The geological stratigraphy in the northwestern Ma Shi Chau

Abstract

Before the year 2000, the sedimentary and volcano-sedimentary rocks on Ma Shi Chau (MSC) have long been regarded as the formations in Permian and Jurassic respectively. Geotechnical Engineering Office (GEO) had modified the geological model and changed the Northeastern side of MSC, the pre-volcanic sedimentary rocks, to an older rock formation, from Permian Tolo Harbour Formation to Devonian Bluff Head Formation. The reasons of change are mainly from structural evidence and lithological evidence.

But they are still indirect evidences; no fossil was found in the sedimentary sequences at the hinging wall till to the northwestern shore of Ma Shi Chau. Fossils were only found along the southeastern shore of Ma Shi Chau for the footwall of the reverse fault, which are of Permian age.

Through the analysis of the photomicrographs, the Unknown group is difficult to show which formations, the Devonian or Permian is closer to as the Unknown group have similar characteristics of both. For the geochemistry analysis by using different graphs, the patterns of each group are revealed.

From the graphs produced, generally the Unknown group have pattern closer to the Permian Tolo Harbor Formation. But there is still no direct evidence that the Unknown group is within the Permian Tolo Harbor Formation. By the geochemical data it is more likely for the Unknown group to be close the Permian rocks.