1 - HKU Libraries

ashes, in contrast with the coarser Yim Tin Tsai Formation and more vitric Shek Lung KungMember.Around Yi 0, the Shing Mun Formation tuffs are commonly altered, and in places are stronglysilicified and brecciated, as on Kai Kung Shan. West of Yi 0, a pyritiferous, lithic lapilli-bearingash crystal tuff (HK9967, 01550952) contains quartz pyroclasts, up to 2mm, set in aquartz-sericite matrix. The alkali feldspar is replaced by calcite, and the biotite by muscovite andopaque minerals. There are relict lithic fragments in the matrix, and possible pseudomorphs afteramphibole. Adjacent exposures (HK 9966, 0155 0981) comprise dark grey, lithic lapilli-bearing,ash crystal tuff with abundant, ragged to sharp-edged, porphyritic lapilli up to 12 mm long.These tuffs are more typical of the Shek Lung Kung Member; but the intense alteration in thisarea makes it impossible to identify the member with confidence.Volcanic and Sedimentary EnvironmentAs the Shing Mun Formation contains thin, persistent siltstone and sandstone beds it is assumedthat the depositional environment was relatively stable. The sedimentary rocks probably representlahars or mudflows that accumulated in intermontane basins close to volcanic centres. Thetuffaceous content of most of the sediments indicates local erosion of the volcanic sequence.Some lithic clasts in the tuffs have reaction rims, which suggests a high emplacement temperatureand water content.Shek Lung Kung MemberThe Shek Lung Kung Member is broadly equivalent to agglomerate mapped by Allen & Stephens(1971). The lithology is predominantly an ash, to lapilli-ash crystal vitric tuff (Plate 7) thatcharacteristically displays a flow-lineation and sharp-edged porphyritic lapilli. The memberoutcrops only in northeast Lantau Island, but may also be present near Yi 0 in the west. It liesimmediately above Yim Tin Tsai Formation outcrops, although the contact is not exposed.DetailsSiu Ho Wan - Ta Pang Po. Between Siu Ho Wan and Sham Shui Kok, the Shek Lung Kung Member compriseslapilli-ash crystal tuffs. The tuff is poorly exposed, but can be correlated with excellent coastal exposures of themember to the east. It forms narrow, easterly-trending bands of country rock between dykes. Between Sham ShuiKok and Ta Pang Po, the member can be identified with confidence and is characterized by sharp-edged porphyriticlapilli with a linear alignment. There is local hydrothermal alteration of the tuff east of Ta Pang Po, but for themost part it is fresh and unaltered. North of Ta Pang Po, there are uniform lapilli-ash crystal tuffs with somepebble-like lithics. However, there are also sharp-edged porphyritic lapilli, a characteristic feature of the Shek LungKung Member.West of Yam O Wan. For part of the exposed coastline, between Yam Tsai Wan and Ta Pang Po, the tuffs lackthe porphyritic lapilli characteristic of the Shek Lung Kung Member, and are ash crystal tuffs, with prominentmafic minerals. To the east and west, flow-fabric and abundant sharp-edged porphyritic lapilli are typical. At LukKeng Tsuen, the characteristics of the Shek Lung Kung Member are again apparent, with a lineation defined bymafic minerals, and small, angular poiphyritic lapilli. Although apparently scarce, these characteristic lapilli persistto the southwest. In Luk Keng Bay the tuff has a flow-lineation which is sub-horizontal, or plunges gently to thesouthsouthwest. The lithic lapilli are characteristically sharp-edged and porphyritic. On Cheung Sok Tsui, the ShekLung Kung Member consists of a light grey, weathering to light creamish-brown, variably ash to lapilli-ash crystaltuffs. They display a prominent lineation, defined by porphyritic lapilli and laths of biotite, which plunge gently tothe southsouthwest.East of Yam O Wan, At Tai Yam, the tuffs are dominated by porphyritic, aphanitic lapilli, 10 to 30 mm in length.These lapilli define a flow-lineation, exactly as seen at the Shek Lung Kung type locality. The rock is alsocharacterized by its pale, glassy appearance, with well-formed mafic crystals. To the east, the flow-lineation ispresent, but lapilli are not obvious.42