Volcanic rocks of the Orca Group and Ghost Rocks Formation

Unit symbol: Togv
Age range Tertiary, Eocene to Paleocene (66 to 47.8 Ma)
Lithology: Igneous - Volcanic
Group name: Volcanic rocks of the Orca Group and Ghost Rocks Formation
The volcanic rocks of the Orca Group consist of thick to thin tabular bodies of altered tholeiitic basalt that have pillowed, massive, or crudely columnar flows and also include pillow breccia, aquagene tuff, and diabase or gabbro sills; pillows have palagonitic and amygdaloidal chilled margins (Winkler and Plafker, 1993). Also includes mafic sheeted-dike complexes that consist of dark-green, gray, and brown, aphanitic to porphyritic, chiefly basaltic, but locally gabbroic to dioritic dikes (Tysdal and others, 1977, Tysdal and Case, 1979, Winkler and Plafker, 1993). Minor interbedded mudstone and siltstone included locally. Commonly contains green, gray, or red chert in interstices between pillows; rarely includes interpillow clots of pink limestone or black mudstone. In the Knight Island area, felsic plagioclase-quartz dikes occur locally and plagioclase-clinopyroxene-olivine dikes are present but not common (Tysdal and Case, 1979). The dikes are commonly 1 to 2 m thick, vertical or nearly so, and generally strike north (Tysdal and others, 1977, Tysdal and Case, 1979). The dikes contain greenschist facies mineral assemblages ascribed to ocean-floor metamorphism by Bradley and Miller (2006). The dikes crosscut one another, intrude the adjacent pillow basalt, and, on Knight Island, locally intrude sedimentary rocks of the Orca Group (Tysdal and others, 1977). Pillow basalt screens are common in the up-to-2-km-wide transition zone between the pillow basalt and sheeted dike units (Tysdal and others, 1977; Tysdal and Case, 1979; Miller, 1984; Bradley and Miller, 2006). Xenoliths of gabbro and peridotite are present locally on Knight Island (Richter, 1965; Nelson and others, 1985). Small irregular pods, veins, and dikes of plagiogranite are also present in the dike complex north of Bay of Isles on Knight Island (Nelson and others, 1985). The dikes are intruded by and also intrude the gabbro in the transition zone between the gabbro and sheeted dike units (Tysdal and Case, 1979; Tysdal and others, 1977). The dikes make up the topographically high and rugged core of Knight and Glacier Islands (Tysdal and others, 1977). Whole rock K/Ar ages on greenstone, reported by Miller (1984), are 38.8±1.9 and 35.0±1.3 Ma on Knight Island; Miller (1984) interpreted these ages to represent minimum ages for accretion due to heating during the accretionary event that may have caused argon loss. The Ghost Rocks Formation, exposed along the Pacific Ocean side of the Kodiak Island archipelago consists of tholeiitic basalt that occurs within both sandstone- and argillite-rich subunits of the Ghost Rocks Formation. Rocks are typically altered by shearing and low-grade metamorphism. The basalt is included in this unit and, according to Moore and others (1983, p. 270), “* * * these lavas cannot have been derived from a single source and in many respects exhibit chemical affinities to magmas found in a variety of tectonic environments.” Mafic and ultramafic intrusive rocks of the Orca Group are included in unit Togum

Source map information

Source map Nelson, S.W., Miller, M.L., Haeussler, P.J., Snee, L.W., Phillips, P.J., Huber, Carol, 1999, Preliminary geologic map of the Chugach National Forest Special Study Area, Alaska: U.S. Geological Survey Open-File Report 99-362, 1 sheet, scale 1:63,360.
Symbol Tod
Unit name Ophiolite of Glacier Island - Sheeted dikes
Description Mafic sheeted dikes are basaltic in composition and form the lower oceanic-crust part of the ophiolite of Glacier Island. The dikes are diabasic, equigranular, and porphyritic. Phenocrysts of plagioclase and clinopyroxene are common. Facings on chilled margins and other field observations indicate that the dikes were intruded into, between, and across pre-existing dikes. Most dikes are vertical in orientation. The chemistry from typical mid-ocean ridge basalt (Crowe and others, 1992) and the ophiolite is part of a 80 km long belt of ophiolitic rocks that starts at Elrington Island to the south and includes Knight Island, Glacier Island, and the Ellamar area
Lithology Igneous

Correlated geologic units

Label Tosd
Description Orca Group, sheeted dikes, Paleocene and Eocene?
Geologic age Paleocene to Early-Eocene
Geologic setting Extrusive
Lithology Form Importance
Hypabyssal-basalt < Mafic-hypabyssal < Hypabyssal < Igneous Dike or sill Major