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Source map Wilson, F.H., 2007, Unpublished data.
Symbol Dhf
Unit name Hunt Fork Shale
Description Split unit to more nearly match Till and others (2007). These polygons are the more strongly foliated portion of the unit. Twice metamorphosed sedimentary and volcanic rocks; Middle to Upper Greenschist facies. Metamorphic facies and texture increases southward; mostly phyllite and limestone in north; schist and marble in south. Lower Endicott Group- Carbonaceous siliceous clastic rocks. Grades downward into Beaucoup Formation in some places; unconformably overlies upper Middle Devonian and older rocks in other places. Black slate and phyllite, minor fossiliferous limestone; lithic wacke locally in upper part; basal quartz-chert clast conglomerate and sandstone.
Lithology Sedimentary

Correlated geologic units

Label Dhfm
Description Endicott Group, Hunt Fork Shale, metamorphosed
Geologic age Late-Devonian
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Phyllite < Metaclastic < Metasedimentary < Metamorphic Major
Marble < Metacarbonate < Metasedimentary < Metamorphic Indeterminate, major
Metasandstone < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Metasiltstone < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major