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 Berg, H.C., 1973, Geology of Gravina Island: U.S. Geological Survey Bulletin 1373, 41 p., 1 plate, scale 1:63,360.
Symbol Trnv
Unit name Volcanic member of the Nehenta Formation
Description Altered, partly spilitic, basaltic and minor andesitic massive flows, pillowed flows, pillow breccia, and subordinate calcareous tuff and tuffaceous limestone. Pillows up to 8 feet in diameter, usually about 1 foot, with interpillow limestone. Dark green, fine-grained, with relict porphyritic, intersertal, and trachytic textures. Phenocrysts are clinopyroxene and sericitized plagioclase, in a groundmass of altered plagioclase, epidote-clinozoisite, calcite, and potassium feldspar, with common amygdules filled with calcite-chlorite and quartz. some tuff has sufficient K-feldspar to be classified as dacite. Locally includes thin beds of slate, siltstone, graywacke, calcareous grit, and conglomerate. Unconformably overlies Paleozoic rocks and underlies limestone with Norian Halobia.
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