Download Self-guided tour to Ma Shi Chau

EASC1XXX Introduction to Hong Kong Geology
Self-guided Field Trip to Ping Chau, Mirs Bay
Department of Earth Sciences, The University of Hong Kong
Objectives
This fieldtrip will focus on the following topics:
1. Geological structures
2. Geological history of Ma Shi Chau
3. Coastal landform
General geology of Sai Kung
Ma Shi Chau is located in the middle of Tolo Harbour (Figs. 1 and 2). It is an
island of an area of 1.5 km x 500m, connected with Yim Tin Tsai with a sand bar.
Fig. 1 Geological map of Hong Kong, with Ma Shi Chau indicated in a red box.
Tombolo
Figure 2 (a) Geological map showing Ma Shi Chau and Yim Tin Tsai.
Figure 2 (b) A key indicating the rock types and geochoronological
ages of different rock units.
Regional setting
During Permian period (299-251 Ma), the area of present Ma Shi Chau was part
of a shallow water basin on a continental edge. Sediments or rock fragments from
surrounding land were transported and deposited in the basin, accumulating layers of
sediments. Under process of lithification (compaction and cementation), the
sedimentary rocks were formed. Variation of sea level changes depositional
environment as well as the current motion. Relative strong current (e.g. river, coastal
current) can carry coarser sandy sediments while relative weak current or quiet
depositional environment (e.g. delta area, lake) can only transport fine sediments.
Gradual increase in grain sizes of the Permian sedimentary rock in Ma Shi Chau from
southeastern part to northern part of the island indicates variation of sea level as well
as depositional environment in Ma Shi Chau area in the past.
During Jurassic period (201-146 Ma), vigorous volcanic activities happened in
the northeast of Hong Kong, with volcanic eruptions and magma intrusions in the area
(Li, 2007). Granitic rocks (cooled down from magma intrusion) are found in the
northern side of Yim Tin Tsai and volcanic sediments are presented on western and
northwestern part of the Ma Shi Chau (Li, 2007). Major crustal movement following
the intrusions and volcanic activities led to the formations of folds, faults on the island
(Li, 2007).
Figure 3 A tombolo connecting Yim Tin Tsai and Ma Shi
Chau.
Figure 4 Permian reddish siltstone in Ma Shi Chau.
Tombolo
As a result of longshore drift, a tombolo, or a
sand bar, is formed, attaching Ma Shi Chau to
Yim Tin Tsai (Figures 2 and 3). The tombolo
was once under water during high tide and
exposed during low tide. For a convenient
pathway to Ma Shi Chau, villagers hauled
boulders and sediments along the tombolo,
resulting in a well-exposed sand-bar even
during high tide.
Sedimentary rocks of Ma Shi Chau
The sedimentary outcrop of Ma Shi Chau
was believed to be formed at Permian
(Figure 4). Marine fossils such as malluscs,
corals, bryozoans, brachiopods, crinoids,
and some plant fossils were found in the
sedimentary layers and used to determine
the approximate age of the sedimentary
rocks (Lee et al., 1997).
Quartz vein
Networks of white veins are found in
outcrops of Ma Shi Chau. When fluid
passed through the tiny fissures of
outcrops, minerals were deposited from
the solution along the fissures, forming
these vein-like structures. In Ma Shi
Chau, most of the veins are quartz
veins which are relatively more
resistant to weathering (Figure 5).
Figure 5 Network of quartz veins in Ma Shi Chau.
A fault
Ma Shi Chau has experienced complicated
tectonic movement, resulting in a series of
folds and faults. A fault (Figure 6) is a
planar crack in a rock, along which relative
displacement of opposite sides of the fault
occurs. It is caused by compressional or
tensional events during earth movement:
Figure 6 A normal fault with the hanging wall (right) sliding down
relatively to the footwall (left).
Normal fault indicates an extensional
event, with the hanging wall sliding down
relatively to the footwall (Figure 7a).
Reverse fault indicates a compressional
event, with the hanging wall moving up
relatively to the footwall (Figure 7b).
Strike-slip
fault
shows
a
(a)
(b)
(c)
(d)
parallel
displacement of the opposite sides, without
vertical motion (Figure 7c-d).
Figure 7 Faults and the corresponding motions: (a) A normal fault with
horizontal extension. (b) A reverse fault with horizontal compression.
(c-d) Strike-slip faults with horizontal parallel motions
(Credit: http://cdn.readtapestry.com/stories/lVt8rn40G/index.html).
A fold
Fold is a form of deformation in which
original horizontal layers are bent or
curved. Figure 8 shows a fold with one
limb parallel to the coastal line while
another running towards the sea. The
corner at which the limbs change
direction is the hinge. The horizontal
layers was once folded and later tilted
at an angle from the horizon. The hinge
line is described to plunge to a direction
Hingeline
Limb
Figure 8 A plunging fold at the coast of Ma Shi Chau.
and the fold is thus called plunging
fold.
Sausage structure
This shows the way in which different rock
types deform in response to pressure
differently. The discontinuous brown layer
was originally a single layer, interbedded
with the white layers. When these layers
were compressed, the brownish harder and
more brittle layer broke into elongated and
discontinuous blocks, while the whitish
softer layer “flowed” and filled the gaps
between the blocks. These discontinuous
blocks are known as sausage structures.
Figure 9 Brownish sausage structures in softer whitish layers.
References:
Lee, C.M., Chan, K.W., Ho, K.H., 1997. Palaeontology and stratigraphy of Hong Kong vol. 1.
Beijing: Science Press, 206 p. plus 57 plates (In Chinese).
Li, X.C., 2007. Permian Sedimentary Rocks – Tolo Harbour Formation. In: Hong Kong Geology
Guide Book. Hong Kong: Geotechnical Engineering Office Civil Engineering and Development
Department.
Geological time-scale. Geological Society of Hong Kong.
Assignments:
1. Measure 50 dip/srike measurements along the southeast coastal areas of Ma Shi
Chau. Make sure that the measurements are at least 2-m apart and at different
locations.
2. Mark the measurements on a topographic map (Attachment 1) and interpret the
possible structures (e.g. folds, faults) on the map.