McCann Hill Chert, Road River Formation, and Troublesome unit of Weber and others (1992)

Unit symbol: DOka
Age range Lower Devonian to Ordovician (485.4 to 393.3 Ma)
Lithology: Sedimentary
Group name: Shale, chert, and argillite
McCann Hill Chert consists of “thin-bedded and laminated dark- to light-gray chert, siliceous shale, and minor chert gritstone. Contains plant fragments, poorly preserved spores, and rare conodonts, gastropods, and cephalopods. Basal part of formation has beds of dark-gray bioclastic limestone with remarkable varied fauna including corals, brachiopods, trilobites, tentaculitids, fish, and ostracodes of Early Devonian (Emsian) age” (Brabb and Churkin, 1969). Thickness 200 to 800 feet except near Jones Ridge in the Charley River quadrangle, where the basal part of the McCann Hill Chert was placed in the Ogilvie Formation (Blodgett, 1978; Clough and Blodgett, 1984, 1987). Road River Formation consists of “dark-gray graptolitic shale with lesser amounts of grayish-black laminated chert and very minor dark-gray limestone, greenish-gray dolomite, chert arenite, and conglomerate. Chert, chert arenite, and chert conglomerate occur mainly in basal part of formation. Graptolites indicate that most series of Ordovician and Silurian are represented, and that the youngest rocks are Early Devonian (pre-Emsian). Thickness 400 to 900 feet” (Brabb and Churkin, 1969). The Troublesome unit of Weber and others (1992) consists of rhythmically interbedded dark-gray to black cherty argillite to chert, and thin beds of black to gray siliceous slate in the Livengood quadrangle. Associated with extensive mafic intrusive and extrusive rocks. Unidentified Radiolaria are the only reported fossils (Weber and others, 1992). Depositionally overlain by Frasnian carbonate buildups at base of Quail unit of Weber and others (1992), unit Dq here, hence Middle Devonian or older. As mapped here, also includes Pzc unit of Foster and others (1983) in the Circle quadrangle, which consists of chert interlayered with argillite, and rare marble and quartzite, intruded by diorite and gabbro (Foster and others, 1983). Radiolaria from unit Pzc are not age-diagnostic but are reported as “possibly of Mississippian age” (Foster and others, 1983); this constraint is, however, less compelling than the pre-Frasnian age for the Troublesome unit in its type area

Source map information

Source map Foster, H.L., Laird, Jo, Keith, T.E.C., Cushing, G.W., and Menzie, D.W., 1983, Preliminary geologic map of the Circle quadrangle, Alaska: U.S. Geological Survey Open-File Report, 83-170-A, scale 1:250,000.
Symbol Pzc
Unit name Chert and argillite
Description Gray, black, and banded chert interlayered with gray, green, olive, black, gray, and tan argillite, rare gray marble, white quartzite, and white calcareous quartzite. Locally intruded by dark-greenish-gray to black diorite and gabbro.
Lithology Metamorphic

Correlated geologic units

Label Dtr
Description Troublesome unit, rhythmically interbedded cherty argillite to chert and thin beds of slate, Devonian(?), maybe equivalent to McCann Hill Chert
Geologic age Ordovician to Early-Devonian
Geologic setting Undivided
Lithology Form Importance
Chert < Chemical < Sedimentary Bed Major
Argillite < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Slate < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Quartzite < Metaclastic < Metasedimentary < Metamorphic Minor
Greenstone < Metaigneous < Metamorphic Incidental