Hong Kong Geology Guide Book Geotechnical Engineering Office ...

Microscope view of coarse ash crystal tuff: Cracks
in phenocrysts formed by reduced pressure
Altered entrained clast in coarse ash
crystal tuff: Concentric structures
due to recrystallisation
2) Rock types and textures exposed on the cut slope are typical of those found in the
dyke swarm in this district. Therefore, this exposure provides an opportunity to
examine and appreciate the properties of dyke rocks and intrusive processes.
3) Cross-cutting relationships can also be studied by paying careful attention to
contact zones between the different rock types.
4) Try to prepare a simple geological map of the cut slope, including measurements
of the width and orientation of the different rock types within the dykes. Record
the occurrence of any other structures, and examine variations in rock weathering
in different sections of the cut-slope.
Comments
The main geological feature in the area between northeastern Lantau and Tsing Yi is
the Lantau Dyke Swarm. Based on an interpretation of several geological sections in
the district, three phases of dyke intrusion have been identified. Phases I and II were
rhyolitic dyke intrusions, whereas Phase III comprised intermediate to mafic
intrusions, mainly basaltic andesite. It is possible that the Phases I, II and III dyke
intrusions were the result of differentiation from the same magma source. Typical
exposures of dykes intruded in Phase I can be seen at Sham Shui Kok, whereas dykes
of Phase II are well-developed at Penny’s Bay.
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