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 Trcv
Unit name Volcanic rocks of the Chapin Peak Formation
Description Basaltic volcanic rocks gradationally overlie sedimentary rocks. Sedimentary rocks range up to 3 feet thick; pillow flows average 10 feet thick. The volcanic rocks consist of deuterically altered basaltic and minor andesitic pillow flows, pillow breccia, and limestone and aquagene tuff-cemented volcanic breccia. The rocks weather dark green and dark brown; fresh surfaces are dark gray to purplish gray. Flows and clasts are fine-grained, and have relict intersertal, trachytic, diabasic, and rarely, vitroclastic and porphyritic texture. Minerals include plagioclase, augite, calcite, chlorite, epidote, quartz, green hornblende, and prehnite(?). Sedimentary rocks include limestone-cemented granule to boulder conglomerate containing clasts of limestone, basalt, siltstone, argillite, and minor metamorphic rocks and amphibolite. Also dark gray siltstone, argillite, and brownish-gray silty limestone.
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