Poorly consolidated surficial deposits

Unit symbol: QTs
Age range Quaternary, Pleistocene, and uppermost Tertiary (5.333 to 0.0117 Ma)
Lithology: Unconsolidated
Group name: Unconsolidated and poorly consolidated surficial deposits
Silt to coarse-gravel and semi-consolidated sandstone to conglomerate are widespread as an erosional remnant deposit throughout Alaska. Genetically, unit includes deposits of fluvial, glaciofluvial, colluvial, eolian, and shallow-marine deposits and includes local tuffaceous deposits. Unit includes several named formations, including the Faneto Formation of the Aleutian Islands, the Kougarok Gravel of the Seward Peninsula, the Chariot, Saligvik, and Ilyirak Gravels of the Point Hope region, and the Gubik Formation of the North Slope as well as the informally named Holokuk gravel of Bundtzen and others (1999) in southwest Alaska. Some deposits are folded or tilted, reflecting recent tectonic movement. Some marine deposits are richly fossiliferous. Age control is generally sparse; fossils may not be age-diagnostic. The tilted Holokuk gravel of Bundtzen and others (1999) was interpreted by Bundtzen to be an outwash deposit sourced from the glaciated highlands southwest of the Kuskokwim River on the basis of pebble count and clast studies. Boulder-rich conglomeratic deposits on Adak Island, mapped as Tertiary by Coats (1956a), are probably Quaternary in age, certainly no older than Pliocene. Also included in map unit is fossiliferous marine sandstone of northern Adak Island (Coats, 1956a). On Amchitka Island, bedded sand and gravel, composed of hornblende andesite fragments, occurs at an elevation of 180 m. Some beds contain subangular cobbles and boulders, whereas others contain well-rounded cobbles and boulders up to 0.6 m in diameter. Powers and others (1960) interpreted this as a beach and nearshore marine deposit. Also on Amchitka Island, a small area of tilted sedimentary rocks (dipping about 12ยบ SE.) is found at South Bight (Powers and others, 1960). These consist of 60 m of carbonaceous sandy silt, fine to medium sand, and pebbly sand to sandy fine gravel, in random order, a few inches to 0.6 m thick, which grades upward to 45 m of less well-bedded gravel. Fragments of carbonized wood are common in silt layers. Semiconsolidated marine beach deposits consist of poorly bedded, soft, pebbly siltstone cap sea cliffs of volcanic rock on Hagemeister Island and contain shallow-water marine fossils of Pliocene or Pleistocene age. The Gubik Formation consists of marine and fluvial deposits of well to poorly sorted and well to poorly stratified silt, sand, and gravel. Locally includes wood and woody material (Nelson and Carter, 1985). Thickness more than 10.5 m, probably less than 60 m (Reiser and others, 1980)

Source map information

Source map Szumigala, D.J., Newberry, R.J., Werdon, M.B., Athey, J.E., Stevens, D.S.P., Flynn, R.L., Clautice, K.H., and Craw, P.A., 2002, Geologic map of the Eagle A-1 quadrangle, Fortymile Mining District, Alaska: Alaska Division of Geological and Geophysical Surveys Preliminary Interpretive Report 2002-1a, 1 sheet, scale 1:63,360.
Symbol Qat1
Unit name OLDEST TERRACE ALLUVIUM
Description Elongate deposits of well-sorted, well-rounded to subangular, generally poorly stratified gravel, sand, and silt, possibly of glaciofluvial origin, forming elevated benches bordering major streams. Especially prominent and continuous on north side of Walker Fork valley and lower Canyon Creek. Maximum tread elevation approximately 200 to 220 m above the present streams. Thickness highly variable, ranging from thin gravel veneers on bedrock to a reported maximum of 40 m at Napoleon Creek in the Eagle A-2 Quadrangle (Yeend, 1996). Drilling by a placer miner on the north side terrace of Walker Fork near Boundary showed maximum gravel thicknesses of approximately 13 m. Maximum cobble size observed was 0.5 m diameter, but most large clasts are in the range of 0.1 to 0.2 m diameter. Mostly clast-supported, with a medium to coarse, subangular sand matrix. Locally stained orange by limonite. While generally massive and structureless, exposures may locally preserve bedding structures, boulder lags, and ice-wedge casts up to 2 m tall. Locally thickly mantled by ice-rich, primary and redeposited eolian silt. Surface generally smooth and heavily vegetated. Bench gravels are auriferous and have been successfully mined at Napoleon Creek and upper Walker Fork
Lithology Unconsolidated

Correlated geologic units

Label QTg
Description High-level gravel
Geologic age Pliocene to Pleistocene
Geologic setting Sedimentary, continental
Lithology Form Importance
Gravel < Coarse-detrital < Unconsolidated Bed Major
Sand < Coarse-detrital < Unconsolidated Bed Indeterminate, major
Silt < Fine-detrital < Unconsolidated Bed Indeterminate, major