Associated volcanic rocks

Unit symbol: SOv
Age range Silurian, Llandovery, and Ordovician (485.4 to 433.4 Ma)
Lithology: Sedimentary
Group name: Sedimentary and volcanic rocks of Prince of Wales Island
Felsic to mafic volcanic rocks and minor interbedded argillite and volcaniclastic conglomerate. Includes basaltic to andesitic pillow flows, porphyritic andesite, andesitic to rhyolitic breccia and tuff, and agglomerate (Eberlein and others, 1983; Brew and others, 1984; Gehrels, 1992; Brew, 1996). Unit is locally metamorphosed to subgreenschist or lower greenschist facies (Gehrels, 1992). Ordovician plutons (predominately Middle Ordovician) of unit Ogi are coeval and were probably the magma source of the volcanic rocks (Gehrels and Saleeby, 1987). On Prince of Wales Island, includes pale-colored dacitic and rhyolitic breccia, tuff-breccia, tuff, and extrusive domes (Brew, 1996) and andesitic and basaltic flows and breccia (Herreid and others, 1978; Rubin and Saleeby, 1992). Age constrained by stratigraphic relations with sedimentary rocks of the Descon Formation (included in unit SOd), intrusive relations with plutons of Late and Middle Ordovician age (unit Ogi), and K/Ar ages of volcanic rocks that indicate early Silurian and Late Ordovician emplacement (Eberlein and others, 1983; Gehrels and Saleeby, 1987)

Source map information

Source map Brew, D.A., compiler, 1996, Geologic map of the Craig, Dixon Entrance, and parts of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2319, 53 p., 1 sheet, scale 1:250,000.
Symbol SOdb
Unit name Descon Formation - Basaltic to andesitic volcanic rocks
Description Augite-bearing basaltic to andesitic pillow flows, pillow breccia, agglomerate, breccia, and tuff-breccia; some massive lava flow deposits.
Lithology Igneous

Correlated geologic units

Label SOdvm
Description Descon Formation, basaltic to andesitic volcanic rocks
Geologic age Ordovician to Llandovery
Geologic setting Extrusive
Lithology Form Importance
Basaltic-andesite < Mafic-volcanic < Volcanic < Igneous Flow, pillows Major
Basaltic-andesite < Mafic-volcanic < Volcanic < Igneous Pyroclastic, tuff Major
Basaltic-andesite < Mafic-volcanic < Volcanic < Igneous Volcaniclastic, volcanic breccia Minor
Felsic-volcanic < Volcanic < Igneous Incidental
Limestone < Carbonate < Sedimentary Lens Incidental