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Pan Long Wan Formation - Krp
The recently defined Pan Long Wan Formation (Table
5.6), which is up to 420 m thick, crops out mainly to the south
of Siu Chung Lam Wan on the Clear Water Bay Peninsula (Figure
5.16). It incorporates the Tai Miu Wan Member and the lower
part of the Clear Water Bay Formation, as previously recognized
by Strange
et al., 1990. The formation probably extends
for some distance beneath the seabed to the north of Siu Chung Lan
Wan, but is not seen in exposure and probably diminishes rapidly
in thickness in this direction. The formation comprises a basal
trachydacite lava (Tai Miu Wan Member), and units of fine ash tuff,
up to 140 m thick but generally <50 m thick, separating further
trachydacite lavas (Figure
5.23).
The Tai Miu Wan Member (Strange et al., 1990), which is up to 150 m thick, is
a distinctive bluish grey trachydacite lava with finely laminated
flow banding and locally large hexagonal cooling columns. Characteristic
feldspar phenocrysts, set in a very fine-grained groundmass, are
common whereas quartz phenocrysts are rare. The member is thickest
in the vicinity of the easterly-striking Tin Ha Shan Fault, towards
the southern end of the Clear Water Bay Peninsula, and diminishes
rapidly in thickness towards the north.
The fine ash tuff units of the Pan Long Wan Formation, which are
weakly eutaxitic and contain small angular aphanitic clasts, decrease
in thickness from south to north. Towards the north end of its outcrop,
the formation mainly comprises dark bluish grey, very fine-grained
lava with scattered small euhedral feldspar crystals. The lava is
flow banded and locally flow folded in the north (Plate
5.30), whereas in the south it is mostly autobrecciated.
The formation has not yet been dated radiometrically, but is included
as part of the Repulse Bay Volcanic Group on the basis of whole-rock
geochemistry (Campbell
& Sewell, 1998).
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