Late Early Triassic microbial reefs of the western United States: a description and model for their deposition in the aftermath of the end-Permian mass extinction

Abstract

Research in the western United States to reinvestigate and characterize the previously reported stromatolites of the latest Early Triassic (Spathian) Virgin Limestone Member of the Moenkopi Formation has shown that these microbial build-ups formed patch reef mounds. Detailed field analysis of these reef mounds and the surrounding beds has established that they attained 1–2 m of topographic relief above the seafloor. A petrographic study of these microbial mounds illustrates that they consisted of microbial fabrics, as well as voids with early marine cements and formed in the absence of in situ metazoans. These reef mounds formed during the aftermath of the end-Permian mass extinction that spanned the Early Triassic. Due to the devastation of marine metazoans at this extinction, the Early Triassic has often been deemed a reef gap because no metazoan reefs are found at this time. Whereas colonial metazoan reefs are globally absent from Lower Triassic strata, microbial build-ups similar to these described from the western United States have been found in south China, Armenia, southern Turkey and Greenland. A regional Early Permian to late Early Triassic unconformity exists in the study region. However, analysis of these microbial reefs within a global context suggests that conditions favouring microbial reef development must have existed as long as 4–8 m.y. after the end-Permian mass extinction. A depositional model of these microbial mounds indicates that flooding of the shelf with anoxic and/or CO 2-rich deep waters may have fostered their occurrence.