Mangaqtaaq formation of Anderson and Watts (1992) and Ulungarat formation of Anderson (1993)

Unit symbol: Du
Age range Upper Devonian (393.3 to 358.9 Ma)
Lithology: Sedimentary
Group name: Endicott Group, undivided
These units are considered the base of the Ellesmerian sequence; however, it is not clear if they belong to the Endicott Group or represent a precursor. The Mangaqtaaq formation of Anderson and Watts (1992) is interpreted as a restricted marine or lacustrine limestone. Anderson and Watts (1992) described it as consisting of as much as 200 m of black algal limestone, sandstone, and interbedded mudstone, where the lower 80 to 100 m consists of cyclic repetitions of 3- to 10-m-thick intervals of interbedded algal limestone and sandstone with thin intervals of black mudstone; the upper part of the unit is rhythmically interlaminated black mudstone and siltstone. Anderson (1991b) described the unit in the Demarcation Point quadrangle; Reiser and others (1971) describe a roughly similar unit in the Mount Michelson quadrangle to the west, where it is mapped underlying the Kayak Shale (Mk) and reported as pre-Mississippian. The Mangaqtaaq overlies the Ulungarat formation of Anderson (1991a) at a sharp contact interpreted by Anderson to be an unconformity and it is, in turn, unconformably overlain by the Kekiktuk Conglomerate (Mek). Age is poorly constrained as Late Devonian or Early Mississippian (Anderson, 1991b) on the basis of plant fossils in its lower part. The Ulungarat formation of Anderson (1991a) consists of a coarsening- and thickening-upward, shallow-marine to nonmarine fluvial sequence, not unlike some other formations of the Endicott Group, in particular, the Ear Peak Member of the Kanayut Conglomerate. As redefined by Anderson (1993), the unit has four informal members. The lowermost, unit A, is marine, green-gray-weathering mudstone and black phyllite that gradually contains an upward-increasing proportion of chert arenite and calcarenite sandstone that contains variable amounts of shallow-marine invertebrate skeletal material. The second member, unit B, consists of “* * * chert granule to pebble conglomerate, chert arenite, and siltstone in channelized fining-upward intervals in an overall coarsening-upward sequence” (Anderson, 1993). The third member, unit C, a cliff-former, consists of poorly sorted, clast-supported chert pebble to cobble conglomerate in thick channel-fill deposits (Anderson, 1993). “Brown-red mudstone and interbedded thin sandstone beds underlie, are lateral to, and overlie the cliff-forming conglomerates” (Anderson, 1993). The uppermost member consists of mottled red mudstone that has sparse, laterally discontinuous sandstone lenses. The lowest member contains abundant Middle Devonian marine invertebrate fossils, whereas the upper nonmarine members contain undated plant fossils, whose age is only constrained by the overlying, unconformable Mississippian Kekiktuk Conglomerate. As much of the mapping of the eastern Brooks Range predates the definition of this unit, it is possible that this unit is potentially equivalent to the lower member of the Kanayut Conglomerate as mapped in the Arctic and Killik River quadrangles

Source map information

Source map Brosge, W.P., Reiser, H.N., Dutro, Jr. J.T., Detterman, R.L., and Tailleur, I.L., 2001, Geologic map of the Arctic quadrangle, Alaska: U.S. Geological Survey Map I-2673, 38 p. pamphlet, 2 sheets, scale 1:200,000.
Symbol MDsc
Unit name Unnamed sheared conglomerate
Description North Slope subterrane: Composed of chert and argillite pebbles as long as 1 cm. Unit weathers brown and yellow and contains minor black shale and siltstone. May be equivalent to upper, plant-bearing part of the Ulungarat Formation (informal name) of Anderson (1991a, b) mapped 145 mi east of the Arctic quadrangle
Lithology Sedimentary

Correlated geologic units

Label Du
Description Ulungarat Formation, a coarsening- and thickening-upward, shallow-marine to nonmarine fluvial sequence. Base of the Ellesmerian sequence, Middle to Upper Devonian
Geologic age Middle-Devonian
Geologic setting Sedimentary, deltaic-and-nearshore
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Calcarenite < Arenite < Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Lithicarenite < Arenite < Sandstone < Clastic < Sedimentary Carbonaceous Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major