Hyd Group igneous rocks, undivided

Unit symbol: Trhgv
Age range Upper Triassic (247.2 to 208.5 Ma)
Lithology: Sedimentary
Group name: Hyd Group, undivided
Basaltic pillow flows, pillow breccia, flow-banded rhyolite, and rhyolitic tuff, which intertongue with calcareous interbeds, flow breccia, banded ash-flow tuff, and subordinate andesitic breccia and aquagene tuff. Locally, unit consists of massive greenstone, pillow greenstone, pillow breccia, and volcanic breccia that contain lenses of mafic to intermediate tuff, felsic tuff that has quartz crystals, as well as limestone, conglomerate, sandstone, argillite, and rare bedded chert. Amygdaloidal basalt flows that contain calcite-, chlorite-, and chalcedony-filled amygdules are locally present. Rocks in most areas are moderately metamorphosed and deformed and may be altered, bleached, or spilitic and contain zones of hydrothermal alteration. Locally, greenstone is augite and (or) hornblende phyric and commonly pyritic (Karl and others, 1999). Unit consists of rhyolite on Annette Island (Berg, 1982; Gehrels and others, 1987), rhyolite of the Puppets Formation and basalt of the Chapin Peak Formation on Gravina Island (Berg, 1982; Berg and others, 1978, 1988; Gehrels and others, 1986, 1987); and rhyolite and subordinate basalt of the Keku Volcanics and basalt and andesite of the Hound Island Volcanics in Keku Strait (Muffler, 1967; Brew and others, 1984). Unit also includes small metagabbro bodies in the Petersburg quadrangle (Karl and others, 1999)

Source map information

Source map Karl, S.M., Haeussler, P.J., Himmelberg, G.R., and Brew, D.A., 2002, Unpublished data.
Symbol Trhv
Unit name Hyd Group volcanic rocks
Description Dark green fine-grained basalt with plagioclase and augite phenocrysts, locally in felted plagioclase groundmass. Pillow forms are common and well-exposed in Windfall Harbor and Gambier Bay. Mafic volcanic rocks include interpillow limestone and volcanic breccia. Locally flows are amygdaloidal with calcite, chlorite, and chalcedony amygdule fill. Locally altered, bleached, or spillitic. Locally as thick as 1000 m.
Lithology Igneous

Correlated geologic units

Label Trhv
Description Hyd Group, including Chapin Peak Formation marine volcanic rocks and subordinate calcareous sedimentary rocks
Geologic age Carnian to Norian
Geologic setting Extrusive
Lithology Form Importance
Basalt < Mafic-volcanic < Volcanic < Igneous Flow, pillows Major
Andesite < Mafic-volcanic < Volcanic < Igneous Volcaniclastic, volcanic breccia Indeterminate, major
Limestone < Carbonate < Sedimentary Bed Minor