Abstract

Microfacies analysis of five patch reefs from the Martin Bridge Formation (Upper Triassic/Norian, Summit Point, Oregon, United States) reveals that microbialite fabrics dominate reef construction versus corals or other large metazoans. Other reef bioconstructors include characteristic Late Triassic branching corals and diminutive calcifiers (solenoporacean red algae, foraminifera, and sponges). The patch reefs exhibit dense growth fabrics and had elevation above the seafloor, making them ecological reefs or true reefs. Robust binding and encrusting organisms inhabited higher-energy areas, whereas red algae, followed by phaceloid coral colonies, inhabited zones of decreasing water energy, defining a subtle zonation consistent with wave energy in the paleoenvironment.

The dominance of microbial fabrics and diminutive bioconstructors make the Summit Point reefs distinct among Upper Triassic reefs from northeastern Panthalassa. Summit Point was originally identified as a Dachstein-type reef (e.g., high diversity, dominated by corals and sponges, massive bedding, large reef cavities). However, the dominance of microbial fabrics over larger metazoans, the (comparatively) modest faunal diversity, and lack of abundant, multigenerational epibionts or cements clearly differentiates the Summit Point reefs from the Dachstein reefs of northern Europe. The Summit Point reefs do, however, bear a striking resemblance to some Middle Triassic (Anisian) reefs from the Tethys, particularly with the abundance of microbialite fabrics and diminutive, binding or encrusting bioconstructors.

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