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Source map Wilson, F.H., 2010, Unpublished data.
Symbol OSsl
Unit name Ordovician-Silurian platy limestone and shale, Dillinger Subterrane Facies Rocks
Description Thick succession of thin- to medium-bedded, finely laminated platy lime mudstone and dark-gray fissile shale and lesser amounts of siltstone. Platy limestones commonly with planar trace fossils, shales commonly contain abundant and well-preserved graptolites. Limestones commonly appear to be turbiditic in origin. Discrete intervals dominated by either shale or limestone noted in exposures along Hoholitna River, although at present this unit has not been subdivided due to the lack of detailed stratigraphic study. In the Door Mountains of the east-central part of the Sleetmute A-2 quadrangle, the unit is exposed as limestone resistant ridges of thin- to medium-bedded, platy limestones. Apparently the shakier interval are more susceptible to erosion, and do not form the crests of the NW-trending strike ridges which characterize the unit here. Graptolites from outcrops of the unit (identified by Claire Carter and Gil Raasch) along the Hoholitna River (fossil localities 1, 2, and 5) indicate Early Ordovician to undifferentiated Silurian ages for this unit. Two age definitive conodont collections, also from along the Hoholitna River (fossil localities 2 and 4) were identified by N.M. Savage to indicate an Early Ordovician (Arenig) age. Thickness of the unit of the uncertain, but minimally it is estimated to be at least 350-400 m thick. Lithologically, the OSsl unit closely resembles many other contemporaneous deep-water, basinal to slope successions exposed in southwest and west-central Alaska. The East Fork Hills Formation of Dutro and Patton (1982) of the southern part of the Medfra quadrangle is one such unit, and it includes beds ranging in age from Early Ordovician to Middle Devonian. Assigned by Dutro and Patton (1982) to belong to a separate tectonostratigraphic terrane they termed the East Fork terrane, we regard this entity to belong to Dillinger subterrane of the Nixon Fork terrane. Other correlative rocks include the deep-water, basinal succession of the Post River Formation established by Churkin and Carter (1996) in the Terra Cotta Mountains of the southeastern McGrath quadrangle, which forms a major part of the stratigraphic succession of the Dillinger subterrane in that region
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