Facies architecture and across-shelf variability of an extensive Lower Triassic (Smithian) microbial carbonate mound complex in the western U.S.

Abstract

Widespread Lower Triassic microbial carbonates occur after the end-Permian mass extinction (EPME) and are commonly attributed to reduced metazoan competition after the EPME, or to paleoceanographic conditions that suppressed metazoan abundance and increased ocean carbonate saturation. Testing these hypotheses requires direct spatial (versus temporal) linkages between Lower Triassic microbial deposits and lithologic or geochemical proxy evidence for environmental perturbations. This study uses facies within and associated with an extensive Lower Triassic (Smithian) microbial carbonate mound complex, which developed across a > 400-km-wide middle-to-inner shelf in southern Utah, U.S.A., to assess potential controls on microbial carbonate development.Middle-shelf microbial mounds (1–2 m tall) are composed of stromatactis-rich peloidal boundstones that are laterally linked by microbial intermound beds. Inner-shelf microbial mounds (<1 m) are composed of microbial laminites that are linked by flat-lying microbial laminite intermound beds. Both mound types nucleated atop non-mound microbial carbonates in deep- to shallow-subtidal environments and aggraded during sea-level rise. During sea-level fall, mounds broadened, shortened, and terminated growth when higher current energies inhibited substrate stability and mat nucleation. Middle- and inner-shelf mound morphologies and facies differences reflect across-shelf accommodation space variations and proximity to nearshore terrigenous sediment influx.Sparse, but persistent benthic fauna and bioturbation in all microbial facies and the lack of lithologic indicators of bottom-water anoxia indicate sufficient O2 levels to support animal life. The broad Utah shelf study area implies that shelf-edge, upwelling-derived, carbonate-saturated waters did not control microbial carbonate precipitation. Regional controls on microbial mound complex development include deposition during sea-level rise along a wide shallow shelf that maximized the areal extent of clear-water, low-energy subtidal environments and promoted growth of prolific photosynthesizing microbial communities. Probable global controls include the post-EPME reduction in skeletal metazoan competition, which permitted microbial mats to flourish, and elevated global-ocean carbonate saturation states and promoted extensive carbonate precipitation.