Mississippian bioherms of southwestern Missouri and northwestern Arkansas

Lenticular bodies of massive limestone, flanked by thin crinoidal beds, crop out in southwestern Missouri and northwestern Arkansas. They are within the St. Joe limestone of the Lower Mississippian. Strata underlying these bodies typically are down-warped, those on the flanks lapping against, finally passing completely over the limestone core. Dips away from the “reef” are commonly around 5 degrees, but reach as much as 30 degrees. Conglomeratic detritus, originating from the core, occurs in flanking beds at their contact with it. Thin sections of these masses show no more than 15 per cent of the limestone to consist of fossil remains. These are chiefly encrusting bryozoans and crinoid fragments, with occasional ostracods and brachiopods. The matrix is microspar, its particles ranging from about three to 10 microns in diameter. A minor amount of disseminated clay was observed. Flanking beds and inter-reef limestones differ in being composed of 50 to 75 per cent fossils; the matrix is more variable and in places coarser than in the core. The clay content, although minor, is apparent along partings. The lenticular cores, or bioherms, are interpreted as minor elevations caused by entrapment of fine mud by non-calcareous plants in an environment of low energy. Reef-building calcareous organisms are virtually absent. Wave and current action as a formative agent are negated by the presence of fine mud in the bioherm. Genetic interpretations of certain bioherms from different areas and varying parts of the Paleozoic and Tertiary were discussed and rejected as an explanation for these phenomena --Introduction, pages 1-2