Older carbonate strata of the Porcupine River sequence of Brosgé and Reiser (1969) and equivalent units

Unit symbol: DZkb
Age range Middle Devonian to Neoproterozoic? (635 to 382.7 Ma)
Lithology: Sedimentary
Group name: Clastic and carbonate rocks of the Yukon Flats Basin
Poorly known unit that consists of a thick sequence of limestone and dolostone and minor calcareous shale and black chert, located largely on the northeast side of the Yukon Flats basin. The only formally defined unit of the sequence is the Salmontrout Limestone (Churkin and Brabb, 1965; Oliver and others, 1975), a dark-gray massive biohermal limestone of Early or Middle Devonian age that is about 600 ft thick. Brosgé and Reiser (1969) also included several other carbonate rock units in the Porcupine River sequence, which covered much of the southern part of the Coleen quadrangle and which likely extend into the northern Black River quadrangle. These consist of their unit Dd, about 300 m of dark-gray, fine- to medium-grained dolostone that contains minor black chert; light-gray limey dolostone and limestone; and yellow and gray very fine-grained laminated dolostone and chert that contains minor red and green shale and sandstone; as well as their unit SOCl, which is an interbedded gray, laminated-to-mottled limey dolostone and dark gray dolostone that contains minor black chert, light gray coarse-grained dolostone, and light- to dark-gray calcilutite. This subunit, SOCl, which includes light-gray coral and limestone cobble conglomerate, is at least 600 m thick and includes Silurian and Ordovician corals and snails and Cambrian(?) trilobites. Gray, foliated, thin-bedded, platy, and recrystallized micritic limestone of early(?) Paleozoic age in the Table Mountain quadrangle (C.G. Mull, written commun., 2012) was suggested by Mull to possibly be equivalent to the Ogilvie Formation of the adjacent Yukon (which is mapped here as unit Dof), but seems a better fit here. Other map units included here are (1) unit Pzl of Brosgé and Reiser (1969), which consists about 120 m of black silty calcareous shale, black fine-grained, thin-bedded limestone, and brown siltstone which contain siltstone concretions and nodular cherty dolostone; (2) unit DCl of Brabb (1970) that consists mainly of massive limestone and dolostone several thousand feet thick, minor red and green argillite, and black chert that includes a few hundred feet of quartzite along Lower Ramparts of Porcupine River; and (3) units Dl and Sl of Brabb (1970), which are medium-gray to grayish-black, fine- to coarse-grained, locally dolomitic, crinoidal limestone. The Amy Creek unit of Weber and others (1992) is tentatively included here, but it also bears some affinity with units on this map such as the Jones Ridge Limestone (OCjr) and possibly the Nanook Limestone (included in unit DZnl) and Katakturuk Dolomite (Pxls) of northern Alaska. As defined by Weber and others (1992), the Amy Creek unit of inferred Silurian to Proterozoic age consists of light- to medium-gray, medium-grained; mudstone, wackestone, and packstone that all contain interbedded chert; black carbonaceous argillite; and dolostone. The Amy Creek unit contains interbedded and interlayered minor lime mudstone, light- and dark-gray chert, gray marble, gray, fine-grained quartzite, basaltic greenstone, lenses of tuff, tuffaceous siltstone, shale, and minor volcaniclastic graywacke. Basaltic flows and flow breccia at least 100 m thick occur locally in shaly rocks. Lithology and algae strongly resemble Proterozoic or early Paleozoic dolostone of the western part of the Charley River quadrangle. In the Circle quadrangle (Foster and others, 1983), the igneous part of the unit consists of dark-greenish- or bluish-gray, medium-fine-grained calcareous basalt that locally has well-developed pillows. Thin limestone layers at the base of the basalt. Upper units in basalt are amygdaloidal, overlain by greenish-brown breccia of calcareous basalt that has an opaline matrix. Black, moderately coarsely crystalline limestone overlies basaltic portion of unit. Much of this unit corresponds to unit DCpu of Till and others (2006a), who stated that “lithofacies and faunal data suggest that most of DCpu formed in a relatively shallow-water shelf or platform setting, but upper Silurian and Lower Devonian graptolitic shale, limestone, and chert in the Porcupine River area (Churkin and Brabb, 1967; Coleman, 1987) accumulated in a slope or basin environment and may correlate with the Road River Formation (Churkin and Brabb, 1965; Brabb and Churkin, 1969), which is included in unit DOka in the southern part of the province. Conodonts and megafossils in DCpu have chiefly Laurentian (North American) affinities (Oliver and others, 1975; Rohr and Blodgett, 1994; Dumoulin and others, 2002; Blodgett and others, 2002)”

Source map information

Source map Reifenstuhl, R.R., Dover, J.H., Newberry, R.J., Clautice, K.H., Liss, S.A., Blodgett, R.B., and Weber, F.R., 1998, Geologic map of the Tanana A-1 and A-2, central Alaska: Alaska Division of Geological and Geophysical Surveys Public Data File 98-37a, 19 p., 1 sheet, scale 1:63,360.
Symbol PzPac
Unit name Cherty-argillite and chert
Description Heterogeneous unit of dominantly black to dark gray chert and siliceous to carbonaceous argillite with well-developed phyllitic to subphyllitic slate-like cleavage, and containing one or more dark gray limestone layers or lenses. The geochemical signature of this unit is typical of the chert in the Amy Creek unit (Haug and others, 1997) of Weber and others (1988). Limestone locally crops out, and is dark gray to very dark gray, up to 3 m thick, and is here included in the cherty-argillite and chert unit
Lithology Metamorphic

Correlated geologic units

Label SZa
Description Amy Creek unit, siliceous dolomite, chert, and basaltic greenstone, minor limestone, shale..., Silurian(?) to Late Proterozoic
Geologic age Ediacarian to Silurian
Geologic setting Undivided
Lithology Form Importance
Mafic-volcanic < Volcanic < Igneous Flow, pillows Major
Dolostone < Carbonate < Sedimentary Bed Major
Chert < Chemical < Sedimentary Bed Minor
Limestone < Carbonate < Sedimentary Bed Incidental
Shale < Mudstone < Clastic < Sedimentary Bed Incidental
Siltstone < Clastic < Sedimentary Bed Incidental