Conglomerate, sandstone, and lignite

Unit symbol: TKs
Age range lower Tertiary to Upper Cretaceous (83.6 to 33.9 Ma)
Lithology: Sedimentary
Group name: Conglomerate, sandstone, and lignite
Sandstone, mudstone, thin coal seems, and conglomerate exposed in the Charley River and Rampart areas. In the Charley River area, the unit consists of poorly consolidated sandstone, grit, pebble-to cobble-conglomerate, and carbonaceous mudstone with coal seams and occurs just east of the Kandik basin and along the Tintina Fault Zone. Late Cretaceous, Paleocene, and Eocene pollen have been recovered from exposures within and immediately south of the map area (Miyaoka, 1990) and from shallow core holes (unpublished oil industry data, 2002, cited in Till and others, 2006a). In the western Rampart area, along the Victoria Creek Fault Zone, quartz- and chert-rich fluvial conglomerate, sandstone, and mudstone are typical, and palynoflora of probable Maastrichtian age (Farmer and others, 2003) as well as of early Tertiary age (Chapman and others, 1982) have been collected. Younger part of the unit likely correlative to rocks of unit Tsu, sedimentary rocks, undivided. Rocks of this unit also occur in a 25-m-thick section along the Sethkokna River in the northeast Medfra quadrangle (Patton and others, 1980). There, the conglomerate contains clasts of quartz, chert, felsic volcanic rocks, and talc schist, which Patton and others (1980) provisionally assigned a latest Cretaceous age (Campanian-Maastrichtian) on the basis of pollen in the lignite beds. Unit is overlain by rhyolite and dacite flows. On the Seward Peninsula, Till and others (2011) mapped two separate sedimentary sequences as their unit TKs. Here, only the more northern Late Cretaceous to Tertiary rocks are included; the older middle-Cretaceous rocks are assigned to map unit Kcc of this map. The rocks included here from the unit TKs of Till and others (2011) consist of gray and brown siltstone, mudstone, sandstone, coal, and minor conglomerate poorly exposed in narrow slices along the Kugruk Fault Zone in the northeastern and southeastern Bendeleben quadrangle; it has been explored for coal and uranium (Retherford and others, 1986; Dickinson and others, 1987). Pollen assemblages of Late Cretaceous and Tertiary (Eocene to early Miocene?) ages have been found in finer grained parts of the sequence (Till and others, 1986; Haga, in Retherford and others, 1986)

Source map information

Source map Werdon, M.B., Newberry, R.J., Szumigala, D.J., and Pinney, D.S., 2001, Geologic map of the Eagle A-2 quadrangle, Fortymile mining district, Alaska: Alaska Division of Geological and Geophysical Surveys Preliminary Interpretive Report 2001-3A, 1 sheet, scale 1:63,360.
Symbol Ts
Description Poorly- to well-indurated, nonmarine sedimentary rocks of early Tertiary age, as established by plant fossils (Foster, 1976). Typically consists of conglomerate, sandstone, shale, amber-bearing coal, siltstone, and graywacke. Includes local felsic tuff, usually rhyolite composition, with distinctive quartz and sanidine phenocrysts in a light colored, fine-grained matrix or fine-grained, layered ash tuff. Total thickness of unit in excess of 140 m. Silicified basal conglomerate with cobbles to 1 m is over 40 m thick in lower Napoleon Creek. Magnetic susceptibility is very low, <0.1 x 10 SI (Système International). Petrified wood, within massive black chalcedony containing abundant plant fossils, is locally present in the Chicken area. Silicified and altered regional correlatives of this unit contain epithermal gold prospects, as at Ptarmigan Hill, 10 km west of Eagle,Alaska, and Grew Creek, near Ross River, Yukon Territory (Newberry and others, 1998b). Spatial association with placer gold-bearing gravels in the Chicken and Napoleon Creek areas suggests that this unit may be gold-bearing as well. The unit is usually truncated by high-angle faults; unconformable contacts with older units are only rarely observed. The age of this unit may extend into the Cretaceous (based on Ar/ Ar biotite plateau age of 105.4 Ma and white mica plateau age of 108.8 Ma on tuffaceous? material, east side of Ingle Creek, Eagle A-3 Quadrangle; Layer and others, 2001)
Lithology Sedimentary

Correlated geologic units

Label TKcg
Description Conglomerate, sandstone, and lignite
Geologic age Campanian to Eocene
Geologic setting Sedimentary, continental
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Major
Mudstone < Clastic < Sedimentary Carbonaceous Indeterminate, major
Lignite < Coal < Sedimentary Bed Minor