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 Gehrels, G.E., 1992, Geologic map of southern Prince of Wales Island, southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2169, 23 p., 1 sheet, scale 1:63,360.
Symbol SOdbv
Unit name Descon Formation, basaltic to andesitic volcanic rocks
Description Dark to light green basaltic to andesitic pillow flows, pillow breccia, and tuff breccia, with subordinate tuff, volcaniclastic strata, and volcanic rocks of silicic composition. Pillows range up to 1 m across, interpillow carbonate and chert are quite rare. Bedding in pillow breccia is well-developed. Locally contains interbedded volcaniclastic graywacke and black siliceous argillite. Basalt is spilitic with highly altered plagioclase (andesine) phenocrysts up to 4 mm long, with variable albitic and sericitic alteration. Clots of chlorite, epidote, and opaque minerals are probably alteration products of ferromagnesian phenocrysts. Matrix includes plagioclase microlites, chlorite, epidote, leucoxene, and opaque minerals. Lower greenschist facies to sub-greenschist facies metamorphism. Rocks do not have a foliation and plagioclase microlites are randomly oriented. Protolith textures such as pillows and breccia fragments are only slightly flattened.
Lithology Metamorphic

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