Table 5.6 - Field and petrographic characteristics of formations belonging to the Repulse Bay Volcanic Group


'Rhyolitic' Subgroup

UNIT Mount Davis Formation Long Harbour Formation
Colour light grey to grey light grey to pinkish grey
Composition rhyodacite to rhyolite rhyodacite to rhyolite
Lithologies crystal tuff and tuffite crystal tuff and rare eutaxite
Dominant texture lapilli-bearing coarse ash lapilli-bearing coarse ash
Crystals feldspar, quartz and biotite pink feldspar, quartz and biotite
Lithics lava and tuff tuff and rare granite
Glass/Pumice rare rare
Fabric massive massive to crudely bedded
Thickness 950 m >600 m

'Trachytic' Subgroup

UNIT Ngo Mei Chau Formation Ap Lei Chau Formation Che Kwu Shan Formation Mang Kung Uk Formation Pan Long Wan Formation
Colour pale bluish grey pale grey pale grey black to brown bluish grey
Composition trachyte to rhyolite trachyte to rhyolite trachyte to rhyolite trachydacite to rhyolite trachydacite to rhyolite
Lithologies vitric tuff, tuffite, limestone, lava vitric tuff, tuff-breccia, tuffite vitric tuff, tuff-breccia crystal tuff, tuffite, epiclastic breccia lava, vitric tuff
Dominant texture welded fine ash welded fine ash welded fine ash fine ash porphyritic and fine ash
Crystals quartz and feldspar quartz and feldspar quartz and feldspar feldspar and quartz feldspar and quartz
Lithics - - - rare accretionary lapilli -
Glass/Pumice common common common rare rare
Fabric ± eutaxite ± eutaxite ± eutaxite well-bedded ± eutaxite and flow-banded
Thickness >450 m >1250 m 500 m 300 m 420 m