Dolostone, limestone, orthoquartzite, and minor chert

Unit symbol: CZls
Age range Cambrian and Neoproterozoic? (1000 to 497 Ma)
Lithology: Sedimentary
Group name: Farewell platform facies
Exposed in the Sleetmute and Taylor Mountains quadrangles are two separate Middle Cambrian limestone subunits and in the Sleetmute quadrangle is a presumed upper Proterozoic unit of mixed lithology (Blodgett and others, 2000, and unpub. data). Also included, in the McGrath quadrangle, are red beds and carbonate rocks of the Khuchaynik Dolostone, Lone Formation, Big River Dolostone, and Windy Fork Formation of Babcock and others (1994). The uppermost and thickest of the Middle Cambrian limestone subunits in the Taylor Mountains quadrangle composed of medium- to thick-bedded, commonly light-gray to dark-gray, rarely pink-weathering (light-gray fresh) lime mudstone that has locally abundant, well developed wavy styolites. Minor green-gray shale intervals present locally. Trilobites are locally abundant and diverse in this subunit and are indicative of a late Middle Cambrian age (Palmer and others, 1985; Babcock and Blodgett, 1992; Babcock and others, 1993; St. John, 1994; St. John and Babcock, 1994, 1997). The lower limestone subunit is poorly exposed and consists only of scattered rubble-crop of coquinoid limestone (lime wackestone to packstone) that has an abundant and diverse trilobite fauna (agnostids notably common) and ancillary acrotretid brachiopods, hyoliths, and cap-shaped fossils of early Middle Cambrian age. Thickness of this subunit is uncertain, but probably at least 5 m. Faunas from both subunits are most closely allied biogeographically with coeval faunas from the Siberian Platform. Stratigraphically below these are medium-bedded, medium-gray, orange-weathering dolostone, limestone, orthoquartzite, and minor chert in the Sleetmute quadrangle. Dolostone has locally abundant floating quartz grains, is locally trough cross stratified, but also has well developed parallel laminations, low domal stromatolites, and local paleokarst intervals. Total thickness of unit uncertain, but is at least 300 to 400 m in thickness. Several repeated sedimentary cycles observed in unit. The Khuchaynik Dolomite is medium-gray, light- to medium-gray-weathering dolostone at least 228 m thick that contains numerous packstone or grainstone beds and locally numerous discontinuous bands of gossan are present. Lone Formation is dominantly thin- to medium-bedded siltstone and fine- to coarse-grained sandstone at least 107 m thick. Contains interbeds of lime mudstone or dolomudstone as much as 12 m thick. Usually weathering maroon, it locally weathers earthy yellow, tan, white, gray, gray-green, or reddish-brown. “Sedimentary features in sandstone include planar crossbeds, symmetrical ripple marks, load casts, and siltstone intraclasts” (Babcock and others, 1994). Contact with Big River Dolostone is conformable and sharp; the Big River Dolostone is a distinct bed of earthy yellow-weathering dolomitic lime mudstone that lacks coated grains. Distinguished from Windy Fork Formation by presence of much more sandstone and dolostone and much less siltstone. Big River Dolostone is light- to medium-gray-weathering dolomudstone that has a fenestral fabric and numerous packstone to grainstone beds composed of coated grains. Unit contains two distinctive white-weathering dolostone bands in the upper part of unit. Distinguished from the overlying Khuchaynik Dolostone by these distinctive white bands, the presence of poorly sorted, large, irregular grains, and the lack of major sulfide deposits. Windy Fork Formation is “dominantly thin- to medium-bedded siltstone and fine- to coarse-grained sandstone that weather earthy yellow or orange-brown, at least 84 m thick. Some sandstone shows planar crossbeds. Interbeds of lime mudstone or dolomudstone are a minor part of unit” (Babcock and others, 1994). Each of these units contains finely disseminated pyrite throughout. Age of unit in the Sleetmute quadrangle is thought to be upper Proterozoic based on distinctive and very similar or identical lithologies shared with units in the McGrath quadrangle of presumed late Proterozoic age (Babcock and others, 1994; R.B. Blodgett, written commun., 2000)

Source map information

Source map Babcock, L.E., Blodgett, R.B., and St. John, J., 1994, New Late(?) Proterozoic age formations in the vicinity of Lone Mountain, McGrath quadrangle, west-central Alaska: U.S. Geological Survey Bulletin 2107, p. 143-156.
Symbol
Unit name Khuchaynik Dolostone
Description At least 228 m thick dolostone containing numerous packstone or grainstone beds composed of small (< 1 mm) coated grains and locally small vugs where the grains have been dissolved. Dolostone is medium-gray and weathers light- to medium-gray. Locally dolostone recrystallized to sugary texture and numerous discontinuous bands of gossan are present
Lithology Sedimentary

Correlated geologic units

Label CZds
Description Oolitic limestone, Khuchaynik Dolomite of Babcock and others, Big River Dolostone of Babcock and others
Geologic age Neoproterozoic to 2
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Dolostone < Carbonate < Sedimentary Calcareous Major
Limestone < Carbonate < Sedimentary Calcareous Major