Download Self-guided tour to Sai Kung

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) History of High Island Reservoir and geographical factors considered during the
construction
(2) Physiography and Geology of Sai Kung
(3) Coastal landforms
General Physiography and Geology of Sai Kung
Sai Kung area is a hilly
terrain deprived of lowlands
(Fig. 1), indicative of a
submerged landscape with
dismembered rivers and
drowned
valleys.
The
original topography of the
area prior to the construction
of the reservoir (Fig. 2)
shows a long and narrow
waterway trending in a
WNW-ESE direction with
rivers entering at right
angles,
reflecting
a
rectangular drainage system
developed along existing
geological structures.
Fig. 1 3D view of Sai Kung area derived from
SRTM data and processed with Global mapper
The High Island area in Sai Kung
Peninsula is occupied predominantly
by volcanic rock formations formed
during the Jurassic-Cretaceous time.
During this ‘Yenshanian Movement’,
northward subduction of a plate under
South China formed an island arc in
the coastal province of South China,
causing extensive intrusion and
extrusion of acid rocks. Some of the
volcanic rocks formed by such violent
eruption can be seen at Sai Kung.
Fig. 2 High Island area in 1964 before construction of reservoir
Sai Kung area was probably a caldera,
a volcanic depression formed by
subsidence of the ground as the
magma chamber below was drained,
during the Jurassic Period. Volcanic
centers were present around the
caldera. Thick layers of volcanic
ashes, lavas and tephra were formed
on the surface during several eruption
episodes. These were ‘Plinian’ type
eruptions, which can last up to a few
weeks. A sequence of ash over 100 m
thick was deposited within the caldera.
The major rock types in Sai Kung are
pyroclastic rocks including tuff and
volcanic breccias, rhyolite, and
sedimentary rocks such conglomerate,
sandstone and mudstone.
Fig. 3 Convergent tectonic setting of Hong Kong area during
the Jurassic time
Fig. 4 Jointed columns with mafic dike intruded along a kind-band
in east dam area
Fig. 5 Toppled rock columns near Pak Lap
Spectacular columnar joints and
kink bands were formed within the
ash layer, seemingly only shortly
after the volcanic eruption. These
columns are best exposed in the
East Dam area, Ninepin Islands
and on sea cliffs on several outlying
islands. The jointed columns vary in
geometry and size, with the majority
of the columns being 5- or 6-sided,
the ideal geometry for regularly
spaced cooling centers. A careful
examination of the jointed columns
shows that they are occasionally
bent, with ‘kink-bands’ inclining
consistent at about 40o to the
southeast (Fig. 4). The formation of
the joint patterns and the
significance of the kink-banding
will be discussed on the trip.
Toppling failures are commonly
observed in the jointed columns
(Fig. 5). The last phase of the
volcanic activities is represented by
intrusion of mafic dikes, sometimes
along pre-existing kink bands in the
jointed rocks.
The post-Cretaceous tectonics is
mainly characterized by faulting
events. We now know that during the Cenozoic, the coastal province of South China underwent
several phases of extension including at least a phase of N-S extension and a phase of NE-SW
extension. The multiple sets of fault and lineaments found in the Sai Kung area are associated
with these extensional events. The current tectonic movement is controlled by a E-W directed
compression, associated with the collision of the Philippines with Eurasia along Taiwan.
Coastal Landforms
Remarkable erosional landforms can be observed along the coast of Sai Kung Peninsula. The
distribution of beaches and headlands reflects the energy conditions. Erosional landforms
including sea caves, arches and sea stacks often develop along pre-existing lines of weakness.
Depositional features are confined to coves and small bays.
The many islands in Sai Kung Sea were once
peaks of a hilly region during the last Ice Age.
Global climatic warming since about 12000 year
ago caused the sea level to rise, drowning the lowlying areas. The irregular and jagged inlets and
bays in Sai Kung were conceivably once a valley
system during the Ice Ages. The great water
depths around the islands and rivers without fully
developed lower course morphology are evidence
for the drowned landscape.
The islands in Sai Kung are marked by the
presence of numerous sea caves and arches.
Among the more picturesque and notable ones are
the Tiu Chung Chau Twin Cave, Basalt Island
Cave, Sha Tong Hou Cave and Muk Min Cave.
Most of these sea caves and arches developed
along pre-existing fractures and faults, along
which wave erosion and removal of material is
intensified in rocks weakened by the fractures and
faults. The best example is illustrated in Muk Min
Fig. 6 Two faults can be observed to control the
Cave where two faults controlling the
development of Muk Min Cave near Pak Lap
development of the cave can be readily observed
(Fig. 6).
A sand bar, called tombolo, connecting the small island to Sharp Island formed as water
current flow is retarded; the reduction in current velocity causes sediments in the currents to be
deposited. The tombolo, like several others in Sai Kung area, is probably part of a ridge
connecting the two peaks.
High Island Reservoir Project
The construction of the High Island Reservoir commenced in 1971 in an attempt to alleviate the
water supply problem of Hong Kong. The reservoir has a capacity of 280 million cubic meters.
Major geotechnical works to build the reservoir included the construction of the east main dam,
now standing at 64m above sea level, the cofferdam and the outer layer of concrete dolosse to
attenuate wave action. A few villages, Lan Nai Wan in particular, were submerged as the
reservoir was filled.
Resources
Hong Kong Geological Survey http://www.cedd.gov.hk/eng/about/organisation/int.htm
Water Supplies Department http://www.wsd.gov.hk/en/text/edu/fish/hi_rev.htm
Recommended stops for the field trip:
STOP 1
STOP 2
STOP 3
STOP 4
West Dam & Yuen Ng Fan
East Dam
Trail towards Mok Min Cave from Pak Lap
Muk Min Cave
Major legends
QHH
Quaternary
marine mud
ms
Quaternary
marine sand
JHI
JurassicCretaceous
fine ash tuff
JCB
JurassicCretaceous
trachydacite &
rhyolite lava
1
2
Entrance to Pak Lap
3
4
Assignments:
1. Are all the polygonal surfaces are in hexagonal shapes? If no, why?
2. Measure the diameters of the polygonal surfaces at Stops 2 and 4 and summarize the
measurement in a table.
3. What do the difference of size and shape tell you about the cooling history of these
volcanic rocks in Sai Kung?
4. Describe different kinds of dams built at Stop 1 (West Dam) and Stop 2 (East Dam).
What are the purposes of these dams? Summarize their features in a table.