Introduction to Tolo Channel
Tolo Channel is a graben with water filled on
it. The both sides of it, which are very straight, are resulted from several
normal faults. The mountains of channel are consisted of rocks formed in Devonian (400 million years ago), Jurassic (180 million years ago)
and Cretaceous (120 million years ago). Sedimentary rocks formed during
Devonian and early Jurassic firstly, and then tectonic subduction happened and
caused volcanic eruption, which resulted in igneous rock intrusive. Thus, green
volcanic tuff can be observed at the bottom of the Portland Island. After
igneous intrusion, sedimentary rocks deposed during Cretaceous and create the
unconformity (see sketch of Portland Island). Combination of several normal
faults within Devonian and Jurassic rocks formed the graben, known as the Tolo
Channel now, and along the Tolo Channel, unconformity can be recognized at Pat
Sin Leng and Ma On Shan, where Cretaceous rocks met Jurassic rocks and Jurassic
rocks met Devonian rocks, respectively.
1.1 Location of
Stratigraphy: Early and middle Jurassic sedimentary rocks.
Distribution: Confined to small
exposures on both sides of Tolo Channel and south of Yuen Long, around the
foothills of Tai Tong.
Lithology: The Tolo Channel
Formation is made of grey laminated siltstone with intermeddled fossiliferous
Environment: A shallow marine, sub
littoral environment of deposition is inferred for the Tolo Channel Formation.
Apart from sandstones, pebbly sandstones and
conglomerates of Devonian age exposed along the north-western shoreline of the
Tolo Channel, rocks visible on Ma Shi Chau are also representatives of
Palaeozoic sedimentary strata found in Hong Kong. These rocks, which are called
the Tolo Harbour Formation, are of Permian age.
They are predominantly exposed around Tolo Harbour and adjacent areas,
including Central Island, Ma Shi Chau, The Chinese University of Hong Kong as
well as the western coast of Three Fathoms Cove. Also, Ma Shi Chau is the
typical locality of the Tolo Harbour Formation. The rocks are well exposed on
the island, comprising a sequence of rocks, including mudstone, sandstone and
siltstone, which have been folded by earth movements.
Formation of Tolo Channel
The Tolo Channel Formation is mainly composed of
light-colored siltstones with rare sandy lenses alternating with thinly bedded
fossiliferous black mudstone containing pyritous nodules. At the
type locality, 1 km northeast of Fung Wong Wat, the strata dip reasonably (c.
60o) to the
northwest although fossil evidence suggests they’re overturned.
The formation has an uncovered thickness of 75 m and is faulted
at its base in opposition to the Bluff Head Formation.
The top of the formation isn’t uncovered.
At Nai Chung, the Tolo Channel Formation strata are highly
contorted and comprise highly cleaved, coarsely laminated siltstone alternating
with finely laminated fossiliferous mudstone .
2.3 Formation of
1.) South Coast of Pak Kok Shan.
The Tolo Channel Formation plants out within the intertidal area of a headland on the south coast of Pak Kok Shan. The formation
is seventy five m thick
and consists of thinly bedded
black mudstone with gray to dark gray siltstone
containing pyrites nodules. The strata dip northwest at 60o howeverfossil proof suggests that they may be overturned. A quartzphyric rhyolite dyke,
trending northeast, intrudes the succession parallel to the coast. at
the south side of the dyke, black mudstones with overwhelmed and silicified sandstone are exposed over a width of 5 m and that they make biggeroffshore.
2) Lo Wu Wat.
South of To Tau Tsui, the Tolo Channel
Formation is uncovered at low tide in
a slimoutcrop, most
effective 10 m extensive, at the foreshore alongside the north coast of Chek Mun Hoi Hap (Tolo
Channel). The thinly bedded siltstone with sandstone is dark gray whilst clean, however is yellowish to reddish brown wherein it is weathered.
3) Ma Shi Chau.
The Tolo Channel Formation is exposed at the foreshore on
the southern tip of Ma Shi Chau, wherein it includes pinkish-brown weathered, laminated, micaceous siltstones, dipping
steeply to the west. approximately 150 m of strata are estimated, however the contact with the Permian strata to the southeast is
faulted. The strata are unconformable overlain to the northwest by
means of sedimentary breccia that forms the bottom of
the Tsuen Wan Volcanic institution.
The outcrop has yielded an ammonite fossil indicating a Jurassic age. apart
from their steep dip, thesestrata are typically handiest weakly
deformed, in marked evaluation to
the strata of the adjoiningcrop of
Tolo Harbour Formation.
4) Nai Chung.
