PETROLOGY OF THE MIDDLE SILURIAN (WENLOCK: SHEINWOODIAN) MASSIE FORMATION OF THE CINCINNATI ARCH REGION: ANALYSIS OF UNDOLOMITIZED STRATA FROM SOUTHEASTERN INDIANA

Dolomitization of middle Silurian strata is widespread throughout the Cincinnati Arch region, resulting in moderate to severe alteration of sedimentary fabrics and dissolution of calcareous fossil content in most sections. One location that managed to escape significant dolomitization is the New Point Stone quarry near Napoleon, southeastern Indiana, making this site a regional locus for paleontological research. Most attention has focused on the Wenlock (early Sheinwoodian) Massie Formation, which contains a rich, well-preserved, and distinctive fauna. Nevertheless, little detailed sedimentological information is available for this important unit; the lithologic diversity and detailed petrology of the Massie Formation has not hitherto been explored thoroughly. Four primary lithotypes are recognized: a basal tabular limestone (Ls), associated bryozoan-micrite microbioherms (Mb) that are onlapped by a fossiliferous gray shale (Sh), and an upper calcareous siltstone (CSs). The limestone is a tabular pelbiosparite (Ls_t) capped by an undulating hardground (Ls_h), with both crest (Ls_h-c) and trough (Ls_h-t) sub-environments. Microbioherm cores (Mb_c) are biolithites that can be texturally stromatolitic (Mb_c-s) or thrombolitic (Mb_c-t) and are surrounded by coarse biomicrite flank beds (Mb_f) conspicuously enriched in certain bioclasts (e.g., pelmatozoan debris: Mb_f-p). The main body of shale (Sh_m) is poorly indurated, heavily bioturbated, and rich in fine fossil debris as well as larger, typically compressed macrofossils. A more indurated, calcareous bed (Sh_t) low in the unit is characterized by a similar overall fabric with fewer but uncompressed macrofossils. The base of the shale unit is marked by a texturally complex, concretionary biomicrite (Sh_c) that contains the greatest concentration of macrofossils, although they are commonly compressed, partially disarticulated, and/or otherwise damaged, reflecting exhumation during sediment starvation. The uppermost calcareous siltstone is (CSs) contains quartz silt, fine carbonate debris, and few macrofossils. Collective petrologic evidence supports the interpretation of a stratigraphic succession recording the transition from late transgressive (Ls, Mb) to highstand (Sh) to falling stage (CSs) systems tracts in a third-order sequence.