The Kogruk Formation, 1,500 to 2,000 ft thick, is composed of marine carbonate rocks deposited in normal-marine to shoal-water environments. Carbonate rock types typically are bryozoan, echinoderm packstone and wackestone, and lesser amounts of calcareous mudstone, and ooid grainstone and packstone. Dolomite and other carbonate sedimentary rocks characteristic of intertidal and supratidal environments are absent in the sections studied. The Kogruk Formation was deposited in an open-marine environment on a subsiding shelf on which carbonate deposition and subsidence were near equilibrium. Only minor oscillations in environments of deposition are seen in a typical section. Lithostrotionid coral faunas are best developed adjacent to the shoal-water facies.

Two major coral faunas are recognized. The older, 600-800 ft above the base of the Kogruk Formation, consists of Lithostrotion (Siphonodendron) sinuosum (Kelly), L. (S.) warreni (Nelson), Lithostrotionella mclareni (Sutherland), Thysanophyllum astraeiforme (Warren), Thysanophyllum orientale Thomson, and Sciophyllum lambarti Harker and McLaren. This fauna is of middle Meramecian age. The younger coral fauna is in the highest 600-800 ft of the Kogruk Formation and contains many of the species of lithostrotionids which are present in the lower beds, plus Lithostrotionella aff. L. macouni (Lambe), L. banffensis (Warren), a new species each of Lithostrotionella and Sciophyllum, and two species of Faberophyllum. This fauna is of late Meramecian age.

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