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 Patton, W.W., Jr., Wilson, F.H., Labay, K.A., and Shew, Nora, 2009, Geologic map of the Yukon-Koyukuk basin, Alaska: U.S. Geological Survey Scientific Investigations Map SIM-2909, 26 p., 2 sheets, scale 1:500,000.
Symbol Ont
Unit name Novi Mountain and Telsitna Formations, undivided
Description Novi Mountain Formation: Shallow water deposits. Predominately calcareous siltstone and shale in lower part. Upper part characterized by cycles beginning with massive limestone commonly containing flat carbonate pebbles at the base and grading upward through thin irregularly bedded shaly limestone into calcareous siltstone or shale at the top. Telsitna Formation: Chiefly shallow water limestone and dolomite. The lower part dominated by highly fossiliferous thin-bedded fine-grained limestone containing silty interbeds. Middle part consists chiefly of nonfossiliferous dolomite. The upper part is composed mainly of limestone that ranges from thick bedded and fine-grained to thin-bedded and silty. Stringers and nodules of black chert occur near the top. Eastern part of Medfra quadrangle and southeastern corner of Ruby quadrangle
Lithology Sedimentary

Correlated geologic units

Label Ont
Description Novi Mountain and Telsitna Formations, and unnamed correlative units, massive dolomite and limestone, Late to Middle Ordovician
Geologic age Middle-Ordovician to Late-Ordovician
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Dolostone < Carbonate < Sedimentary Bed Major
Limestone < Carbonate < Sedimentary Bed Major