Hyd Group sedimentary rocks, undivided

Unit symbol: Trhgs
Age range Upper Triassic (247.2 to 201.3 Ma)
Lithology: Sedimentary
Group name: Hyd Group, undivided
Includes rocks locally mapped separately as the Burnt Island Conglomerate (Muffler, 1967), conglomerate and carbonate rocks of the Nehenta Formation, the Hamilton Island and Cornwallis Limestones, and argillite and limestone of the Hyd Group (Karl, 1992; Karl and others, 1999). Breccia and conglomerate of variable composition, locally called the Burnt Island Conglomerate, is as much as 200 m thick and clast composition reflects directly underlying units (S.M. Karl, unpub. data). Locally, where the Hyd Group sedimentary rocks overlie limestone and dolostone of the Pybus Formation (Plps), breccia consists of unsorted angular blocks of limestone and dolostone in calcareous matrix, but where the Hyd Group sedimentary rocks overlie chert of the Pybus Formation, breccia is clast-supported chert pebble-cobble conglomerate in a calcareous matrix. Where it overlies the Cannery Formation, the breccia is a matrix-supported debris flow deposit that has angular blocks of chert, argillite, graywacke, volcanic rocks, white vein quartz, and schist that are as large as 20 m. Breccia and conglomerate are commonly at or near base of Hyd Group, but the position is variable. At Keku Strait, the Hamilton Island Limestone consists of highly folded, generally very thin-bedded, aphanitic, dark-gray, locally dolomitic limestone that contains thin to medium beds of dark-green calcarenite and minor black claystone layers (Brew and others, 1984). Cornwallis Limestone, on Kuiu Island, consists dominantly of medium- to very thick-bedded, medium-gray oolitic limestone and contains chert clasts derived from the underlying Pybus Formation (Brew and others, 1984). The Hamilton Island Limestone reflects a deep-water slope facies environment and is coeval with the shallow-water shelf facies of the Cornwallis Limestone. Other limestone in the Hyd Group varies from dark- to medium-gray, bluish-gray or dark-brown, massive to medium-bedded limestone that may be graphitic, argillaceous, dolomitic, carbonaceous, or conglomeratic. Argillite of the Hyd Group is dark-gray to black, carbonaceous, locally siliceous or calcareous and rhythmically bedded. Subordinate chert, limestone, graywacke, and conglomerate are also present. Limestone may occur in lenses as much as 12 m thick. Carbonaceous beds and limestone locally contain well preserved Middle Triassic (Ladinian) ammonites and Daonella, Ladinian or Carnian conodonts, Late Triassic (late Carnian to early Norian) Halobia, crinoids, corals, mollusks, pelecypods, gastropods, ammonites, Monotis, early Norian Halobia alaskana, middle Norian Halobia cf. H. fallax, H. lineata, and late Norian Heterastridium (Muffler, 1967; Berg and others, 1988)

Source map information

Source map Karl, S.M., 2001, Unpublished data.
Symbol Trhl
Unit name Limestone of the Hyd Group
Description Near Glacier Creek consists of black, argillaceous marble, and gray massive marble, 2-20 m-thick beds. Minor volcanic and black slate layers. In Keku Strait, Hamilton Island Limestone consists of limestone, mudstone, and calcarenite; generally very thin-bedded, dark-gray aphanitic limestone (locally dolomitic), minor black claystone layers and thin to medium beds of dark-green calcarenite; highly folded. The Hamilton Island Limestone is a deep water, slope facies limestone coeval with the shallow water shelf facies Cornwallis Limestone, on Kuiu Island, consisting of limestone and calcarenite; dominantly medium- to very thick-bedded, medium gray oolitic limestone that contains chert clasts from the underlying Pybus Formation. On Admiralty Island, Hyd limestone includes dark brown or dark gray, medium, 5-20 cm bedded, fossiliferous limestone, and thicker bedded sparsely fossiliferous limestone. Maximum thickness estimated at 200 m. In the Duncan Canal area, Hyd limestone includes dark to medium gray, white to light gray-weathering, massive to medium bedded limestone. Locally graphitic, argillaceous, dolomitic, or conglomeratic. Clasts in limestone conglomerate include argillite, tuff, chert, limestone, and fossil debris. On Gravina Island, the consists of dark bluish gray, very fine grained recrystallized limestone (Berg, 1973). The lower part of the calcareous member is distinguished by thinly interlayered dark gray and dark brown carbonaceous limestone and siltstone containing locally abundant Halobia. Interbedded with carbonaceous rocks are sparsely fossiliferous, olive-weathering fine-grained limestone, light gray, locally concretionary sandy to gritty limestone, calcareous sandstone, grit, and conglomerate. The conglomerate contains grit- to cobble-size clasts derived from underlying Paleozoic rocks, including trondhjemite, metarhyolite, limestone, argillite, and siltstone in a sandy to gritty lime matrix. Bedding thickness ranges from an inch to 10 feet. Upper part is thickly interbedded brownish-gray calcareous grit, conglomerate, and sandstone. The conglomerate contains mainly subangular clasts of the underlying Nehenta Formation in a gray siltstone and limestone matrix (Berg, 1973). Most of the limestone is massive and calcitic, but locally it is moderately to thickly bedded and on Annette Island there is a thick section of thin-bedded limestone turbidites. Locally the limestone is dolomitic or contains pods and masses of barite, disseminated pyrite, or quartz veins
Lithology Sedimentary

Correlated geologic units

Label Trhl
Description Hyd Group limestone and dolomite
Geologic age Late-Triassic
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Limestone < Carbonate < Sedimentary Calcareous Major
Dolostone < Carbonate < Sedimentary Calcareous Indeterminate, major
Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Calcarenite < Arenite < Sandstone < Clastic < Sedimentary Calcareous Indeterminate, major