Sedimentary rocks of the Orca Group

Unit symbol: Tos
Age range Tertiary, Eocene to Paleocene (59.2 to 38 Ma)
Lithology: Sedimentary
Group name: Sedimentary and volcanic rocks of the Orca Group, undivided
Thin- to thick-bedded graywacke sandstone, siltstone, mudstone, slate, and locally, minor conglomerate, which display abundant sedimentary structures, such as graded bedding, crossbedding, and ripple marks, along with flute, groove, and load casts, which together indicate deposition from turbidity currents (Tysdal and Case, 1979; Nelson and others, 1985). Graywacke sandstone is more abundant than finer-grained rocks (Tysdal and Case, 1979). Bioturbated limestone lenses and concretions are found locally and, along with conglomerate, are characteristic of the unit (Moffit, 1954; Tysdal and Case, 1979; Nelson and others, 1985). Matrix-supported conglomerate and pebbly mudstone and sandstone lenses are widespread (Winkler and Plafker, 1993). Conglomerate “ranges from matrix-supported pebbly mudstone and sandstone to massive clast-supported pebble, cobble, and boulder conglomerate” (Nelson and others, 1985). The generally well-rounded clasts consist primarily of extrabasinal felsic volcanic and igneous rocks (felsic porphyry and tuff, granitic rocks, and white quartz); and intrabasinal sedimentary and mafic rocks (greenstone, sandstone, siltstone argillite, and limestone (Nelson and others, 1985; Winkler and Plafker, 1993; Winkler and Tysdal, 1977; Moffit, 1954). Conglomerate usually occurs as lenses 90 to 210 m thick, though the thickest lens measured 900 m thick (Nelson and others, 1985). Matrix-supported conglomerate and pebbly mudstone may have been formed by submarine landslides on unstable slopes, whereas inversely and normally graded clast-supported conglomerate beds are channel fill deposits (Winkler and Tysdal, 1977). Thin-section petrography shows that most of the sandstone is feldspathic to feldspatholithic (Nelson and others, 1985) and contains abundant monocrystalline quartz, which indicates a plutonic provenance (Dumoulin, 1987, 1988). Unit metamorphosed to zeolite or prehnite-pumpellyite facies in the Cordova and Middleton Island quadrangles (Winkler and Plafker, 1993); Nelson and others (1985) reported that alteration ranges from diagenetic recrystallization of the matrix to low greenschist-facies metamorphism. In the Seward and Blying Sound quadrangles, unit also includes rocks mapped as siltstone by Tysdal and Case (1979). Their siltstone unit is locally tightly folded and metamorphosed to slate; medium-gray and green lenses of micritic limestone as much as 2 m thick are present locally. At several places in the islands south of Knight Island Passage, Tysdal and Case (1979) describe locally folded and contorted sequences of siltstone and isolated greenstone blocks mixed in with sandstone and siltstone within otherwise uniformly layered sections that they thought were olistostromes. Fossils reported by Nelson and others (1985) include: Alnus (Alder) pollen; foraminifers: Globogerina sp., G. senni, Globogerina sp. (hispid), and Globorotalia sp.; and echinoids: Holaster sp.?, Hypsopygaster sp., and Nucleopygus as well as a crab, Branchioplax washingtoniana, and a pelecypod, Acila decisa, reported by Addicott and Plafker (1971, cited in Nelson and others, 1985), which together suggest a Paleocene(?) to late Eocene age

Source map information

Source map Nelson, S.W., Dumoulin, J.A., and Miller, M.L., 1985, Geologic map of the Chugach National Forest: U.S. Geological Survey Miscellaneous Field Studies Map MF-1645B, pamphlet, 16 p., 1 sheet, scale 1:250,000 (Chugach NF Arc/Info coverage 1993).
Symbol Tos
Unit name Orca Group -- Sedimentary rocks, undivided
Description A sequence of thin- to thick-bedded sandstone, siltstone, and mudstone. Abundant sedimentary structures, such as graded bedding, crossbedding, and ripple marks, along with flute, groove, and load casts, indicate deposition from turbidity currents. Sandstone is more abundant than finer-grained rocks. Limestone lenses or concretions are found locally, and these, along with the earlier mentioned conglomerates, are characteristic of sedimentary rocks belonging to the Orca Group. Thin-section petrography indicates that most of the sandstones are feldspathic to feldspatholithic.
Lithology Sedimentary

Correlated geologic units

Label Tos
Description Sedimentary rocks of the Orca Group, undivided
Geologic age Late-Paleocene to Middle-Eocene
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Sandstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Major
Argillite < Metaclastic < Metasedimentary < Metamorphic Bed Minor
Conglomerate < Clastic < Sedimentary Bed Minor
Limestone < Carbonate < Sedimentary Bed Incidental
Chert < Chemical < Sedimentary Bed Incidental