Cascaden Ridge and Beaver Bend combined correlative units

Unit symbol: Dcr
Age range Devonian (393.3 to 358.9 Ma)
Lithology: Sedimentary
Group name: Clastic and carbonate rocks of the Yukon Flats Basin
Cascaden Ridge unit of Weber and others (1992) is chiefly rhythmically interbedded gray and olive-gray shale, gray siltstone, and graywacke, exposed in Livengood quadrangle. Less abundant components are gray limestone and polymictic granule- and pebble-conglomerate that has a graywacke matrix. Conglomerate clasts include mafic and ultramafic rocks, chert, dolomite, and shale (Weber and others, 1992). Limestone interbeds contain Middle Devonian gastropods, conodonts, brachiopods, and corals (Blodgett, 1992; Weber and others, 1992). Beaver Bend unit of Weber and others (1992) is conglomerate, graywacke, siltstone, and slate in the Livengood quadrangle. Conglomerate is polymictic, clast-supported granule- to pebble-sized and has clasts that include chert, quartz, quartzite, mafic and felsic volcanic rocks, argillite, slate, siltstone, and sandstone. Graywacke is fine- to medium-grained; framework grains are mostly chert and quartz, and subordinate slate. The Beaver Bend unit has yielded unidentifiable plant fragments; it was assigned a probable Devonian age by Weber and others (1992). In the western Circle quadrangle, nonfossiliferous chert-pebble conglomerate that was mapped as part of the MzPzat unit by Foster and others (1983) was considered to be equivalent to the Beaver Bend unit of Weber and others (1992) (F.R. Weber, unpub. data, cited in Wilson and others, 1998). In the north-central Circle quadrangle, north of the Tintina Fault, two units mapped by Foster and others (1983) are included here: unit Pzcg, chert-pebble conglomerate; and unit Pzcc, chert, chert-pebble conglomerate, and minor limestone. Limestone of unit Pzcc yielded Late Devonian (Famennian) conodonts

Source map information

Source map Foster, H.L., Laird, Jo, Keith, T.E.C., Cushing, G.W., and Menzie, D.W., 1983, Preliminary geologic map of the Circle quadrangle, Alaska: U.S. Geological Survey Open-File Report, 83-170-A, scale 1:250,000.
Symbol unk
Unit name Chert
Description Various chert symbols shown on Circle map in a variety of rock units. No further description in text. Assigned to Cascaden on the basis of location.
Lithology Sedimentary

Correlated geologic units

Label Dcb
Description Cascaden Ridge, Beaver Bend combined correlative units, shale, siltstone, sandstone, conglomerate, and minor limestone, Middle Devonian
Geologic age Middle-Devonian to Devonian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Shale < Mudstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Major
Conglomerate < Clastic < Sedimentary Bed Minor
Quartzose-sandstone < Sandstone < Clastic < Sedimentary Bed Minor
Limestone < Carbonate < Sedimentary Bed Incidental