Port Graham formation of Kelley (1980)

Unit symbol: Trpg
Age range Upper Triassic, Norian (228 to 208.5 Ma)
Lithology: Sedimentary
Group name: Limestone and volcanic rocks of the Kenai Peninsula
Informal unit that dominantly consists of dark-gray, carbonaceous limestone and silty limestone that has varying amounts of silica cement (Kelley, 1980). Other common lithologies include fine-grained, dark-gray, siliceous to limy mudstone, silty sandstone, and dark-gray to dark-olive-gray, thin- to medium-bedded chert that has mudstone partings (Kelley, 1980). Limy beds tend to be most common in the lower (middle Norian) part of the unit (R.B. Blodgett, written commun., 2007; and unpub. data of Humble Oil Company [now Exxon-Mobil] reported by R.B. Blodgett); whereas the upper part, of late Norian age, is composed of considerably more volcaniclastic rock fragment-rich and shaly beds. Volcaniclastic fragment-rich beds contain a diverse, but uncommon molluscan fauna that consists of both bivalves and gastropods; shaly beds tend to have a monotaxic fauna of monotid bivalves. Fossils are locally abundant, as reported by Kelley (1980): mostly thin-shelled mollusks, but also corals, echinoids, ammonites, and trace fossils. Martin and others (1915) and Martin (1926) reported bivalves Halobia cf. H. superba Mojsisovics, Pseudomonotis (now placed in genus Monotis) subcircularis Gabb, Nucula?, and coral Astrocoenia? sp. Silberling and others (1997) provided a detailed analysis of known Late Triassic bivalve fauna known from the Port Graham area southwest of Seldovia and reported that middle Norian age Halobia lineata and H. dilitata are in collections reported by Martin (1915). Two different species of Late Triassic Monotis were reported by Silberling and others (1997), Monotis (Pacimonotis) subcircularis and Monotis (Monotis) alaskana, and late middle Norian ammonite Steinmannites. Silberling and others (1997) indicated rocks of this unit are unique because both middle Norian and late Norian strata represent pelagic strata, whereas elsewhere, for example in the Kamishak Formation, middle Norian strata represent shallow water facies (Whalen and Beatty, 2008; Blodgett, 2008). Kelley (1984) and Bradley and others (1999) assigned an upper age limit of Early Jurassic to unit, although no fossils of this age are known. Early Jurassic fossils do occur in upper part of the overlying Pogibshi formation of Kelley (1980)

Source map information

Source map Bradley, D.C., Kusky, T.M., Haeussler, P.J., Karl, S.M., and Donley, D.T., 1999, Geology of the Seldovia quadrangle: U.S. Geological Survey Open-File Report 99-18, 1 sheet, scale 1:250,000.
Symbol JTrp
Unit name Port Graham formation of Kelley (1980)
Description Dark-gray, carbonaceous limestone and silty limestone, also includes tuff, tuffaceous sedimentary rocks, and chert. Most of unit considered Norian in age.
Lithology Sedimentary

Correlated geologic units

Label Trpg
Description Port Graham formation of Kelley (1980)
Geologic age Norian
Geologic setting Undivided
Lithology Form Importance
Limestone < Carbonate < Sedimentary Carbonaceous Major
Volcanic < Igneous Pyroclastic, tuff Indeterminate, major
Mixed-clastic < Clastic < Sedimentary Tuffaceous Indeterminate, major
Chert < Chemical < Sedimentary Bed Minor