THE IMPORTANCE OF MICROBIALITES AND OTHER MICROFAUNA AS FRAMEWORK ELEMENTS IN UPPER TRIASSIC REEFS FROM EASTERN PANTHALASSA

Scleractinian corals and calcareous sponges became significant reef builders in the Late Triassic (~235–200 Ma); however, these large, characteristic metazoans were not always the dominant elements in Upper Triassic reefs. Here we discuss the construction of two reefs from Eastern Panthalassa; the upper Carnian Pardonnet Hill patch reefs from British Columbia, and the lower Norian Summit Point reefs from Oregon. The reef microfacies have been evaluated quantitatively in order to achieve a rigorous and unbiased assessment of the dominant components and framework elements. This dataset also allows for a quantifiable comparison of the construction and paleoecology of these two reefs with other reef ecosystems.

Both the Pardonnet Hill and Summit Point reefs have areas of framework dominated by microbialites (bindstones) and in many sections microbialites are volumetrically more abundant than corals or calcareous sponges. Both of these observations suggest that microbialites are structurally more important than corals or sponges, particularly since there are large areas of both reefs that are devoid of either corals or sponges. Additionally, there are patches built either entirely or in part by solenoporacean (red) calcareous algae and encrusting foraminifers. The microfacies data from the Pardonnet Hill and Summit Point reefs supports the reinterpretation of these reefs as microbial reefs, coral-microbial reefs, or microbial-coralgal reefs; similar to some Upper Jurassic coral–thrombolite reefs from Europe. These two reefs differ considerably from other North American reefs, such as the Mina coral reefs (Nevada) or the Lime Peak calcisponge reefs (Yukon). Nevertheless, since there are few Triassic reefs from Northeastern Panthalassa, regional trends in Upper Triassic reef ecology and comparisons to Tethyan reefs may need to be revisited in light of this new classification.