Kenai Group, undivided

Unit symbol: Tkn
Age range Tertiary, Miocene to Oligocene (28.1 to 3.6 Ma)
Lithology: Sedimentary
Group name: Nearshore and nonmarine sedimentary rocks in southern Alaska
Coal-bearing clastic unit in vicinity of Cook Inlet that consists of, in descending stratigraphic order: Sterling, Beluga, and Tyonek Formations and Hemlock Conglomerate. According to Calderwood and Fackler (1972), unit is at least 8,000 m thick in the subsurface of Cook Inlet. Individual formations are typically estuarine and nonmarine clastic sedimentary rocks. Sterling Formation is interbedded, weakly lithified sandstone, siltstone, mudstone, carbonaceous shale, lignite coal, and minor volcanic ash. Beluga Formation is similarly nonmarine, interbedded, weakly lithified sandstone, siltstone, mudstone, carbonaceous shale, coal, and minor volcanic ash (Bradley and others, 1999). Calderwood and Fackler (1972) reported that a distinctive feature of the Beluga Formation is its lack of massive sandstone beds and massive coal seams that characterize the underlying Tyonek Formation; however, lignitic to subbituminous coal seams can be as much as 4 m thick, though more typically are 2 m or less thick in the upper part of Beluga Formation. The contact between Beluga and overlying Sterling Formation may be an unconformity, but in any case can be difficult to pinpoint (Calderwood and Fackler, 1972; Turner and others, 1980). Tyonek Formation is carbonaceous nonmarine conglomerate and subordinate sandstone, siltstone, and coal (Winkler, 1992; Bradley and others, 1999) and is identified by massive sandstone beds and lignitic to subbituminous coal beds as much as 9 m thick (Calderwood and Fackler, 1972). Hemlock Conglomerate consists of fluvial conglomeratic sandstone and conglomerate that contains minor interbeds of siltstone, shale, and coal and is lithologically transitional with Tyonek Formation, leading to some confusion; Hemlock Conglomerate is best known from the subsurface. Calderwood and Fackler (1972) included West Foreland Formation within the Kenai Group; however, it was separated as a distinct unit by Magoon and others (1976). Swenson (1997) has proposed an alternative stratigraphic column for Cook Inlet basin that recognizes the time-transgressive nature of the units, wherein all units of Kenai Group and West Foreland Formation overlap somewhat in age. Dallegge and Layer (2004) suggested that the age range of the stratigraphic units be revised based on 40Ar/39Ar dating of tephra from within Kenai Group. In particular, they document the time-transgressive nature of the formations and that the Tyonek Formation may be as old as 49 Ma (early Eocene, Ypresian) in the Matanuska Valley, making its lower part age-equivalent with the Hemlock Conglomerate and West Foreland Formation. According to R.G. Stanley (written commun., 2009) the type sections for West Foreland Formation and the subdivisions of Kenai Group are not in outcrop but rather in the subsurface located in several different wells, both onshore and offshore. These wells, in turn, are located many miles from each other in an area of complicated structure and lateral facies changes; therefore, correlation of these subsurface type sections with the surface outcrops is poorly documented, difficult, and controversial

Source map information

Source map Weber, F.R., 1961, Reconnaissance engineering geology for the selection of highway route from Talkeetna to McGrath, Alaska: U.S. Geological Survey Open-File Report 61-169, 2 sheets, scale 1:250,000.
Symbol 15
Unit name Poorly consolidated, bedded rocks, prob. Sterling
Description Loosely cemented conglomerate, sandstone, clay shale, and lignite. Conglomerate composed of well-rounded pebbles of varied, resistant rock types; generally four inches in diameter or less. Conglomerate layers usually near top of poorly consolidated rock section, occur near the mountains, and are frequently a reddish color because of iron-staining or oxidation. Sandstone, clay shale, and lignitic coal make up major part of section. Sandstone, common in Susitna lowland, is yellowish gray, well-sorted, fine to medium grained; grains are subangular. Coal is soft brownish-black, and occurs in beds or is recognizable as fossil tree stumps.
Lithology Sedimentary

Correlated geologic units

Label Tsf
Description Sterling Formation
Geologic age Late-Miocene to Zanclean
Geologic setting Sedimentary, continental
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major
Shale < Mudstone < Clastic < Sedimentary Carbonaceous Incidental
Lignite < Coal < Sedimentary Bed Incidental