Flysch

Unit symbol: Kfy
Age range Upper and Lower? Cretaceous (125 to 89.8 Ma)
Lithology: Sedimentary
Group name: Flysch
Monotonous sequence in central Alaska of intensely deformed and locally highly metamorphosed turbidites; includes dark-gray to black argillite; fine- to coarse-grained, generally dark-gray graywacke, siltstone, and shale turbidites; thinly bedded and dense cherty argillite; dark-gray polymictic pebble conglomerate; subordinate black chert pebble conglomerate, a few thin layers of dark-gray to black radiolarian chert; and thin, and dark-gray impure limestone interbeds described by Csejtey and others (1992) and Reed and Nelson (1980), among others. Commonly referred to as the Kahiltna terrane or assemblage (Nokleberg and others, 1994) or “black crap.” Includes rocks mapped by Reed and Elliot (1970) as units Km, Mzu, and Mzs. In Tyonek quadrangle, coquina beds yielded Buchia sublaevis Keyserling of Valanginian age (Solie and others, 1991b). Inoceramus hobetsensis Nagao and Matsumoto, of middle Turonian age, was also found within unit (W.P. Elder, written commun. to D. Bradley, 1989). Csejtey and others (1992) reported additional fossils from unit ranging from Valanginian to Cenomanian as well as several collections of radiolarians that could only be described as Jurassic or Cretaceous. Fossils indicate unit contains both Lower and Upper Cretaceous rocks; 40Ar/39Ar age determinations on hornblende from igneous clasts in the conglomerate yield Cenomanian ages (Layer and Solie, 2008), which suggests a maximum age of latest Early Cretaceous age. Detrital zircon studies by Dwight Bradley and Peter Haeussler (see Wilson and others, 2009, 2012; and Hults and others, 2013) have yielded a variety of minimum age results from Barremian to Aptian and Turonian to Santonian. Studies by Ridgway and others (2002), Kalbas and others (2007), and Hampton and others (2007) provided information indicating multiple provinces for unit and reported detrital zircon analyses that suggest the Kahiltna assemblage is, at least in part, Aptian or Albian or younger in age. Hults and others (2013) suggest unit represents multiple discrete basins separated by a major suture zone. In Talkeetna quadrangle, similar rocks were assigned a Jurassic and Cretaceous age (unit KJs of Reed and Nelson, 1980), however, the Jurassic fossils were derived from spatially and lithologically distinct rocks not part of this Kfy map unit. The Kahiltna assemblage, as generally mapped, is widespread in southern Alaska, and often the repository for miscellaneous dark-colored sedimentary rock units, and as such, does not represent a coherent package of rocks

Source map information

Source map Haeussler, P.J., 2007, Unpublished data.
Symbol Ks
Unit name Turbiditic sedimentary rocks
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
Lithology Sedimentary

Correlated geologic units

Label KJf
Description Flysch sequence, Early Cretaceous and Late Jurassic
Geologic age Aptian to Turonian
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Lithic-wacke < Graywacke < Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major
Limestone < Carbonate < Sedimentary Bed Minor
Chert < Chemical < Sedimentary Bed Minor
Conglomerate < Clastic < Sedimentary Bed Minor