Post River Formation and correlative units

Unit symbol: SCpl
Age range middle Silurian, Wenlock, to Upper Cambrian (497 to 427.4 Ma)
Lithology: Sedimentary
Group name: Farewell basinal facies clastic rocks
Best described by Churkin and Carter (1996), the Post River Formation consists mainly of fissile shale, mudstone, and silty and argillaceous limestone divided into five members. At the base is (1) a lower siltstone member, as thick as 300 m, characterized by thin beds of cross-laminated calcareous siltstone and argillaceous limestone rhythmically interbedded with shale and argillite. Above this is (2) a relatively noncalcareous mudstone member, at least 75 m thick, overlain by (3) another calcareous siltstone and argillaceous interval only 30 m thick, that is thinner than, but otherwise closely lithologically resembles the lower siltstone member. Overlying these is (4) the formally defined, 220-m-thick Graptolite Canyon Member, a nearly pure, dark-gray shale and siliceous shale that contains abundant graptolites and forms most of the upper two-thirds of the formation. The uppermost part of the Post River Formation is (5) the limestone member, a dark, laminated limestone, probably not much more than 18 m thick, interbedded with thin beds of black graptolitic shale (Churkin and Carter, 1996). Age control from graptolites indicates that most of the Ordovician and early Silurian is represented. The Lyman Hills formation of Bundtzen and others (1997a), equivalent to the lower siltstone of the Post River Formation as defined by Churkin and Carter (1996), consists of silty limestone and shale, is commonly cross-laminated, and locally contains Bouma ‘cde’ intervals. Age of the Lyman Hills formation of Bundtzen and others (1997a) is constrained by uppermost Cambrian conodonts and Ordovician and Silurian graptolites. Exposed primarily in the Lime Hills and McGrath quadrangles, unit likely extends into Talkeetna quadrangle where it is mapped as part of undivided unit DCd

Source map information

Source map Bundtzen, T.K., Harris, E.E., and Gilbert, W.G., 1997, Geologic map of the eastern half of the McGrath quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 97-14a, 34 p., 1 sheet, scale 1:125,000.
Symbol SOsh
Unit name Post River Sandstone; graptolitic shale, siltstone, and chert
Description Medium- to dark-gray, fetid, fissile, isoclinally folded, carbonaceous shale, siltstone, and black bioturbated, siliceous siltstone and chert. Local distinctive sulfurous plumes. One of the most complete Ordovician-Lower Silurian graptolite successions in the world. Thought to be the equivalent of Mudstone, Upper Siltstone, and Graptolite Canyon member of Churkin and Carter (1996). Also equivalent to Road River Formation in east-central Alaska. Dillinger Subterrane
Lithology Sedimentary

Correlated geologic units

Label SCpl
Description Post River Sandstone and Lyman Hills Formations and correlative units, silty limestone and siltstone, Early Ordovician
Geologic age Furongian to Wenlock
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Chert < Chemical < Sedimentary Bed Major
Shale < Mudstone < Clastic < Sedimentary Bed Major
Phyllite < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Quartzite < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Limestone < Carbonate < Sedimentary Bed Indeterminate, major
Sandstone < Clastic < Sedimentary Bed Indeterminate, major