The Ordovician-Silurian Unconformity in Southeastern Indiana

Abstract

"In southeastern Indiana the Whitewater Formation (Upper Ordovician) is overlain disconformably by the Brassfield Limestone (Lower Silurian). Rocks adjacent to the contact contain evidence of a complex erosional and depositional history. Near the end of Ordovician time, withdrawal of the sea resulted in widespread erosion of Whitewater rocks, the surface of which became extensively pitted. During the earliest stage of Silurian transgression the Ordovician rocks were bored by marine organisms, and at two localities both pits and borings became traps for sediment (omission facies) that is otherwise unrepresented in the section. The omission facies was then phosphatized, and at one locality the remaining depressions became encrusted with thinly laminated phosphorite. The crusts and nearby planar rock surfaces were then encrusted by bryozoans, the growth of which was followed by a final minor episode of sedimentation and phosphatization. At studied localities where phosphatization did not occur, the eroded Whitewater surface is overlain either by conglomeratic Brassfield Limestone containing Whitewater clasts or by a burrowed micritic unit. Events associated with earliest phases of the Silurian transgression occurred in a relatively quiet water environment that lay between the Ripley Island positive area and an offshore high-energy zone where Brassfield crinoidal sands accumulated."