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Source map Karl, S.M., and Baichtal, James, 2013, Unpublished data.
Symbol Trhv
Unit name Hyd Group, volcanics
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 on Admiralty Island. 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. In Glacier Creek area, pillow basalt, locally vesicular, locally has calcite-filled amygdules. Aphanitic, locally porphyritic. Includes flows, agglomerate, and volcanic breccia. Contains thin limestone beds, and hydrothermal deposits of jasper and magnetite. Massive, dark green, blocky andesite ranges to well-foliated andesitic tuff. Associated rhyolite or dacite tuff, and black argillaceous limestone and slate. In Keku Straits, Hound Island volcanics, consist of basaltic breccia and pillow lava flows, andesitic volcanic breccia, aquagene tuff, tuff breccia, minor thin-bedded limestone. In Duncan Canal includes massive greenstone, pillow greenstone, pillow breccia, and volcanic breccia, with lenses of mafic to intermediate tuff, felsic tuff with quartz crystals, limestone, conglomerate, sandstone, argillite, and rare bedded chert. Locally greenstone is augite and/or hornblende phyric. Commonly pyritic. Weathers reddish brown. On Gravina Island, includes Chapin Peak Formation, which Consists chiefly of marine basaltic volcanic rocks and subordinate intertonguing sedimentary rocks that crop out mainly on western Gravina Island. Characterized by basaltic pillow flows, calcareous agglomerate and volcanic breccia, and aquagene tuff. The volcanic and volcaniclastic rocks intertongue gradationally with limestone and calcareous clastic sedimentary rocks that are much less abundant and that occur mainly near the base of the formation. 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.Secondary minerals include chlorite and epidote. Mineralogy and textures suggest either widespread deuteric alteration or prehnite-pumpellyite facies metamorphism. The Chapin Peak Formation is at the top of the section, the base of the Triassic section is the Nehenta volcanic member (Berg, 1973), consisting of 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 in diameter, 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