Sedimentary environment of Middle Ordovician iron oolites in northeastern Kansas, U.S.A.

Abstract

The petrographical and mineralogical composition of Middle Ordovician (Simpson Group) rocks in northeastern Kansas (Nemaha and Marshall counties) suggest their deposition in a near-shore environment that was affected by both major and minor transgressional and regressional events. Two iron oolite (goethite) horizons found locally in the lower part of the St. Peter Sandstone delineate the position of an ancient shoreline. The sedimentation of Middle Ordovician rocks in northeastern Kansas was influenced by a positive structure (paleohigh) associated with the Midcontinent Rift System, that extended into southern Nebraska. This structure consisting mostly of Precambrian granites, high in magnetite, was the probable source of iron. The absence of St. Peter Sandstone in certain areas of northeastern Kansas is attributed to non-deposition rather than subsequent erosion.