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 Redman, E.C., Gilbert, W.G., Jones, B.K., Rosenkrans, D.S., and Hickok, B.D., 1985, Preliminary bedrock-geologic map of the Skagway B-4 quadrangle: Alaska Division of Geological and Geophysical Surveys Report of Investigations 85-6, 1 sheet, scale 1:40,000.
Symbol uPzvb
Unit name Basalt
Description Basalt, rarely shows signs of metamorphism. Pillows, vesicles, calcite-filled amygdules. Aphanitic flows and agglomerate. Contains replacement pods of jasper and magnetite, and thin limestone beds. Glacier Creek area.
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