Chugach flysch

Unit symbol: Kaf
Age range Upper Cretaceous (83.6 to 66 Ma)
Lithology: Sedimentary
Group name: Chugach accretionary complex
Extensive unit includes Valdez Group and Kodiak and Shumagin Formations, which extend from southeast Alaska to the western end of the Alaska Peninsula around the Gulf of Alaska. Formations generally consist of very fine- to medium-grained, medium-light-gray to medium-dark-gray, highly indurated, lithic graywacke and siltstone that generally increases in metamorphic grade to the northeast (Dusel-Bacon and others, 1993, 1996). At Sanak Island and in the Shumagin Islands in southwest Alaska, Shumagin Formation is largely not metamorphic, except for some contact metamorphism around the Sanak and Shumagin plutons. Kodiak Formation displays very low-grade metamorphism, and the Valdez Group is locally metamorphosed to amphibolite facies in the northeastern part of its exposure range. Shumagin Formation consists of interbedded sandstone, siltstone, mudstone, and shale at least 3,000 m thick (Burk, 1965; Moore, 1974a). Sandstone beds in unit vary in thickness from 2 cm to 20 m, are graded, and contain abundant shale and siltstone chips (Moore, 1974a; Detterman and others, 1996). Thin (<10 cm) grayish-black mudstone interbedded with sandstone; however, locally mudstone forms dominant lithology. Thin-bedded siltstone, mudstone, and sandstone sequences are rhythmically bedded and have sharp upper and lower contacts indicative of turbidity-current deposition in deep-sea-fan and abyssal-plain environments. Fossils are uncommon in Shumagin; existing collections indicate early Maastrichtian age (J.W. Miller, written commun., 1983–88, oral commun., 1991), which is age-equivalent to upper parts of Hoodoo, Kaguyak, and Chignik Formations (units Khk, and Kcs, respectively). Shumagin is a trench- and abyssal-plain-facies equivalent of Hoodoo Formation (Mancini and others, 1978); Plafker and others (1977) correlated Shumagin Formation with Kodiak Formation of the Kodiak Islands and with the Valdez Group of south-central Alaska. Thin-section analysis of samples by John Decker (DGGS, oral commun., 1985), suggests close lithologic correlation with Kodiak Formation on Kodiak Island. According to Decker, protolith for Shumagin was derived from north and transported along paleo-Aleutian trench, though a component of apparently local derivation has same provenance as Hoodoo Formation. Kodiak Formation consists of medium- to thick-bedded graded bed sequences, averaging 1 m thick, of arkosic wacke and shale that has occasional beds of pebbly conglomerate. Flute casts and complete Bouma sequences indicate deposition by turbidity currents below wave base. Moore (1969) designated the type section along the west shore of Uyak Bay. Nilsen and Moore (1979) indicated that the unit is approximately 5,000 m thick, in contrast to the estimate of 30,000 m originally suggested by Moore (1969). Nilsen and Moore (1979) found that the unit is repeated structurally by folding and faulting, which makes internal stratigraphic correlation difficult. Nilsen and Moore (1979) used a turbidite facies and facies association scheme that generally followed the system of Mutti and Ricci Lucchi (1972, 1975, cited in Nilsen and Moore, 1979) and Nelson and Nilsen (1974, cited in Nilsen and Moore, 1979); under this system the Kodiak Formation consists largely of the basin-plain and slope-facies associations. The basin-plain facies is the structurally lowest part of the Kodiak Formation and is characteristic of most of the unit on the southeast side of Kodiak Island (Nilsen and Moore, 1979). Slope facies occur primarily on the northwest side of Kodiak Island and are primarily thick mudstone sequences and “* * * may form imbricate slices juxtaposed during multiple phases of synsedimentary slumping * * * [and may] contain chaotically oriented blocks, slabs, and disordered fragments of hemipelagic mudstone that probably slid from upper slope depositional sites to the lower slope or base of slope under the influence of gravity” (Nilsen and Moore, 