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 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., 2 sheets, scale 1:250,000.
Symbol Trhb
Unit name Hyd Group Burnt Island Conglomerate
Description Hyd Group Burnt Island Conglomerate - conglomerate, calcarenite, and limestone; crudely bedded, poorly sorted calcite-matrix pebble conglomerate with clasts of either bluish-green and black argillite, graywacke, and chert derived from the Cannery Formation, or of chert and limestone from the Pybus Formation.
Lithology Sedimentary

Correlated geologic units

Label Trhb
Description Hyd Group breccia and conglomerate
Geologic age Middle-Triassic to Triassic
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Conglomerate-sandstone < Mixed-clastic < Clastic < Sedimentary Olistrostrome Indeterminate, major