Descon Formation and other sedimentary and volcanic rocks of Prince of Wales Island

Unit symbol: SOd
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
Unit includes the sedimentary and volcanic rocks of the Descon Formation of Eberlein and Churkin (1970) and coeval metamorphic rocks of the informally named Moira Sound unit (Slack and others, 2007; Ayuso and others, 2007). The Descon Formation consists of volcaniclastic mudstone and graywacke turbidites, subordinate conglomerate, sandstone, and shale, and minor limestone, chert, and basalt flows and breccia at least 2,600 m thick (Eberlein and Churkin, 1970; Herreid and others, 1978; Gehrels, 1992; Brew, 1996). Graywacke is largely basaltic detritus in a noncalcareous chlorite-rich matrix and commonly shows graded bedding. Conglomerate and sedimentary breccia includes crudely layered porphyritic-andesite breccia and polymictic conglomerate that has clasts of chert, felsic volcanic rock, graywacke, and gabbro (Brew, 1996); metamorphic rock clasts are conspicuously absent. The Moira Sound unit is dominantly siltstone, mudstone, graywacke turbidite, and carbonaceous argillite, alternating with thick sections of mafic, intermediate-composition and silicic volcanic rocks with minor conglomerate and limestone, all metamorphosed to greenschist faces (Slack and others, 2007; Ayuso and others, 2007). Conglomerate in the Moira Sound unit contains metamorphic rock clasts thought to be derived from the underlying Wales Group (CPxwg), which is a distinct difference from the conglomerate of the Descon Formation. Both units are dated by graptolites of Ordovician and early Silurian (Llandovery) age (Churkin and others, 1970). The Descon Formation is conformably overlain by the late Silurian (Llandovery to Ludlow) Heceta Limestone (unit Shpa of Eberlein and Churkin, 1970). The Descon Formation contains Middle to Late Ordovician detrital zircons (Gehrels and others, 1996), whereas the Moira Sound unit contains detrital zircons reflective of the underlying Wales Group (S.M. Karl, unpub. data). Although the base of the Descon Formation has not been observed, Gehrels and Saleeby (1987) suggested that it may unconformably overlie the Wales Group (unit CPxwg) because the Wales metamorphic suite has been deformed and metamorphosed more than the Descon Formation. While this inference might be true for the Moira Sound unit on southern Prince of Wales Island (mapped as Descon by Gehrels and Saleeby, 1987), this is probably not true for the Descon Formation proper. Locally subdivided into units SOdc and SOv

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 SOdg
Unit name Descon Formation - Volcaniclastic graywacke, siltstone, mudstone turbidites, and minor sedimentary breccia, limestone, and polymictic conglomerate
Description Mainly graywacke and mudstone with interbedded massive basalt lava and pyroclastic flows. Graywacke consists of basaltic detritus in noncalcareous chlorite-rich matrix; graded bedding common. Conglomerate and sedimentary breccia range from crudely layered augite-phenocryst-rich andesite breccia to polymictic types with clasts of chert, felsic volcanic rock, graywacke and gabbro.
Lithology Sedimentary

Correlated geologic units

Label SOdu
Description Descon Formation, undivided sedimentary and volcanic rocks
Geologic age Ordovician to Llandovery
Geologic setting Undivided
Lithology Form Importance
Mafic-volcanic < Volcanic < Igneous Major
Mudstone < Clastic < Sedimentary Bed Major
Graywacke < Sandstone < Clastic < Sedimentary Bed Major