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 Riehle, J.R., Detterman, R.L., Yount, M.E., and Miller, J.W., 1993, Geologic map of the Mount Katmai quadrangle and adjacent parts of the Naknek and Afognak quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2204, 1 sheet, scale 1:250,000.
Symbol Th
Unit name Hemlock Conglomerate
Description Poorly indurated fluvial sandstone and conglomerate and subordinate siltstone, shale, coal, and tuff. Yellowish-brown to olive-gray. Conglomerate has mainly chert, quartz, and plutonic-rock clasts and less abundant clasts of metamorphic rocks and silicified volcanic rocks. Rare tree stumps in growth position; abundant plants fossils in fine-grained strata. Locally metamorphosed where adjacent to plutons and sills of unit Tab and Ti. Measured thickness of 558 m on north shore of Kukak Bay at Cape Nukshak is probably maximum in map area. Unit at Cape Nukshak is probably late Oligocene and may be as young as early Miocene in age on the basis of plant fossils (J.A. Wolfe, written commun., 1988). Disconformable on unit Tc, unconformable on unit Kk or unit Jn
Lithology Sedimentary

Correlated geologic units

Label Th
Description Hemlock Conglomerate
Geologic age Late-Oligocene to Oligocene
Geologic setting Sedimentary, deltaic-and-nearshore
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Graywacke < Sandstone < Clastic < Sedimentary Bed Indeterminate, major