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 Karl, S.M., Haeussler, P.J., Himmelberg, G.R., Zumsteg, C.L., Layer, P.W., Friedman, R.M., Roeske, S.M.,and Snee, L.W., 2015, Geologic map of Baranof Island: U.S. Geological Survey Scientific Investigations Map SIM-3335, pamphlet 82 p., 1 sheet, scale 1:250,000.
Symbol KJkv
Unit name Volcanic rocks and chert of the Kelp Bay Group
Description Dark green, fine-grained mafic volcanic rocks with meter-scale lenses of ribbon chert and subordinate mudstone and sandstone. Volcanic rocks include pillow basalt flows, breccia, and massive sills. Massive greenstones are fine-grained, holocrystalline to devitrified hypocrystalline; some contain subophitic textures and may be sills; some may have volcaniclastic protoliths. Primary minerals include plagioclase and clinopyroxene; secondary minerals include epidote, chlorite, albite, quartz, carbonate, prehnite, and pumpellyite. Geochemical analyses of mafic volcanic rocks indicate compositions range from alkaline to tholeiitic, and trace elements plot in ocean floor, within plate, and island arc fields (Karl, 1982). High Ti contents suggest some basalts may represent seamounts. Variations in thickness, texture, chemistry, and associated sedimentary rock types indicate the basalts represent a variety of magmatic sources (Karl, 1982). The volcanic rocks and chert do not have a penetrative fabric. Locally greenstone has cataclastic or mylonitic textures. Mafic volcanic rocks associated with ribbon chert are mapped in Katlian Bay, Rodman Bay, Hanus Bay, and Kelp Bay. Chert beds are one to 15 cm in thickness, in sections several to tens of meters in thickness. Unit is lithologically correlated with Waterfall Greenstone. Unit is correlated with chert and volcanic rock units in the McHugh Complex in southern Alaska (Nelson and others, 1987; Karl and others, 2011), and the Pacific Rim and Leech River Complexes on Vancouver Island (Brandon and others, 1983)
Lithology Igneous

Correlated geologic units

Label JTrkg
Description Kelp Bay Group, undivided
Geologic age Jurassic to Turonian
Geologic setting Melange
Lithology Form Importance
Melange < Tectonite Melange, blocks Major