Kelp Bay Group, undivided

Unit symbol: KTrm
Age range Cretaceous to Triassic (252.2 to 89.8 Ma)
Lithology: Tectonite
Group name: Kelp Bay Group, undivided
Consists of volcanic rocks, sandstone, chert, the mélange of the Khaz Complex (formerly Khaz Formation) of Karl and others (2015), the Pinnacle Peak Phyllite and the phyllite of Rodman Bay. Primarily exposed in the Sitka and Port Alexander quadrangles, also included here is an outlier of similar rocks associated with the Tarr Inlet Suture Zone in the Mount Fairweather and Skagway quadrangles. The undivided Kelp Bay Group consists of phyllite, quartzite, greenschist, greenstone, graywacke, and graywacke semischist and has been locally subdivided and mapped as two informal members, a sandstone member and a volcanic rocks and chert member, as well as the more formally defined members. The sandstone member consists of altered and recrystallized dull-green graywacke, volcanic wacke, turbiditic argillite, and subordinate polymictic conglomerate. Unit also includes gray and green, thin- to medium-bedded volcaniclastic turbidites and interbedded tuff (Karl and others, 2015). The volcanic rocks and chert member consists of dark-green, fine-grained pillow basalt flows and breccia that has meter-scale lenses of ribbon chert, subordinate mudstone and sandstone, and massive greenstone that may be sills or may have a volcaniclastic protolith. Geochemical analysis of the volcanic rocks indicate that compositions range from alkaline to tholeiitic, and trace elements plot in ocean floor, within plate, and island arc fields (Karl, 1982). The volcanic rocks and chert do not have a penetrative fabric, though locally the greenstone has a cataclastic or mylonitic texture (Karl and others, 2015). Karl and others (2015) reports that the volcanic rocks and chert member can be correlated with chert and volcanic rock units in the McHugh Complex in southern Alaska. Both the Kelp Bay Group and the McHugh Complex are considered part of the Chugach accretionary complex. Here we also include small slivers of serpentinite and partially serpentinized peridotite on Baranof Island that were found enclosed in the phyllite unit

Source map information

Source map Johnson, B.R., and Karl, S.M., 1985, Geologic map of western Chichagof and Yakobi Islands, southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1506, pamphlet, 15 p., 1 sheet, scale 1:125,000.
Symbol Kw
Unit name Waterfall Greenstone
Description Dominantly greenstone with lesser amounts of graywacke, greenschist, radiolarian chert, and marble. Generally recrystallized to epidote, chlorite, albite, calcite, and prehnite with occasional relict plagioclase and augite phenocrysts. Chert is coeval with chert collected from greenstone in the McHugh Complex, which is also considered to be a component of the melange facies of the Cretaceous accretionary terrane of Plafker and others (1977). Part of the Freeburn assemblage. NOTE: More extensive description in pamphlet.
Lithology Metamorphic

Correlated geologic units

Label KJkw
Description Waterfall Greenstone and other greenstone associated with the Khaz Formation or Freeburn assemblage, southeast Alaska
Geologic age Jurassic to Early-Cretaceous
Geologic setting Melange
Lithology Form Importance
Greenstone < Metaigneous < Metamorphic Greenschist Major
Chert < Chemical < Sedimentary Bed Minor
Limestone < Carbonate < Sedimentary Bed Incidental
Graywacke < Sandstone < Clastic < Sedimentary Bed Incidental