Much of the Pacific Rim, including most of North America west of the Rocky Mountains and much southern Alaska, is made up of a series of allochthonous accreted terranes. Alaskan accreted terranes contain Paleozoic and Mesozoic carbonate rocks with numerous paleogeographic affinities. Difficulties in determining the paleogeographic positions of these accreted terranes hinder our ability to decipher the tectonic history of Alaska. Carbonate-producing organisms are particularly sensitive to climate and oceanographic conditions and therefore are significant indicators of biogeography, paleogeography, and oceanography. This study provides detail on the stratigraphy and petrography of a Permian limestone unit from the allochthonous Chulitna terrane in southcentral Alaska and provides insight into the paleogeography and tectonic history of the terrane.

Recent studies contrast facies assemblages typical of cool-water carbonate environments with those typical of warm-water settings. Photosynthetic carbonate producing organisms, abundant lime mud and non-skeletal carbonate grains, and little mechanical compaction due to early cementation characterize warm-water carbonate facies. Cool-water carbonates are dominated by heterotrophic carbonate producing organisms, have few non-skeletal carbonate grains, little or no lime mud, and commonly compact to a greater degree than warm-water lithofacies. The differences between warm- and cool-water carbonate facies assemblages allow for more refined environmental interpretations of ancient limestones than would otherwise be possible.

Our petrographic examination reveals that the limestone unit contains features characteristic of cool-water deposition. Heterotrophic bryozoans and echinoderms dominate the biotic assemblage to the exclusion of phototrophic carbonate producing organisms. Planispiral forams, echinoid spines, a single solitary rugose coral, spicule molds and fragments of brachiopods and bivalves occur in minor amounts. The assemblage lacks non-skeletal grains, contains little carbonate mud, lacks evidence of former aragonitic components, and contains mechanical compaction features. Triassic limestones from the Chulitna terrane contain facies indicative of a low latitude tropical to subtropical environment. Accumulation of cool-water carbonate facies during Permian time followed by deposition of warm-water facies during the Triassic requires a change in latitude, climate, or oceanography. Differentiation of warm- and cool-water depositional regimes in Permian and Triassic limestones of other North American Cordilleran accreted terranes will generate new insight into the paleogeographic setting of the terranes, and provide information about the distribution of calcareous organisms throughout the ancestral Pacific Ocean.