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., and Baichtal, James, 2013, Unpublished data.
Symbol Trh
Unit name Hyd Group, undivided
Description Sedimentary and volcanic rocks. Unit consists of basal breccia, overlain by medium-bedded dark brown limestone and sooty carbonaceous limestone, sooty carbonaceous slate and argillite, black chert, black limestone, calcareous graywacke turbidites, and volcanic sandstone and conglomerate. Pillow basalt, volcanic breccia, and banded rhyolite form lateral facies to limestone, argillite, and volcanic sandstone. Unit is characterized by meter scale lateral and vertical facies changes. Unit has unconformable base and top, estimated at 1500 m total thickness. Metamorphosed intermediate to mafic volcanic rocks; phyllite, greenschist, greenstone, minor semischist; weathers light to dark green; probably several thousand meters thick. Described by Lathram and others (1959; 1965) in part and described as "..albite epidote chlorite schist, schistose graywacke, slate, conglomerate, phyllite, and minor augite bearing schistose volcanic flow breccia and andesite flows ...marble ...quartz conglomerate and breccia
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