Tachilni, Bear Lake, Chuniksak, Nevidiskov, and Chirikof Formations

Unit symbol: Tms
Age range Tertiary, Miocene and upper Oligocene (28.1 to 5.333 Ma)
Lithology: Sedimentary
Group name: Sedimentary rocks of southwest Alaska
Exposed on the southwestern Alaska Peninsula and Attu Island, these formations represent a quiescent stage between active volcanism in the Meshik arc (Wilson, 1985) or the informal early series of Marlow and others (1973), which was renamed “Lower Series” by Vallier and others, (1994) and the modern Aleutian magmatic arcs and are distinct from other local Tertiary units because of greater abundance of nonvolcanic debris and better sorting. The marine Tachilni Formation consists of about 60 m of gray to brown, poorly consolidated, crossbedded subgraywacke sandstone commonly interbedded with volcanic-pebble conglomerate and siltstone (Waldron, 1961; Detterman and others, 1996). Unit is richly fossiliferous, containing 36 genera of bivalves and 11 genera of gastropods (Detterman and others, 1996) that yield a late Miocene age assignment (Marincovich, 1983). Tachilni is unconformably overlain by late Tertiary and Quaternary volcanic and volcaniclastic rocks that are presumably partially correlative with Milky River Formation (unit QTvs). No lower contact of formation is known, although it may overlie the Belkofski Formation. The Bear Lake Formation (Burk, 1965; Allison and Addicott, 1973), locally 300 to 500 m thick, consists of inner-neritic marine and nonmarine, moderately well-sorted, moderately well-rounded, dark brown to pale yellowish-brown sandstone, conglomerate, siltstone, and shale (Wisehart, 1971; Nilsen, 1984). Conglomerate contains well-rounded clasts that consist of 40 to 55 percent quartz and chert, 20 to 30 percent volcanic fragments, 10 to 15 percent felsic plutonic clasts, and the remainder lithic sedimentary clasts. Bear Lake Formation is also abundantly fossiliferous and contains mainly pelecypods, gastropods, and echinoids of late Miocene age (Louie Marincovich, Jr. [USGS], and C.W. Allison [Univ. of Alaska], written commun., 1978, cited in Detterman and others, 1981a); most lived nearshore in shallow water less than 100 m deep (Marincovich, 1983, and written commun., 1985, cited in Detterman and others, 1996). The contact between Bear Lake Formation and underlying Meshik Volcanics (unit Tmv) or Stepovak and Tolstoi Formations (units Tarcs and Ttk, respectively) varies from disconformity to angular unconformity. Contact between Bear Lake and overlying Milky River Formation (QTvs) generally is disconformable; however, contact locally is an angular unconformity. Chuniksak Formation is at least 600 m of fine-grained, well-bedded, laminated siliceous shale, argillite, calcareous argillite, chert, siliceous siltstone, sandstone, sandy shale, graywacke, and minor pebble conglomerate exposed on Attu Island. Unit ranges widely in color, including dark-gray, yellow, brown, green, purple, and red and weathers to red angular chips and blocks (Gates and others, 1971). Diatoms and foraminifers sparsely scattered throughout unit; fossil mollusks and plants occur in narrow concretion-bearing zones near the middle of the measured section. Best inferred age for the unit is probably Miocene (Gates and others, 1971), although plant fossils, identified by Roland Brown (cited in Gates and others, 1971, p. 736), were suggested to be probably early Tertiary, and possibly Eocene in age. Uppermost part of unit has been removed by erosion, and locally, unit is highly deformed and faulted. Also included here are the rocks mapped as the basement rocks of Shemya Island (Gates and others, 1971, p. 776), which are lithologically similar to, and coeval with, the Chuniksak Formation. Nevidiskov Formation of Attu Island consists of coarse graywacke and conglomerate in a generally fining-upward sequence on Attu Island (Gates and others, 1971). Conglomerate clasts as much as 0.7 m in diameter are derived from the basement rocks of Attu Island (unit TKkf), which include porphyritic basalt, graywacke, and cherty argillite. In upper part of unit, lenses of argillite are more common, and contact with argillite of the Chuniksak Formation is gradational. Both the Nevidiskov and Chuniksak Formations are less deformed than the basement rocks, whose erosion was probably the source for these two map units. A single Pecten fossil from near the base of the Nevidiskov Formation was of indeterminate age. Gates and others (1971) consider the Nevidiskov Formation to be perhaps of early Miocene or late Oligocene age because of its conformable contact with the overlying Chuniksak Formation of probable Miocene age. The Chirikof Formation on Attu Island consists of boulder and pebble conglomerate, coarse sandstone, carbonaceous shale and sandstone, and possibly a single lava flow (Gates and others, 1971). Boulder conglomerate contains rounded boulders, whereas fragments in the pebble conglomerate and coarse sandstone are angular to round. The pebble conglomerate contains fragments “* * * of argillite, basalt, chert, and albite granite in a crudely bedded and poorly sorted sand matrix” (Gates and others, 1971, p. 738). The sandstone contains “* * * grains of quartz, plagioclase, argillite, chert, basalt and carbonaceous fragments cemented by limy, carbonaceous, and siliceous material. Interbedded with the coarse sand are beds of coaly shale with many leaf and stem imprints and a few silicified tree limbs” (Gates and others, 1971, p. 738). Plant fossils from the carbonaceous shale beds include some of the same species found in the Chuniksak Formation and suggest a similar age

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 Tta
Unit name Tachilni Formation
Description Composed of gray to brown, poorly consolidated, cross-bedded, subgraywacke sandstone commonly interbedded with volcanic-pebble conglomerate and siltstone (Detterman and others, 1996). Sandstone is composed of 30-35 percent angular quartz, 30 percent volcanic rock fragments, 10-15 percent feldspar, and 5 percent pyroxene and amphibole in a clay matrix.
Lithology Sedimentary

Correlated geologic units

Label Tta
Description Tachilni Formation, Chuniksak Fm.
Geologic age Miocene
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Graywacke < Sandstone < Clastic < Sedimentary Bed Major
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major