Unga, Belkofski, and Unalaska Formations

Unit symbol: Tuu
Age range Tertiary, Miocene (28.1 to 11.62 Ma)
Lithology: Sedimentary
Group name: Sedimentary rocks of southwest Alaska
The Unga Formation (Dall, 1882, 1896; Detterman and others, 1996) is about 275 m thick, 25 percent or more of which is volcanic rocks (lahar deposits, debris-flow deposits, and tuff). Volcanic rocks are dominant in upper part, whereas carbonaceous shale and coal are restricted to lower part; sandstone and conglomerate throughout unit are composed of poorly sorted and typically loosely consolidated volcanic debris. Unit is geographically restricted to Unga Island, the Pavlof Islands, Deer Island, and along the Pacific coast of Alaska Peninsula adjacent to Unga Island. Fossils are locally abundant but are restricted to thin zones that contain numerous specimens of a few genera. Petrified wood, including logs and stumps in growth position, is common and typically associated with debris-flow deposits that engulfed then-existent forests of Metasequoia sp. (Eakins, 1970). Neither top nor base of formation is exposed, and, therefore, its relation to other units is not well defined. However, because the formation has similar structural attitudes as underlying and overlying units, we infer that it disconformably overlies Stepovak Formation (Ts) and is disconformably overlain by late Miocene volcanic rocks (Tv). Belkofski Formation (Kennedy and Waldron, 1955; Burk, 1965; see also, McLean, 1979), exposed along Pacific coast of the southwestern Alaska Peninsula, is about 1,830 m thick and consists of tuffaceous, volcaniclastic sandstone, siltstone, and conglomerate and contains interbeds of tuff and volcanic breccia. Unit is mainly gray or greenish-gray to gray-brown on mainland; on offshore islands, rocks are dominantly red, pink, and purple and all are very well indurated. Stratigraphic relation to other units on Alaska Peninsula is not known with certainty; however, the most likely correlative unit, on the basis of lithologic similarity and apparent stratigraphic position, is Unga Formation. Lower contact of Belkofski Formation is nowhere exposed; upper part of unit is overlain unconformably by volcanic flows of late Miocene age. We infer that the Belkofski Formation, which is commonly intruded by plutonic rocks, is a contact-metamorphosed equivalent of the Unga Formation. A K/Ar age determination on a clast from volcanic agglomerate, mapped as Belkofski Formation, is 11.79±0.41 Ma (Wilson and others, 1994). The Unalaska Formation, as mapped by Drewes and others (1961), is “* * * a thick sequence of coarse and fine sedimentary and pyroclastic rocks intercalated with dacitic, andesitic, and basaltic flows and sills, cut by numerous dikes and small plutons * * *.” Pillowed flows are common along southern Unalaska Island, as are abundant sills. Age control is sparse and is primarily based on a single collection of a fossil relative of a sea cow identified as the genus Cornwallis or Desmostylus of probable Miocene or late Oligocene age. Also included in this map unit on nearby Umnak Island is bedded argillite and tuff, keratophyre flows, and albitized intrusive rocks; the argillite, which has characteristics possibly suggesting turbidity current deposition, contains carbonized impressions suggestive of leaf fragments (Byers, 1959). Sparse radiometric ages on volcanic or intrusive rocks associated with this unit on Unalaska and Umnak Islands yield late Oligocene ages, suggesting a longer age range for the parts of this unit in the Aleutian Islands, in contrast to the Alaska Peninsula part of the unit. Exposed on the southwestern Alaska Peninsula and Unalaska and Umnak Islands, these units may represent the waning stages of the Meshik or “Lower Series” magmatic arc along the paleo-Aleutian trench. The Aleutian Islands part of unit potentially may correlate with the rocks of unit Tarcs, such as the Gunners Cove or Banjo Point Formations, although these are thought to be slightly older

Source map information

Source map Wilson, F.H., Weber, F.R., Dochat, T.M., Miller, T.P., and Detterman, R.L., 1997, Revised geologic map of the Cold Bay and False Pass quadrangles, Alaska Peninsula: U.S. Geological Survey Open-File Report 97-866, 34 p., 1 sheet, scale 1:250,000.
Symbol Tbe
Unit name Belkofski Formation
Description Consists of about 1,830 m, of gray or greenish-gray to gray-brown, tuffaceous, volcaniclastic sandstone, siltstone, and conglomerate containing interbeds of tuff and volcanic breccia along northwest shore of Belkofski Bay. This unit is conformably overlain by the Tachilni Formation and, on the basis of lithologic similarity and apparent stratigraphic position, is likely correlative with the Unga Formation (unit Tu; Detterman and others, 1996). Lower contact of the Belkofski Formation is nowhere exposed.
Lithology Sedimentary

Correlated geologic units

Label Tbe
Description Belkofski Formation
Geologic age Late-Oligocene to Middle-Miocene
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Minor