Stratigraphy and optically-stimulated luminescence dating of sub-aerially exposed Quaternary deposits from two shallow bay in Hong Kong
Episodes of subaerial exposure recorded in Quaternary sedimentary sequences on continental shelves are indicators of sea-level change. In this study, the Quaternary superficial deposits from two shallow bays in Hong Kong are examined in order to identify the timing of such episodes. Two rotary boreholes in Tai O Bay penetrated a succession of marine and terrestrial deposits formed during the last four interglacial-glacial cycles. The interglacial periods and the glacial periods are found to be represented by siliciclastic-dominated shallow marine–estuarine deposits and colluvial–alluvial fan deposits, respectively. An excavation of a coastal land reclamation in Deep Bay revealed Holocene intertidal–subtidal estuarine deposits overlying aeolian deposits of the last glacial age. Optically stimulated luminescence dating carried out has confirmed the last glacial age of the terrestrial unit unconformably overlain by the Holocene marine unit but only minimum ages can be obtained from the pre-last glacial terrestrial units. In one of the Tai O boreholes studied, the density and moisture content distribution is found to reflect the episodes of subaerial exposure of the pre-Holocene interglacial marine units. In the palaeo-desiccated crusts of these units, density increases through iron cementation resulting from acid-sulphate soil development while moisture content decreases as a consequence of desiccation. Quaternary terrestrial deposits occurring on continental shelves are therefore datable to a greater degree of certainty than their subaerial counterparts.
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