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The Geology of Hong Kong (Interactive On-line)
Tuen Mun Formation - Ju

Volcanic rocks thought to be pre-Middle Jurassic, are restricted to the northwest of the New Territories where they outcrop to the west and north of Tuen Mun (Figure 5.7). Previously, on 1:20 000-scale geological map Sheets 5 and 6 and in Memoir 3, two formations were recognized: the Tsing Shan Formation in the west and the Tuen Mun Formation in the east. However, on the basis of extensive new borehole data from poorly exposed parts of the Tuen Mun area, it is proposed here to rationalize the lithostratigraphy into one formation only, the Tuen Mun Formation.

The Tuen Mun Formation is fault bounded and commonly strongly foliated suggesting that it may have been tectonically emplaced as a single block. Ar–Ar dating of a small rock fragment and a single amphibole crystal from the Tuen Mun Formation have indicated ages of 181 ± 3 Ma and 182 ± 35 Ma respectively. However, this interpretation is complicated by a strong thermal overprint (Evensen & York, 1998) so that significant uncertainty remains over the age of formation.

The lower part of the Tuen Mun Formation comprises a largely epiclastic and volcaniclastic sequence (Figure 5.8). In the west, fine-grained, cross-bedded, well-graded quartzitic sandstone, metasiltstone and phyllite predominate, with subordinate tuff, tuffite and conglomerate. In the east, tuff and tuffite are more pronounced and the sandstones and siltstones include a minor volcaniclastic component. A silica-rich detrital source is therefore inferred for the lower part of the formation, with fluvial deposition localized in narrow basins or channels. Some volcaniclastic material was derived from distant or small volume andesitic stratovolcanoes.

The upper part of the formation (>2000 m thick) is mainly composed of a sequence of andesitic lavas (Plate 5.17) with some interbedded lapilli-bearing ash crystal tuffs (Figure 5.8). These indicate that, compared with the lower part of the formation, the volcanic sources were either nearer, or that their activity had increased. There may be an unconformity at the base of this upper part of the formation. This is marked locally by conglomerates and breccias, but the contact is also faulted in places. A source vent has been inferred for some of the lavas. This lies to the north of Tuen Mun, where marble and other Palaeozoic clasts also occur in the lavas.