Eagle Creek and Hasen Creek Formations

Unit symbol: Peh
Age range lower Permian (298.9 to 272.3 Ma)
Lithology: Sedimentary
Group name: Skolai and Mankomen Groups, undivided
These two argillic formations are part of the Mankomen and Skolai Groups, respectively. Eagle Creek Formation (Richter, 1976) is thin-bedded argillite and siliceous siltstone, with interbedded calcareous siltstone and sandstone, biomicritic limestone, and pebble to cobble conglomerate. Eagle Creek Formation is cut by gabbroic dikes and sills and is abundantly fossiliferous; it includes specimens of brachiopods, cephalopods, corals, foraminifera, and bryozoans. Richter (1976) reported that thin lenses (less than 30 m) of carbonaceous shale, calcareous argillite, and chert that contain the Middle Triassic pelecypod Daonella separate the Eagle Creek Formation from the overlying Nikolai Greenstone. These lenses are not separately mapped here from the Eagle Creek Formation, but are not considered part of the Eagle Creek Formation. Hasen Creek Formation (MacKevett and others, 1978) is weakly metamorphosed, diverse, thin-bedded argillite with less abundant lithic graywacke, conglomerate, shale, chert, schist, marble, and limestone (carbonates are mapped separately as unit Pehls, described below). The argillite and shale are shades of gray or yellowish brown, fine-grained, silica-rich rocks. The graywacke and conglomerate are both variegated, have calcite-rich matrices and multicolored clasts. The Hasen Creek Formation is also cut by Triassic gabbro, the Chitina Valley batholith (unit KJse), and Tertiary hypabyssal rocks

Source map information

Source map Richter, D.H., 1976, Geologic map of the Nabesna quadrangle, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-932, 1 sheet, scale 1:250,000.
Symbol Pe
Unit name Eagle Creek Formation
Description Part of Mankomen Group. Thin-bedded argillite and siliceous siltstone, with interbedded of calcareous siltstone and sandstone, biomicritic limestone, and a pebble to cobble conglomerate. Gabbroic dikes and sills. Equivalent to parts of the Cache Creek Group and the Skolai Group
Lithology Sedimentary

Correlated geologic units

Label Pe
Description Eagle Creek Formation, Early Permian, marine argillite and limestone, lower greenschist facies [equivalent to Hasen Creek Fm.]
Geologic age Cisuralian
Geologic setting Undivided
Lithology Form Importance
Marble < Metacarbonate < Metasedimentary < Metamorphic Greenschist Major
Argillite < Metaclastic < Metasedimentary < Metamorphic Greenschist Major