Younger strata of the Porcupine River sequence of Brosgé and Reiser (1969), undivided

Unit symbol: JMpu
Age range Jurassic to Mississippian (358.9 to 152.1 Ma)
Lithology: Sedimentary
Group name: Clastic and carbonate rocks, Porcupine River region
Unit consists of poorly known clastic rocks and limestone in the Coleen quadrangle in the northeastern part of the state. Youngest part of the sequence included here is red and green laminated chert; partly silicified siltstone and shale; and ferruginous very fine-grained sandstone of presumed Jurassic age. The most common unit of the Porcupine River sequence is gray to black, locally pyritic siltstone and silty shale that has nodules of siltstone of Jurassic and Permian age; Brosgé and Reiser (1969) estimated that it is about 300 m thick. Underlying this is Permian and Carboniferous gray, cherty limestone and dark-gray shaly limestone and chert about 60 to 180 m thick that contains a local basal shale and sandstone. This basal shale and sandstone may be equivalent to unit Css of Brosgé and Reiser (1969), which they describe as yellow, coarse-grained conglomeratic, poorly sorted, partly carbonaceous sandstone and fine-grained hematitic sandstone; as much as 60 m thick; Carboniferous in age. Also includes thick beds of gray chert breccia and minor very fine-grained limonitic sandstone of Jurassic or Permian age. They thought that this part of the unit, which is at least 60 m thick, may be, in part, a silicified dolomite breccia. A Permian brachiopod-bearing gray nodular siltstone and yellow-weathering silty limestone at least 120 m thick is the remaining part of unit. Also included here are several thousand feet of poorly described quartzite or chert, argillite, and dark-gray limestone in the Black River and Charley River quadrangles (Brabb and Churkin, 1969; Brabb, 1970). Till and others (2006a) noted lithologic and faunal similarities of parts of this map unit to the Siksikpuk (TrIPeg) and Kuna Formations (part of the Lisburne Group, unit IPMlg) as described in Dumoulin and others (2004), which suggests ties between this unit and correlative rocks of northern Alaska. Faunal similarities also tie this unit to strata to the south, such as the Tahkandit Limestone (unit Ptl)

Source map information

Source map Brabb, E.E., 1970, Preliminary geologic map of the Black River quadrangle, east-central, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-601, 1 sheet, scale 1:250,000.
Symbol PCs
Unit name Shale
Description Grayish-black shale and argillite and dark-gray limestone. Quartzite and conglomerate occur locally in lower part. Few thousand feet thick.
Lithology Sedimentary

Correlated geologic units

Label Pzca
Description Chert and argillite, eastcentral Alaska, Late Paleozoic
Geologic age Carboniferous to Paleozoic
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Chert < Chemical < Sedimentary Bed Major
Argillite < Metaclastic < Metasedimentary < Metamorphic Minor