Volcanic and sedimentary rocks of Nakwasina Sound

Unit symbol: Trvs
Age range Triassic (252.2 to 201.3 Ma)
Lithology: Igneous - Volcanic
Group name: Massive basalt and greenstone
Consists of intensely folded massive and schistose greenstone, graphitic schist, phyllite, and graywacke. Unit is dominantly greenstone with interlayered graphitic schist or phyllite. Massive greenstone, greenschist, phyllite, limestone, graywacke, and chert are subordinate. Contains lenticular beds of metalimestone similar in color and composition to the Whitestripe Marble (unit Trwm, here) (Loney and others, 1975). Intruded by Early Jurassic diorite on Halleck Island (included in unit KJdg) (S.M. Karl, unpublished data)

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 Pzsv
Unit name Metasedimentary and metavolcanic rocks of Nakwasina Sound
Description Light green, tan, and white, banded, siliceous, felsic, and calcareous semischist, quartzite, metatuff, and marble. Unit includes cm-scale bands of garnet-diopside-biotite schist. Buff-colored, massive aphanitic felsic rocks contain quartz eyes. Felsic and silicic semischist locally have relict volcaniclastic textures. Marble and semischist contain sparse biotite, sericite, and/or chlorite; epidote, diopside, and garnet. Composition and relict textures suggest protoliths include felsic to mafic volcanic breccia, tuff and volcaniclastic rocks, limestone, calcareous mudstone, siliceous mudstone, and chert. Shallow to deep marine depositional environment adjacent to a volcanic arc is inferred; bimodal felsic and mafic volcanic rocks suggest some volcanic rocks represent an interval of arc extension. Because the Goon Dip Greenstone has been correlated with the Nikolai Greenstone (Jones and others, 1977), which overlies the Skolai Group in the southern Wrangell Mountains, the rocks of Pzsv are stratigraphically equivalent to rocks of the Skolai Group
Lithology Metamorphic

Correlated geologic units

Label Trvs
Description Volcaniclastic rocks of Rodman Bay and sedimentary and volcanic rocks of Nakwasina Sound
Geologic age Triassic
Geologic setting Melange
Lithology Form Importance
Melange < Tectonite Melange, blocks Major