At the quay at Nai Chung relatively contorted, dark and pale gray siltstones and
mudstones appear to underlie
heterolithic conglomerates which crop out on the ridge at once to the west. The siltstones and mudstones are mentioned to contain fossils of Jurassic age.
discovery via of an ammonite in sandstone at an outcrop south of Yuen longconfirmed that this location carries lower Jurassic strata. The sandstone mapped close
to Tai Tong, 2 km south of Yuen lengthy, is now therefore labeled as belonging to
the Tolo Channel Formation.
Introduction to Port Island
located at the inlet where Mirs Bay enters Tolo Channel consists of volcanic
rock, tuff, formed in Jurassic and sedimentary rocks formed in Cretaceous.
Sedimentary rocks consisted of siltstone, sandstone and conglomerate, from top
to bottom, and its bedding plane are tilted about 30 degrees above the ground.
A normal fault can be found at the middle of the island. Tectonic joints and
Stress Relief joints are easily to be found on the rocks and it is observed
that rocks with joints were weathered more seriously due to the water
penetration into joints.On Port island , there are brilliant, unique and unique
maroon conglomerate and siltstone gifts. The waters of this area are Hong
Kong’s “Red Sea.” From Wong Chuk Kok Hoi, Hong Shimen, through the
Chu Shek Bay and Cheung Tsui to the Red Platoon.
of Red Earth and Red Sand is called “Danxia Landform” in mainland
China. Examples include Danxia Mountain in Guangdong Province, Wuyishan
Mountain in Fujian Province, Dragon Mountain in Jiangxi Province, etc. Red Island
is the first representative of Hong Kong. Other Redstone attractions include
Bow Island, Ap Lei Chau and “Hong Kong Red Sea”. The most fascinating
of Chocho is its rock and red geology. There are maroon conglomerates and
siltstone on the island, and “red” is a deserved reputation.
What is mystery
behind the Red rock cinnamon ? To put it plainly is a geological phenomenon.
After more than 6 million years of violent magmatic volcanism in Hong Kong,
most places have also become igneous rocks. After the volcanic activity
subsided, there was a long dry period and the climate was extremely hot. Due to
the significant temperature difference between day and night, the rocks in the
mountains weathered avalanches and deposited rapidly. The weathered gravel and
sand alluvial to the lowlands with seasonal heavy rains. The iron-bearing
minerals in the sediments turn into red iron oxide, which binds sediments such
as grit to form red sediments of unique color. In the hot environment, the
oxidation is particularly strong, the gravel in the release of iron, gravel and
sand mixed with varying shades of red or rust color. These alluvial plains and
river sediments form the red or variegated land-clastic sedimentary landscape can
be seen today in Chocho. The composition is dominated by conglomerates,
sandstones and siltstones and is called the “Red Chau Formation.”
Figure 2.4 Port Island
2.4 Port Island
two main facies were recognized inside
the Port Island Formation: a channel facies and a
floodplain facies. The channel facies is dominant and is represented with the aid of crucial and minor
channels. essential channel
infills are among 3 and
6 m thick, and include reddish to
pinkish brown, clast-supported conglomerate, and subordinate sandy conglomerate
and pebbly sandstone. Trough bypass bedding is the
primary sedimentary shape. man
or woman go bedding sets rangefrom 0.three to
three m thick, and
have a one
of a kind concave-upward base. Paleocurrents measured from circulate bedding units suggest flows in
the direction of the east southeast. Minor channels commonly varietyfrom 2
to 4 m thick and
they generally occur one above a
addition they frequentlyoverlie essential channels. The
minor channels are infilled with reddish to pinkish brown medium- to very
coarse-grained sandstone, and in
reality minor conglomerate. Small-scale cross bedding and
faint parallel lamination may
be gift, but in extensive,sedimentary structures are unusual. each minor channel carries bedding that usually dips at a low angle(<10º). The sandstone beds are laterally non-forestall, range from zero.1 to 0.4 m thick and oftentaper or thin down-dip. The floodplain facies include three fundamental sorts of deposit: sandy siltstone, siltstone with calcrete, and sheet sandstone. Sandy siltstone office work bed as plenty as zero.eight m thick, composed of reddish brown, coarse-grained siltstone to 7339ff1fc90882f8f31ca1efdd2ac191- to medium-grained sandstone. The sandy siltstone commonly consists of sand-stuffed cylindrical burrows and, in locations, raredessication cracks.Siltstone with calcrete is quite unusual and consists of horizons of reddish brown structureless siltstone and sandy siltstone, up to 20% of which includes white calcium carbonate nodules. the ones may alsoadditionally occur as peculiar spheres, filaments and rods. Sheet sandstones are as much as 0.2 m thick. They include reddish brown, medium-grained sandstone, forming laterally non-prevent beds with sharp bases.