Late Ordovician-Early Silurian strata extend from the Texas Panhandle across Oklahoma and Arkansas into eastern Missouri and Illinois. The sequence consists of upper and lower sections of organic-detrital limestone separated by calcareous shales (Sylvan, Cason, Maquoketa Shales); the limestones directly above the shales are commonly oolitic (Keel, Noix Formations). The carbonate rocks have a rich megafauna and microfauna, and the shales yield graptolites and chitinozoans. The Ordovician-Silurian boundary, as here defined, usually falls just above the oolite, but locally it occurs in the shale. The faunal and stratigraphic relations suggest continuous deposition through the oolitic beds, although there is substantial lateral gradation between shale and limestone. At most p aces, Late Ordovician (late Ashgillian) strata are overlain by Late Silurian (late Llandoverian) strata; earliest Silurian beds are poorly represented or absent. Physical evidence for an unconformity at this horizon is common, pointing to interruption in deposition accompanied, at least locally, by erosion. Late Ordovician glaciation reported from Africa and South America was possibly of sufficient magnitude to have caused an appreciable lowering of sea level; this could explain the absence of earliest Silurian strata. The Late Ordovician shales are almost devoid of benthic fossils, a condition that might have been caused by various factors including colder water; however, these shales are closely associated with organic-detrital limestones and oolites which bear a large, sessile and vag ant benthos, suggesting temperate to tropical waters.

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