Yakutat Group, undivided

Unit symbol: KJyg
Age range Lower Cretaceous and Upper Jurassic? (163.5 to 66 Ma)
Lithology: Sedimentary
Group name: Yakutat Group, undivided
Heterogeneous assemblage that includes clastic sedimentary rocks, altered volcanic rocks, chert, carbonate, and granitic rocks. Yakutat Group consists of two major subdivisions that are commonly structurally juxtaposed: a flysch facies and a mélange facies (unit Kmy). Shown here, unit is the undivided Yakutat Group and the flysch facies, which locally has been mapped separately (George Plafker, written commun., 2000). Flysch facies is dominantly dense, hard, poorly sorted gray to brown feldspathic to lithofeldspathic sandstone (graywacke), pebble-cobble conglomerate, and shale-chip conglomerate in thick channel deposits or rhythmically interbedded and graded with gray to black siltstone, argillite, or slate. Fossils of Campanian age have been reported from the flysch facies (Plafker and others, 1994). Locally, contains very sparse warm water molluscan fauna and abundant carbonized subtropical plant remains (George Plafker, written commun., 2000). Also included here is the schist of Nunatak Fiord of Plafker (written commun., 2000), which is complexly deformed, interbedded mudstone and sandstone and minor volcaniclastic rocks and conglomerate metamorphosed to greenschist- and epidote-amphibolite facies. At southern extent of outcrop area, unit includes volcanic rocks metamorphosed to as high as amphibolite facies. Crops out between Boundary and Fairweather Fault Systems and intruded extensively by Tertiary granitic plutons (George Plafker, written commun., 2000)

Source map information

Source map Plafker, George, 2002, Unpublished data.
Symbol Kyb
Unit name Metamorphic rocks of the Brabazon Range
Description Mainly amphibolite facies rocks more than 1,000 -m-thick. Grades over narrow interval into zeolite- and prehnite-pumpellyite-grade rocks of the Yakutat Group. Protolith mainly graywacke-carbonaceous pelite sequence recrystallized to quartzofeldspathic schist.
Lithology Metamorphic

Correlated geologic units

Label KJyb
Description Metamorphic rocks of the Brabazon Range (Yakutat Group equivalent)
Geologic age Late-Jurassic to Cretaceous
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Metasandstone < Metaclastic < Metasedimentary < Metamorphic Greenschist Major
Quartz-feldspar-schist < Schist < Metamorphic Greenschist Major
Amphibolite < Metamorphic Amphibolite Indeterminate, major