| Description |
Typically consist of interbedded dark-gray to black, locally greenish-gray, sandstone, siltstone, and shale turbidites. Also locally includes granule- to boulder-conglomerate, dark-gray to black, thinly bedded and dense, cherty argillite, and limestone. Sandstone includes graywacke in beds up to 2 m thick and feldspathic sandstone. Includes rocks mapped by Reed and Elliot (1970) as their units Km, Mzu, and Mzs. Locally hornfelsed on the margin of intrusions. Three fossils were found in this unit in the Tyonek quadrangle. Coquina beds along the lower Chickak River yielded Buchia sublaevis Keyserling of Valanginian age (Solie and others, 1991). A middle Turonian Inoceramus hobetsensis Nagao and Matsumoto was also found within this unit 1 km east of the Skwentna River 3 km north of its confluence with Emerald Creek (William P. Elder, USGS, written commun., 1989 to Dwight Bradley). A float sample of chert from the headwaters of Old Man Creek contained poorly preserved Spumellariina, which yielded a not-tightly constrained age between middle Toarcian (Lower Jurassic) to upper Valanginian (Lower Cretaceous) (E.A. Pessagno, Univ. of Texas, written commun., 2006, to Marti Miller, USGS). 40Ar/39Ar age determinations (Table 1) on hornblende from igneous clasts in the conglomerate yielded Cenomanian ages. Fossils indicate the rocks are both Early and Late Cretaceous in age, and the radiometric ages on the conglomerate clasts indicate latest Early Cretaceous age igneous rocks formed a source for some of the unit. Thus, considering the Barremian to Aptian and Turonian to Santonian detrital zircon ages within unit Kes, to the southwest, it appears there are at least two sequences of Cretaceous sedimentary rocks within unit Ks, which we are unable to distinguish based upon appearance. In the Talkeetna quadrangle to the north, similar rocks were assigned a Jurassic and Cretaceous age (map unit KJs, Reed and Nelson, 1980); however, the Jurassic fossils were derived from spatially and lithologically distinct rocks which we do not consider as part of this map unit. These rocks are traditionally assigned to the informally named Kahiltna assemblage (Nokleberg and others, 1994), more recent studies by Ridgway and others (2002), Kalbas and others (2007), and Hampton and others (2007) have reported detrital zircon analyses that suggest the Kahiltna assemblage is Aptian or Albian or younger, similar to some of detrital zircon analyses reported for unit Kes below. However, these analyses are somewhat in conflict with the Valanginian fossils reported by Solie and others (1991). The Kahiltna assemblage, widespread in southern Alaska, is often the repository for miscellaneous dark-colored sedimentary rock units and as such may not represent a coherent package of rocks. In the southwest part of the Tyonek quadrangle, the nature of the transition from this unit to the Kuskokwim Group is undefined. Both units have similar lithology and character and available mapping is not sufficient to either distinguish the units or to indicate that they should be mapped as the same unit. Locally, the presence of interbedded light tuffaceous deposits indicates contemporaneous volcanism; in these same areas, extremely angular grains of oscillatory zoned plagioclase, volcanic rock fragments, hornblende, epidote, and calcite, suggest derivation from the now buried Jurassic magmatic arc to the south (Reed and Nelson, 1980). Unit as mapped in the Tyonek quadrangle includes rocks mapped by Reed and Elliott (1970) as units Km, Mzu, and Mzs. Locally subdivided |
