Mélange of the Yakutat Group

Unit symbol: KJmy
Age range Cretaceous and Jurassic? (163.5 to 66 Ma)
Lithology: Tectonite
Group name: Mélanges
Yakutat Group is a heterogeneous rock assemblage including clastic sedimentary rocks, altered volcanic rocks, chert, carbonate, and granitic rocks largely outboard of the Fairweather Fault in the vicinity of Yakutat on the Gulf of Alaska. It consists of two major subdivisions that are commonly structurally juxtaposed: a flysch facies, which is described above as unit KJyg, and a mélange facies, which is described here. The mélange facies is characteristically composed of blocks of competent rocks of heterogeneous lithology, as much as several kilometers in size, enclosed in a pervasively sheared matrix of dark-gray to black mudstone, which may be cherty or tuffaceous (George Plafker, written commun, 2000). Mélange blocks are predominantly greenstone, oolitic limestone, marble, granitic plutonic rocks, chert, and graywacke. Greenstone masses are generally massive and structure-less but locally have pillow forms and agglomeratic texture, commonly have epidote-rich pods and stringers, are cut by diorite dikes, and are associated with limestone and marble, as well as pods of radiolarian-bearing ribbon chert. Poorly preserved fossils near Russell Fiord indicate that the marble may be Late Triassic in age. Bodies of foliated, fine- to medium-grained hornblende biotite tonalite are associated with the greenstone; a pluton in Russell Fiord, an arm of Yakutat Bay, yielded a K/Ar hornblende age of 163.3±3.5 Ma (Hudson and others, 1977). Lenticular bodies of massive laumontite-rich volcanic lithic and feldspathic sandstone and lenses as thick as 300 m and beds of well-rounded and sorted pebble and cobble conglomerate as thick as 200 m are locally developed and are thought to be olistostrome deposits exhibiting soft-sediment deformation; they probably represent deep sea channel fills (George Plafker, written commun, 2000). At Russell Fiord near Hidden Glacier unit also includes angular blocks of gray oolitic limestone that has chert nodules

Source map information

Source map Brew, D.A., 1997, Reconnaissance bedrock geologic map of Glacier Bay National Park, Alaska. in Geiselman, Joy, Dunlap, J., Hodge, P. and Albert, D., eds., Glacier Bay Ecosystem GIS Volume 1: U.S. Geological Survey, Biological Resources Division. (Digital map)
Symbol Kym
Unit name Yakutat Group Melange
Description Melange - mixed chert, greenstone, and phyllite. Highly folded / faulted, found in Lituya Province (Brew, pers. comm., 1999).
Lithology Tectonite

Correlated geologic units

Label KJyx
Description Yakutat Group, melange facies
Geologic age Late-Jurassic to Cretaceous
Geologic setting Melange
Lithology Form Importance
Melange < Tectonite Melange, matrix Major