Sheep Creek formation of Bundtzen and others (1997a) and correlative siliciclastic units of the Mystic structural complex

Unit symbol: PDsc
Age range Permian to Devonian (419.2 to 252.2 Ma)
Lithology: Sedimentary
Group name: Sedimentary rocks of the Mystic structural complex
Clastic-dominated but heterogeneous middle third of the Mystic structural complex. Consists of sandstone, conglomerate, siltstone, and argillite, and lesser chert and limestone. Depositional environments are dominantly fluvial, but carbonate platform and deep marine environments are also represented by subordinate rock types in this grouping of units. Includes the clastic parts of the informal Sheep Creek formation of McGrath and Lime Hills quadrangles, an argillite and chert unit thought equivalent to the Sheep Creek formation of Bundtzen and others (1997a; Bundtzen, DGGS, unpub. data, 1997, 1998), and the conglomerate of Mount Dall in the Talkeetna quadrangle (Reed and Nelson, 1980), which is a “dark-brown to yellowish-brown-weathering sequence of conglomerate, sandstone, siltstone, and shale mainly of continental origin that forms a broad, open, east-plunging syncline.” Unit is chiefly massive lenticular beds of conglomerate and sandstone with numerous cut and fill channels, which have yielded a Permian plant assemblage of Siberian affinity (Mamay and Reed, 1984). Conglomerate of Mount Dall of Reed and Nelson (1980) contains clasts of Devonian limestone, as well as chert, chert-pebble conglomerate, and graded sandstone, presumably derived from the White Mountain sequence of Decker and others (1994). Age control is provided by conodonts, fusulinids, brachiopods, corals, and plants. Unit tentatively includes an unnamed shale and chert (T.K. Bundtzen, DGGS, written commun., 1998) that occurs in a limited area in the Lime Hills quadrangle and that hosts the Gargaryah bedded-barite deposit (Bundtzen and Gilbert, 1990) as well as includes dark gray-green to distinctly maroon, pyrite-rich volcanogenic phyllite, and white-weathering, gray-green, banded phyllitic ribbon chert (Bundtzen and others, 1997a) exposed between strands of the Denali Fault System in the eastern McGrath quadrangle. The ribbon chert has yielded Permian through Pennsylvanian Radiolaria and Permian through Mississippian conodonts (Bundtzen and others, 1997a)

Source map information

Source map Bundtzen, T.K., Harris, E.E., and Gilbert, W.G., 1997, Geologic map of the eastern half of the McGrath quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 97-14a, 34 p., 1 sheet, scale 1:125,000.
Symbol PDs
Unit name Sheep Creek Formation; sublithic sandstone, limestone-chert conglomerate, and minor limestone
Description Medium-gray, distinctly brown-red weathered, medium- to coarse-grained, sublithic sandstone and pebble conglomerate that contains clasts of limestone, chert, lithic fragments, volcanic clasts, and polycrystalline quartz. Limestone-chert conglomerate near the top of PDs probably correlative with non-marine, conglomerate-rich Mystic subterrane rocks at Mount Dall in Talkeetna Quad. Farewell Composite Terrane;
Lithology Sedimentary

Correlated geologic units

Label PDsc
Description Sheep Creek Formation and correlative largely siliciclastic units of the Mystic structural complex (Permian to Devonian)
Geologic age Middle-Devonian to Permian
Geologic setting Sedimentary, continental
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Major
Shale < Mudstone < Clastic < Sedimentary Bed Minor
Siltstone < Clastic < Sedimentary Bed Minor