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Source map Karl, S.M., 2013, Unpublished data.
Symbol KJk
Unit name Kelp Bay Group
Description The Kelp Bay Group is a diverse assemblage of tectonized sedimentary and volcanic rocks of Mesozoic age named for rocks at Kelp Bay on northern Baranof Island by Berg and Hinckley (1963). It was redefined by Loney and others (1975) to include the Triassic Goon Dip Greenstone, Whitestripe Marble, Pinnacle Peak Phyllite, greenstone, chert, and limestone of the Waterfall Greenstone, and Jurassic and Triassic schist and limestone on Chichagof Island. On northern Baranof Island, Loney and others (1975) subdivided the Kelp Bay Group into Triassic greenschist, phyllite, and graywacke units, the Jurassic and Triassic Khaz Formation, and schist, gneiss, amphibolite, and greenschist units. Plafker and others (1976; 1977) correlated the younger units of the Kelp Bay Group with the mélange facies of the Chugach accretionary complex, and correlated the Triassic Goon Dip Greenstone and Whitestripe Marble with their upper plate, including greenstone and marble in the Wrangell Mountains. Johnson and Karl (1985) restricted the Kelp Bay Group to the units in the accretionary complex, and excluded the Goon Dip Greenstone and Whitestripe Marble, which were not considered to be part of the accretionary complex, and were correlated with Wrangellia by Jones and others (1977). On northern Baranof Island, Loney and others (1975) included the Paleozoic Nakwasina Group of Berg and Hinckley (1963) in the Kelp Bay Group, and abandoned the Nakwasina Group. These Paleozoic rocks are basement to the Triassic greenstone and marble of the upper plate rocks, and excluded from the Kelp Bay Group, which represents the lower plate rocks, in this report. The Kelp Bay Group is interpreted to represent the mélange facies of an accretionary complex that ranges from Triassic through Late Cretaceous in age (Johnson and Karl, 1985). On Baranof Island it includes the Triassic(?) Pinnacle Peak Phyllite (Trp), the Khaz Complex (KJkk, formerly Khaz Formation), consisting of slatey argillite and tuff that contain blocks of mudstone, graywacke, limestone, chert, basalt, and diorite; graywacke turbidites (KJks), and mafic volcanic and volcaniclastic rocks and chert (KJkv). On Chichagof and Yakobi Islands, the Kelp Bay Group also includes the Freeburn Assemblage, a tectonic collage composed of kilometer-scale, fault-bounded blocks of metasedimentary and metavolcanic rocks that form a continuous belt (Johnson and Karl, 1985), and the Waterfall Greenstone which corresponds in part to KJkv. Meter-scale blocks that resemble Triassic rocks of the upper plate are locally tectonically incorporated by Mesozoic and Tertiary faults into the Khaz Complex on Baranof Island. In this report, the Khaz Formation is renamed Khaz Complex, because it has no internal or external stratigraphic boundaries, and because its genesis and appearance are dominated by tectonic processes rather than depositional processes, analogous to the correlative Uyak Complex of Kodiak Island and McHugh Complex of southern Alaska
Lithology Sedimentary

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