation of +7.4m P.D. and its deposits on the southern coast of Lantau Island at Tung Wan are problematic in their origin. The feature suggests either a former higher sea level, a lower land mass, or just a considerable sand deposits building up over time behind the present beach. By means of field reconnaissance, granometric analysis and archaeological inference, this study contends that whether a sand levee deposit is that of a raised beach has not been proven, nor on the other hand, a storm beach and dune sand origin has been disproven. Under such circumstance, the concept of a previous higher sea level is not supported by field evidence in our study area and is unnecessary in accounting for the sand levee deposits here. The reasons include: 1. A topographic elevation of over +10m P.D. is well within the reach of dune sand deposits. Storm beach deposits associated with present day beaches may have been blown inland by strong wind during typhoon periods to form the levee. 2. A former higher sea level would be expected to leave high level erosional benches or strandlines on the rocky coast as well as marine deposits in the coastal valleys but none are particularly evident in the study area. 3. Textural studies indicated that sand deposits from the levee are obviously different from those from the beach with larger proportion of suspended constituents, better sphericity and lower degree of polishing. 4. Sand deposits associated with archaeological remains have been found in the study site dating back from the present to the early Holocene. Altogether six cultural horizons can be distinguished, the lowest of which is found down to the depth of +2m P.D. Therefore the postulated higher sea level could not have existed and it is more likely for the artifacts to have been buried by dune sand deposits migrating inland.