Clastic and calcareous clastic rocks

Unit symbol: Dyss
Age range Devonian (419.2 to 358.9 Ma)
Lithology: Sedimentary
Group name: Clastic and carbonate rocks of the Yukon Flats Basin
Lithic- and quartz-rich clastic sedimentary and low-grade metamorphic rocks found along the northwest and northern margin of the Yukon Flats basin. Brosgé and others (1973) and Brosgé and Reiser (2000) report gray-green, brown-weathering, fine- to medium-grained, thin- to medium-bedded lithic wacke interbedded with lesser amounts of gray and brown quartz wacke and black micaceous shale and silty shale. Faintly schistose in many exposures, weathers brown to rusty. “Rare gritty beds contain chips of shale. The quartz wacke is also fine- to medium-grained and thin- to medium-bedded. Black shale occurs in thin partings and in beds as much as 25 to 50 feet thick (8 to 15 m) that may form 30 to 50 percent of the unit. The total thickness of the unit is uncertain, but is probably about 4,000 feet (1200 m) * * *. Graywacke and shale on the upper Christian River contains Devonian plants and spores” (Brosgé and Reiser, 2000). Brosgé and Reiser (2000) also describe a schistose wacke unit, which they suggest is probably a “slightly more metamorphosed facies” of their wacke and quartzite and slate units. Till and others (2006a) assigned these rocks to the Brooks Range phyllite and graywacke belt, the so-called Slate Creek terrane. However, just across the Yukon Flats basin to the east is the Nation River Formation (unit Dnr), which has some lithologic and age similarity with these rocks

Source map information

Source map Brosge, W.P., Reiser, H.N., and Yeend, Warren, 1973, Reconnaissance geologic map of the Beaver quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-525, scale 1:250,000.
Symbol Dg
Unit name Graywacke
Description Brown-weathering, gray to olive-gray, fine- to medium-grained, thin- to medium-bedded quartz-chert graywacke and lithic wacke, interbedded black shale, siltstone, and minor ironstone. Rocks are schistose west and north of Twin Sisters and near Lone Mountain. Contains unidentifiable plant fragments in outcrops on west bank of Hodzana River. Age uncertain, but probably Devonian and correlative with plant-bearing lithic wacke of Devonian(?) age on the Christian River.
Lithology Sedimentary

Correlated geologic units

Label Dg
Description Dark-greenish-gray fine-grained impure graywacke
Geologic age Devonian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Graywacke < Sandstone < Clastic < Sedimentary Bed Major