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 Carter, L.D., and Galloway, J.P., 1985 (revised 2005), Engineering-geologic map of the Harrison Bay quadrangle, Alaska: U.S. Geological Survey Open-File Report 2005-1194, 32 p., 2 sheets, scale 1:250,000.
Symbol QTas
Unit name Alluvial and eolian sand and marine sand and silt
Description Highly variable composition, but generally consists of stratified deposits of marine silty sand, gravelly sand, silt, and minor clay, overlain by fluvial silty sand, gravelly sand, and minor organic-rich silt, which in turn locally is overlain by eolian sand. In places, marine deposits do not occur, and only fluvial and eolian deposits are present. The base of the marine deposits is exposed only in bluffs along the right bank of the Colville River, where it is marked by a sparse lag of pebbles, cobbles, and boulders. Rock types in the lag include those characteristic of map units Tsg and Tg. The deposits of two marine transgressions occur and locally may be superposed. Fluvial deposits locally contain buried peat beds, and logs of spruce, larch, and poplar. The lower part of the eolian sand contains ventifacted pebbles of chert and quartz derived from the underlying deposits. Peat and/or peaty, silty sand occurs at the top of the unit
Lithology Unconsolidated

Correlated geologic units

Label QTs
Description Older surficial deposits, undivided
Geologic age Pliocene to Holocene
Geologic setting Unconsolidated
Lithology Form Importance
Sand < Coarse-detrital < Unconsolidated Bed Major
Gravel < Coarse-detrital < Unconsolidated Bed Indeterminate, major
Silt < Fine-detrital < Unconsolidated Bed Indeterminate, major
Boulders < Coarse-detrital < Unconsolidated Bed Minor