Hunt Fork Shale

Unit symbol: Degh
Age range Devonian (393.3 to 358.9 Ma)
Lithology: Sedimentary
Group name: Hunt Fork Shale (Endicott Group)
Mostly shale and sandstone; shale is medium-dark- and olive-gray; sandstone is grayish-green and greenish-gray, mostly fine- to medium-grained, micaceous, and locally ripple crossbedded and or graded, widely distributed across northern Alaska. Unit locally subdivided into three informal members: a shale member that consists of mudstone, shale, and sandstone; a wacke member; and a limestone member (Brosgé and others, 2001; Harris and others, 2009). Shale member weathers black to brown; where locally pyritic, weathers rusty and contains a few ironstone concretions. Mudstone and shale are medium- to medium-dark-gray, very silty, fissile, and interbedded with sandstone. Interbedded sandstone is as much as 25 percent brown-weathering, thin-bedded, fine-grained, partly calcareous sandstone and graywacke that includes both quartz-chert arenite and quartz-chert wacke; sandstone is schistose in southern part of its exposure area, as well as minor thin beds of ferruginous, argillaceous, fossiliferous limestone. Unit displays a cyclic depositional pattern with siltstone grading upward into shale; limestone occurs in upper parts of some cycles (Brosgé and others, 1979; Kelley, 1990a). Wacke member is included here with the Noatak Sandstone, unit Degn. Informal dark-gray limestone member weathers yellow, brown, and gray and is thin- to medium-bedded or nodular and has common algal lumps. Commonly includes some orange-weathering, partly calcareous siltstone and fine-grained sandstone above or below the limestone. Unit is metamorphosed in core of Brooks Range and where found thrust imbricated in the Doonerak Window. Where metamorphosed, it consists of dark-gray to black phyllite and lesser gray-green phyllite with thin layers of siliceous or calcareous metasiltstone, lithic wacke, metasandstone, and minor layers of fossiliferous metalimestone. Locally massive mafic sills and dikes up to 10 m thick are common. Mafic bodies in the unit (both strongly and weakly foliated parts) display lower greenschist-facies minerals (Till and others, 2008a). Fossils include brachiopods (late Frasnian to early Famennian), mollusks, echinoderms, and Middle to Late Devonian conodonts (Brosgé and others, 1979; Till and others, 2008a)

Source map information

Source map Kelley, J.S., 1990, Generalized geologic map of the Chandler Lake quadrangle, north-central Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2144-A, 19 p., 1 sheet, scale 1:250,000.
Symbol Dhs
Unit name Hunt Fork Shale, shale member
Description Mudstone, shale, and sandstone; mudstone and shale are medium- to medium-dark-gray, very silty, fissile, and interbedded with sandstone. Sandstone is brownish- and greenish-gray, fine- to medium-grained, mostly fine-grained, laminated, low-angle crossbedded and wavy laminated in part
Lithology Sedimentary

Correlated geologic units

Label Dhf
Description Endicott Group, Hunt Fork Shale
Geologic age Frasnian to Famennian
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Shale < Mudstone < Clastic < Sedimentary Bed Major
Phyllite < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Shale < Mudstone < Clastic < Sedimentary Carbonaceous Incidental
Siltstone < Clastic < Sedimentary Bed Incidental