RECOGNITION OF A SEA LEVEL FLUCTUATION PRESERVED IN THE HIGH-STAND SYSTEMS TRACT OF THE MID-HARTFORD LIMESTONE (LOWER TOPEKA CYCLOTHEM) IN THE IOWA SHELF REGION, NORTHERN MIDCONTINENT

The lower Topeka (Hartford) intermediate cyclothem spans the interval from the top of the paleosol in the underlying Calhoun Shale to the terrestrial (coal and mudstone) deposits in the middle of the Iowa Point Shale. Transgressive deposits include the upper Calhoun Shale and lower Hartford limestone bed. The mid-Hartford shale represents the condensed interval of the cyclothem, deposited at highstand. The lower part is black and represents maximum transgression at a marine condensed interval. The upper part is light gray and represents early regressive deposits. Later regressive deposits are represented by the upper Hartford limestone bed and marine deposits in the lower Iowa Point Shale.

In northeast Kansas, southeast Nebraska, northwest Missouri and extreme southern Iowa, the lower black facies of the mid-Hartford shale is phosphatic, and has abundant conodonts (greater than 4000 elements per kilogram). Farther north in Iowa the mid-Hartford shale exhibits a two part subdivision of black shale overlain by light gray shale. The lower shale interval is separated from the upper shale interval by a thin limestone. This limestone disappears southward in Iowa, just north of the Missouri border. This limestone represents a minor sea-level drop during the generally highstand phase, and the overlying shale interval represents another transgression. These lithologic changes would be expected in this more shoreward, higher shelf area where minor changes in sea-level could affect the establishment of a pycnocline. When water was deep enough the pycnocline developed, inhibiting vertical circulation and allowing preservation of organic matter that accumulated as black shales. At lower sea-level stand the pycnocline was less well-developed allowing more bottom oxygenation and deposition of gray shale. During limestone deposition the pycnocline may have broken up and water depth was shallow enough to allow benthic carbonate production and preservation.