Pennsylvanian Stratigraphy of Douglas County, Kansas: Glacioeustatically Modulated Cyclic Deposition on a Remarkably Smooth Shelf

Abstract

The Middle and Late Pennsylvanian succession of Kansas consists of two types of cyclothems, lithologic successions that consist of several members that repeat in a particular order. Cyclothems are a few meters to a few tens of meters thick. One type of cyclothem, the Kansas cyclothem, or here the limestone-rich cyclothem, consists of interbeds of limestone and shale, where the amounts of limestone are approximately equal to the amount of shale or dominate the succession. This trip will study one such cyclothem in the Oread Limestone at Stop 3 and possibly another cyclothem at the optional Stop 4. Siliciclastic cyclothems are dominated by shale and may contain an incised valley filled partly by fluvial or estuarine sandstone. The Lawrence Shale, which we will see at Stops 2 and 3, contains such features. While many cyclothems are incomplete, both types of cyclothem may contain dark gray to black, fissile, phosphatic core shale beds, commonly a meter or less thick, and an underlying transgressive deposit beneath. Overlying deposits represent regression or progradation of the shoreline: either lime mudstone grading to grainstone or clay-rich marine shale grading to coarse siltstone or fine sandstone. Paleosols and fossils of marine invertebrates are common and indicate that deposition took place in both marine and non-marine conditions. Some marine deposits show a paleosol overprint. The deposits indicate that the area was a shelf during the Late Pennsylvanian where relief was very low, and very laterally extensive deposits formed as sea level rose and fell. The alternation of shale-rich and limestone-rich intervals plus the feet-per-mile dip to the WNW create the characteristic topographic expression of cuestas with gently sloping caps that are underlain by limestone. While glacial deposits are mostly restricted to northern and east-central Douglas County, Pleistocene glaciers influenced the erosional and drainage patterns of the area.