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 Intruded by Early Jurassic diorite on Halleck Island (included in unit KJdg) (S.M. Karl, unpublished data)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 Trsv
Unit name Metasedimentary and Metavolcanic rocks
Description Gray and green volcaniclastic sandstone, mudstone, tuff, chert, limestone, and intermediate to mafic volcanic rocks. In Nakwasina Sound and near Red Bluff Bay the unit is recrystallized to greenstone, carbonaceous phyllite, marble, and semischist. The unit is composed dominantly of volcanic and volcaniclastic rocks, with interlayered marble and sooty, carbonaceous argillite. Augite and labradorite are relict primary minerals in the greenstone. Secondary minerals include epidote, pyrite, magnetite, and chalcopyrite. Common meter-scale lenses of limestone distinguish unit Trsv from rocks of the Kelp Bay Group. Rock types in Trsv alternate in layers meters to tens of meters thick, and are not chaotically mixed like rocks in the Khaz Complex. Mafic volcanic rocks are more massive, form thicker sections, and lack interbedded chert that characterizes the volcanic rocks of the Kelp Bay Group. On Baranof Island, primary depositional textures are preserved in volcaniclastic rocks, pillowed greenstone, bedded limestone, argillaceous limestone turbidites, mudstone, and bedded chert. Limestone and chert commonly have stylolitic bed partings. Unit represents mostly deep water, basinal sedimentary deposits. Unit is folded, faulted, and locally sheared. Unit thickness is unknown due to faulting. Unit occupies a higher structural position than the Kelp Bay Group. Tectonic blocks of Trsv may locally be faulted into the Khaz Complex; blocks of limestone and carbonaceous mudstone are suspected to be tectonic inclusions, possibly due to subduction erosion recycling processes as described by Clift and Vannucchi (2004) because they contrast with the siliceous argillite of the Khaz Complex and resemble rocks of the upper plate. Mylonitic structures in the Trsv unit at Red Bluff Bay postdate its metamorphic fabric. Trsv is intruded by Jurassic diorite on in Nakwasina Sound and on northeastern Yakobi Island (Johnson and Karl, 1985). Trsv is inferred to represent a depositional facies of massive Triassic volcanic rocks assigned to Wrangellia on the Baranof-Chichagof block (Jones and others, 1977). Trsv is not similar to age-correlative rocks of the Lower Jurassic-Upper Triassic rocks of the McCarthy Formation in the Wrangell Mountains
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