Hyd Group, undivided

Unit symbol: Trhg
Age range Upper and Middle Triassic (247.2 to 201.3 Ma)
Lithology: Sedimentary
Group name: Hyd Group, undivided
Includes carbonaceous argillite, slate, and subphyllite, tuff, volcaniclastic wacke, conglomerate, mafic flows, and limestone. The volcanic rocks consist of basaltic pillow flows, pillow breccia, and breccia, rhyolitic tuff that has calcareous interbeds, flow breccia, banded ash-flow tuff, and subordinate andesitic breccia and aquagene tuff. Unit ranges from marine mafic volcanic and deep marine sedimentary rocks in northern southeast Alaska to more felsic volcanic rock and shallow-water limestone and conglomerate to the south (Taylor and others, 2008). On Admiralty Island, group includes volcanic breccia and deformed, locally metamorphosed, massive and thick-bedded mafic to intermediate flows that have pillows and columnar jointing (Lathram and others, 1965; Brew and Ford, 1985). Volcanic flows are interbedded with, but mostly overlie gray to black limestone, graywacke, slate or argillite, black chert, and conglomerate (Lathram and others, 1965). In the Petersburg to Ketchikan area, group includes mafic volcanic rocks that generally overlie felsic volcanic rocks that consits of latite to rhyolite flows, aquagene tuff, and tuff breccia (Brew and others, 1984, Berg and others, 1988, Karl and others, 1999). The volcanic rocks are interbedded with and commonly overlie carbonaceous argillite, limestone, and volcaniclastic wacke; all overlie a basal conglomerate or breccia (Brew and others, 1984, Berg and others, 1988, Karl and others, 1999, Taylor and others, 2008). The Hyd Group is 200 to 800 m thick and is thought to have been deposited in a rift basin (Taylor and others, 2008). As shown here, the Hyd Group consists of the Nehenta Formation on Gravina Island near Ketchikan, the Barlow Cove Formation on Admiralty Island (Lathram and others, 1965), basalt of the Chapin Peak Formation and rhyolite of the Puppets Formation (Berg and others, 1988), the Hound Island and Keku Volcanics (Brew and others, 1984) in southern southeast Alaska, and the Glacier Creek volcanic rocks of Redman and others (1985) in northern southeast Alaska. Age control is largely based on fossils, which are primarily late Carnian to Norian and there are local occurrences of younger Rhaetian fossils (Green and others, 2003). Karl and others (1999) report an occurrence of uppermost Anisian to Ladinian(?) (Middle Triassic) conodonts from the Duncan Canal area. Premo and others (2010) report a 210.3±0.3 Ma 40Ar/39Ar alteration age on fuchsite that they interpreted as age of intrusion of Hyd Group feeder plutons. Green and Greig (2004) reported a 213±5 Ma U/Pb zircon age on rhyolite from the northern part of the unit near the Canadian border. Gehrels and others (1987) reported a 225±3 Ma U/Pb zircon age from rhyolite on Gravina Island. Where possible, subdivided into the following two units, Trhgs and Trhgv:

Source map information

Source map Karl, S.M., Haeussler, P.J., and McCafferty, Anne, 1999, Reconnaissance geologic map of the Duncan Canal-Zarembo Island area, southeast Alaska: U.S. Geological Survey Open File Report 99-168, 1 sheet, scale 1:125,000.
Symbol Trhsv
Unit name Hyd Group sedimentary and volcanic rocks
Description Green volcaniclastic wacke debris flows and turbidites. Locally calcareous. Grades into volcanic breccia. Intercalated mafic pillow tuff. Finer grained facies occur as alternating lenses of gray and green subphyllite. Wacke contains volcanic lithic fragments, argillite, and limestone clasts, and augite crystals. Conglomerate clasts include volcanic rock, graywacke, siltstone, argillite, chert, granitic rock, and white vein quartz.
Lithology Sedimentary

Correlated geologic units

Label Trhg
Description Hyd Group, undivided
Geologic age Middle-Triassic to Late-Triassic
Geologic setting Undivided
Lithology Form Importance
Rhyolite < Felsic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow, pillows Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Volcaniclastic, volcanic breccia Indeterminate, major
Argillite < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Slate < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Limestone < Carbonate < Sedimentary Bed Indeterminate, major
Limestone < Carbonate < Sedimentary Carbonaceous Indeterminate, major
Chert < Chemical < Sedimentary Bed Indeterminate, major
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Graywacke < Sandstone < Clastic < Sedimentary Bed Indeterminate, major