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 Brew, D.A., Ovenshine, A.T., Karl, S.M., and Hunt, S.J., 1984, Preliminary reconnaissance geologic map of the Petersburg and parts of the Port Alexander and Sumdum 1:250,000 quadrangles, southeastern Alaska: U.S. Geological Survey Open File Report 84-405, 43 p., scale 1:250,000.
Symbol Trk
Unit name Keku Volcanics
Description Keku Volcanics - altered felsic flows and flow breccia; flow-banded, aphanitic, feldspar-microporphyritic; minor mafic flows and flow breccia, volcanic wacke, volcanic conglomerate, green aquagene tuff, and thick-bedded oolitic limestone; lenticular. Unconformably overlies Pybus Fm
Lithology Igneous

Correlated geologic units

Label Trhf
Description Hyd Group felsic volcanic rocks, includes Puppets Formation on Gravina Island
Geologic age Late-Triassic
Geologic setting Extrusive
Lithology Form Importance
Felsic-volcanic < Volcanic < Igneous Flow Major
Felsic-volcanic < Volcanic < Igneous Pyroclastic, tuff Major
Mafic-volcanic < Volcanic < Igneous Volcaniclastic, volcanic breccia Indeterminate, major
Mafic-volcanic < Volcanic < Igneous Flow Minor
Limestone < Carbonate < Sedimentary Bed Incidental
Conglomerate < Clastic < Sedimentary Bed Incidental
Sandstone < Clastic < Sedimentary Bed Incidental