Distribution of moisture contents and thermoluminescence ages in an inner shelf borehole from the new Hong Kong International Airport site, China

Abstract

Offshore boreholes and excavations available through coastal engineering projects in the siliciclastics-dominated inner shelf of the northern South China Sea off Hong Kong have permitted the reconstruction of Quaternary sea-level history dating back to almost 0.5 Ma. In this study, moisture content and thermoluminescence (TL) dating are utilized to help verify ages for marine and terrestrial sediments in a 39.52 m offshore borehole from the new Hong Kong International Airport site. The distribution of moisture contents in borehole samples indicated the presence of three palaeo-desiccated crusts formed by soil development when the marine deposits of Oxygen Isotope (OI) stage 5, OI stage 7 and OI stage 9 were sub-acrially exposed during the low sea-level stand(s) of OI stages 2-4, OI stage 6 and OI stage 8, respectively. Out of the six TL dated borehole samples, three from a terrestrial deposit horizon have yielded finite ages of about 250,000 yr BP suggestive of OI stage 8. In these samples, the TL signal of quartz grains was found to increase well beyond the 250 Gy saturation level with the relatively high radiation fluxes allowing for age determination for only three samples. The three remaining samples of pre-Holocene marine deposits yielded minimum ages each having reached TL saturation. The importance of the moisture content upon the final determination of these sediment ages and the collection and storage of samples for TL dating is clearly demonstrated.