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 Pzpt
Unit name Metatuff member of the Puppets Formation
Description Metatuff member estimated to be about 150 feet thick. Metatuff has microbreccia to coarse blocky fragmental texture, is locally massive and locally foliated, has variable content of potassium feldspar, and is deuterically and hydrothermally altered. Tuff is maroon, pink, purple, red, green, and brown and contains fragments of trondhjemite, metamorphic rocks, shard-shaped fragments, pumice, feldspar porphyry, quartz, and albite. Groundmass is an aphanitic microclastic to microporphyritic aggregate of K-feldspar, quartz, albite, hematite, sericite, and dolomite. Chemistry suggests some of tuff has mafic protolith. Silica-alkali-lime ratio indicates metatuff is mainly latite and rhyodacite with subordinate rhyolite.
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