Farewell platform facies

Unit symbol: DZwp
Age range Upper Devonian, Frasnian to Neoproterozoic (1000 to 372.2 Ma)
Lithology: Sedimentary
Group name: Farewell platform facies
Shallow-marine limestone, dolostone, and less common chert; also contains algal reefs of various ages. Trilobites, conodonts, corals, stromatoporoids, and brachiopods indicate a Middle Devonian to Middle Cambrian age range; a nonfossiliferous dolostone unit below the Middle Cambrian strata has been assigned to late Proterozoic (St. John and Babcock, 1997; R.B. Blodgett, written commun., 1998). Where possible, unit is subdivided into three units (DSwc, Ols, and _Zls), described below; however, available map data are inadequate to derive the distribution of each of these subdivisions throughout the range of outcrop. Included in this undivided unit are rocks of the Ordovician to Cambrian(?) Novi Mountain Formation, which consists of limestone that is variably argillaceous, locally orange-weathering, and locally oolitic; it is overlain by the Telsitna Formation, which is composed of peloidal lime micrite and lesser dolostone. These units had a marine shelf depositional setting that was locally very shallow (Dutro and Patton, 1982). Unnamed units in the western McGrath quadrangle include breccia, argillite, laminated limestone, and mudstone, some of which may have been deposited in deeper-water settings. Also includes dolostone that rests on crystalline basement in the northernmost Medfra quadrangle, which Dutro and Patton (1982) assigned to the Middle and Upper Ordovician Telsitna Formation. However, these strata have yielded gastropod steinkerns that allow the possibility of an age as old as Late Cambrian for this dolostone. Blodgett (written commun., 1998) reports a thin- to medium-bedded, commonly finely laminated, dark-gray platy limestone estimated to be at least 300 to 400 m thick in the southern Sleetmute quadrangle. This unit contains minor distinctive flat-pebble limestone conglomerate that forms distinctive marker beds. Near the top of the unit, immediately below the transition into the algal reef facies of unit Ols (described below), platy limestone contains many thick-bedded, fining-upward, debris-flow deposits, which suggest that the depositional environment rapidly shallowed. Conodonts from uppermost beds of the platy limestone indicate a Middle Ordovician age (N.M. Savage, written commun., 1999). Contact of this platy limestone with the overlying Ols unit appears to be gradational but rapid. Ols is lithologically similar to unit Ol of Gilbert (1981) in the McGrath quadrangle and, in part, is similar to the Novi Mountain Formation and the lower part of the East Fork Hills Formation of Dutro and Patton (1982) in the Medfra quadrangle, which are not mapped separately here. Similar coeval rocks also are exposed in the low hills on the west side of Lone Mountain in the McGrath C-4 quadrangle in west-central Alaska. The undivided shallow marine carbonate units of the now-abandoned Holitna Group were, in the past, typically assigned to the Nixon Fork terrane and are now considered part of the Farewell terrane

Source map information

Source map Miller, M.L., Belkin, H.E., Blodgett, R.B., Bundtzen, T.K., Cady, J.W., Goldfarb, R.J., Gray, J.E., McGimsey, R.G., and Simpson, S.L., 1989, Pre-field study and mineral resource assessment of the Sleetmute quadrangle, southwestern Alaska: U.S. Geological Survey Open-File Report 89-363, 115 p., 1 map, scale 1:250,000.
Symbol Pzl
Unit name Holitna Group
Description Limestone, in partly dolomitized, chiefly massive but thinner bedded in upper zones. Contains small reef-like deposits and local intraformational conglomerate and breccia.
Lithology Sedimentary

Correlated geologic units

Label DZn
Description Undivided shallow marine carbonate units of Holitna Basin area, late Proterozoic to Devonian
Geologic age Neoproterozoic to Middle-Devonian
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Limestone < Carbonate < Sedimentary Bed Major
Dolostone < Carbonate < Sedimentary Bed Minor