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 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 16
Unit name Well consolidated, bedded rocks
Description Graywacke, sandstone, grit, quartzite, conglomerate, shale, slate, argillite, limestone, tuff, etc. Covering many places by till, colluvium, and loess less than 5 feet thick. Probably Kahiltna flysch in Susitna basin and Kuskokwim Group in Kuskokwim basin
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