Geology of Mo Tat Wan

Abstract

A study has been conducted on the southwest and northeast coasts of Mo Tat Wan on Lamma Island. The purposes of this was to examine the different lithologies of Mo Tat Wan to determine their relative ages, investigate geological structures by examining kinetic indicators to determine the directions of fault movements and to construct the geological evolutions over the area. Field mapping and microscopic analysis was from February, 2016 to acquire the data to produce a set of 1:1,000-scale inferred geological maps detailing the relative age relationship between lithologies and locations of geological structures. Key findings are that the estimated width of quartz monzonite and the configuration basaltic andesite dyke located at the southwest coast of Mo Tat Wan are somewhat different from what have been annotated in the 1:20,000-scale HKGS Geological Map Sheet 15 (GEO, 2010). For the aspects related to geological structures, the nature and characteristics of kinematic indicators reveal that the past deformations were brittle to semi-brittle with both sinistral and dextral movements. A post-shearing emplacement of mafic dyke was noted with the infilling of mafic materials into the tension gashes observed at the southwestern coast of Mo Tat Wan. The “autobrecciation” (Strange and Shaw, 1986) of quartzphyric rhyolite dyke located at the northeastern coast of Mo Tat Wan (approximately 80 m southeast to the Mo Tat Wan Public Ferry Pier) was observed to be about 3 m wide and the brecciation zone with deformed quartz crystal was likely to be associated with a NW-SE striking fault.