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 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 SOda
Unit name Descon Formation, argillite and shale
Description Black, well-bedded siliceous argillite and dark gray to black shale. Mm-scale lamination and cm-scale compositional layering. Interbedded with dark gray mudstone, siltstone, and limestone up to 1 m thick. Rhythmic and graded beds. Sedimentary structures suggests unit becomes younger to southwest. Argillite is locally cherty in Winter Bay, Barrier Islands, and Moira Sound. Disseminated pyrite is common.
Lithology Sedimentary

Correlated geologic units

Label SOds
Description Descon Formation fine-grained sedimentary rocks [see also NSA 6820]
Geologic age Ordovician to Llandovery
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Argillite < Metaclastic < Metasedimentary < Metamorphic Major
Black-shale < Shale < Mudstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Minor