A SHALLOWING-UPWARD SUCCESSION OF PERITIDAL MICROBIAL CARBONATES IN THE TYMOCHTEE DOLOMITE (SILURIAN - LUDLOW) OF LOGAN COUNTY, OHIO

A shallowing-upward succession of microbial facies occurs within dolomitic boundstones of the Tymochtee Dolomite (Silurian - Ludlow) cropping out in Logan County, Ohio. A twelve-meter section contains distinct microbial geometries and textures that accreted under regressive conditions in a peritidal setting. The microbial facies developed within a relatively narrow depth range and include, in ascending stratigraphic order: (1) subtidal banded rocks; (2) shallow-subtidal thromboidal rocks; (3) columnar stromatolites that accreted in relatively high-energy, low-intertidal waters; (4) domical stromatolites that accreted in a middle- to high-intertidal setting; (5) laminites that developed in a low-energy, upper intertidal to supratidal environment; and (6) flat-pebble conglomerates derived from the reworking of cohesive or lithified microbial mats in a high-intertidal to supratidal setting. Variation in microbialite form is attributed to both the composition of progenitor microbial communities and changes in environmental conditions resulting from a reduction in relative sea level. High salinity and elevated temperature probably facilitated carbonate production and restricted the presence of metazoans in the depositional setting. The abundant lithologic evidence for the mediation of local sediments by microbial communities and the paucity of megafauna indicates that: (1) microbial colonies overwhelmingly dominated the biota in the depositional setting; (2) the paleoenvironment probably had many of the same characteristics as the benthic microbial ecosystems of the Proterozoic and earliest Phanerozoic; and (3) despite the apparent global reduction in microbialite development during the post-Ordovician, communities of calcifying microbes continued to flourish locally during the Silurian.