Geologic Map of Alaska: Sedimentary and volcanic rocks of the Orca Group, undivided

Sedimentary and volcanic rocks of the Orca Group, undivided

Unit symbol:	Tovs
Age range	Tertiary, Eocene to Paleocene (59.2 to 38 Ma)
Lithology:	Sedimentary
Group name:	Sedimentary and volcanic rocks of the Orca Group, undivided

Consists of locally variable amounts of tholeiitic basalt, and tuffaceous and generally minor turbiditic sedimentary rocks (Tysdal and Case, 1979; Nelson and others, 1985; Winkler and Plafker, 1993; Wilson and others, 2012) in Prince William Sound. Volcanic rocks are pillowed, and massive basalt flows, pillow breccia, and tuff are intermixed and interbedded with turbiditic mudstone, siltstone, and fossiliferous volcanogenic sandstone; locally nonvolcanogenic sandstone is interbedded (Winkler and Plafker, 1993). Locally subdivided into units Tos, below, and Togv, described with the volcanic rocks

Source map information

Source map	Nelson, S.W., Dumoulin, J.A., and Miller, M.L., 1985, Geologic map of the Chugach National Forest: U.S. Geological Survey Miscellaneous Field Studies Map MF-1645B, pamphlet, 16 p., 1 sheet, scale 1:250,000 (Chugach NF Arc/Info coverage 1993).
Symbol	Tops
Unit name	Orca Group -- Pillow basalt and sedimentary rocks
Description	Pillow basalt, massive basalt flows, minor volcanic breccia interbedded with sandstone, siltstone, and shale. Distinguished from the interbedded sedimentary and mafic volcanic rocks unit on the basis of the predominance of pillow basalt in the section. Probably gradational with the pillow basalt unit. Two measured sections on Ellamar Mountain show significant differences in the percent of interbedded volcanic and sedimentary rocks that reflect rapid lensing out of units probably due to the presence of an irregular paleosurface. One section contains 92 percent volcanic rocks and 8 percent sedimentary rocks, and the other section about 1,200 ft laterally distant contains 75 percent volcanic rocks and 25 percent sedimentary rocks.
Lithology	Igneous

Correlated geologic units

Label	Tovs
Description	Sedimentary and volcanic rocks of the Orca Group, undivided
Geologic age	Late-Paleocene to Middle-Eocene
Geologic setting	Sedimentary, slope-and-deep-water

Lithology	Form	Importance
Mafic-volcanic < Volcanic < Igneous	Pyroclastic	Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous	Flow	Indeterminate, major
Greenstone < Metaigneous < Metamorphic	Greenschist	Indeterminate, major
Slate < Metaclastic < Metasedimentary < Metamorphic	Greenschist	Indeterminate, major
Sandstone < Clastic < Sedimentary	Bed	Indeterminate, major
Siltstone < Clastic < Sedimentary	Bed	Indeterminate, major
Mudstone < Clastic < Sedimentary	Bed	Minor
Chert < Chemical < Sedimentary	Bed	Incidental