1979, p. 6). Conglomerate and sandstone channels occur locally within the slope facies associations; Nilsen and Moore (1979) mention occurrences in Uyak Bay in particular, where they are on the order of 50 m thick. Fossils from the Kodiak Formation include Inoceramus kusiroensis of Maastrichtian age. Valdez Group consists of dark-gray, thin- to thick-bedded, moderately to poorly sorted sandstone, siltstone, and mudstone flysch that has been metamorphosed to laumontite to mid-greenschist facies; sandstone is fine- to coarse-grained and mainly composed of plagioclase, quartz, and igneous rock fragments (Tysdal and Case, 1979; Dumoulin, 1987). Unit is a thick sequence of rhythmically alternating, multiply-deformed, metamorphosed sandstone-siltstone turbidites in beds that generally range from a few centimeters to a few meters thick and, locally, massive beds as much as tens of meters thick (Winkler and Plafker, 1981; Nelson and others, 1985; Winkler and Plafker, 1993; Bradley and others, 1999). Point count analysis by Dumoulin (1987) showed Valdez Group sandstone contains 6 to 30 percent quartz, 23 to 45 percent feldspar, and 28 to 68 percent lithic fragments; lithic fragments are dominantly volcanic rocks. Proportion of lithic fragments decreases from west to east, as feldspar and quartz increase (Nelson and others, 1985). Conglomeratic sandstone containing clasts of quartzite, intermediate and felsic volcanic rocks, and rare sandstone, limestone, and granitic rocks is uncommon but widely distributed, occurring at base of some sandstone beds (Bradley and others, 1999; Bradley and Miller, 2006). In some places, primary sedimentary structures such as graded bedding, current-ripple cross-lamination, convolute bedding, and sole markings are preserved (Nelson and others, 1985; Winkler and Plafker, 1993). Inoceramus kusiroensis, I. ulrichi, and I. concentrica, of Maastrichtian age, have been reported (Tysdal and Plafker, 1978; Tysdal and Case, 1979; Nelson and others, 1985; Bradley and others, 1999). Rocks north of Cross Sound in the Glacier Bay region are regionally metamorphosed and range from subgreenschist facies to as high-grade as amphibolite facies, producing graywacke semischist, phyllite, slate, and layered schist, semischist, and gneiss (Brew and others, 1978). These metasedimentary rocks are correlated with strata in the Valdez Group to the northwest by lithologic similarity (Brew and Morrell, 1979a) and by stratigraphic continuity (Plafker and Campbell, 1979; Campbell and Dodds, 1983). More eastern exposures of the Valdez Group contain an increasing proportion of interbedded volcanic rocks; where these igneous rocks dominate, they are described as unit Kafv. A K/Ar date on biotite semischist in the Seward quadrangle of 51.5±1.5 Ma was reported by Nelson and others (1985) and was interpreted as a metamorphic age or a cooling age of Valdez Group schist unit along Placer River Fault. Bradley and others (2009) reported 70-Ma detrital zircons from unit near Anchorage

Source map information

Source map Winkler, G.R., Silberman, M.L., Grantz, Arthur, Miller, R.J., and MacKevett, E.M., Jr., 1981, Geologic map and summary geochronology of the Valdez quadrangle, southern Alaska: U.S. Geological Survey Open-File Report 80-892-A, 2 sheets, scale 1:250,000. and modifications by Winkler, 1997).
Symbol Kvs
Unit name Valdez Group -- Metasedimentary rocks, undivided
Description Weakly to strongly foliated marine argillite, siltstone, sandstone, and conglomeratic sandstone. Lower greenschist facies metamorphism. Probable volcanic source terrane.
Lithology Metamorphic

Correlated geologic units

Label Kvs
Description Metasedimentary rocks of the Valdez Group
Geologic age Late-Cretaceous
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Metagraywacke < Metasandstone < Metaclastic < Metasedimentary < Metamorphic Greenschist Major
Metasiltstone < Metaclastic < Metasedimentary < Metamorphic Greenschist Major
Greenstone < Metaigneous < Metamorphic Greenschist Indeterminate, major
Schist < Metamorphic Greenschist Indeterminate, major