RECOGNITION OF MINOR REGRESSIVE SEA-LEVEL CHANGES DURING THE GENERALLY HIGHSTAND PHASE OF THE DEER CREEK CYCLOTHEM (UPPER SHAWNEE GROUP, VIRGILIAN) OF NORTHERN MIDCONTINENT NORTH AMERICA

The Deer Creek major cyclothem includes the top of the Oskaloosa Shale, Rock Bluff Limestone, Larsh-Burroak Shale, Ervine Creek Limestone, and most of the Calhoun Shale, in ascending order. The Oskaloosa Shale contains a reddish paleosol mudstone and the lower sequence boundary of the cyclothem. The Rock Bluff Limestone is the transgressive limestone characterized by large triticitid fusulinids. The Larsh-Burroak Shale is the condensed interval, deposited at highstand. In southwest Iowa and southeast Nebraska the lower black fissile, phosphatic shale facies (Larsh) is separated from the upper, generally lighter colored, shale facies (Burroak) by a thin limestone (Haynies Limestone). The Haynies disappears southward in Kansas and northwest Missouri where only a light-gray fossiliferous shale occurs at this horizon. The Haynies Limestone represents a minor sea-level drop during the generally highstand phase. In southwest Iowa, southeast Nebraska, northwest Missouri, and northeast Kansas a gray shale occurs within the black fissile facies of the Larsh Shale and within the black facies of the Burroak Shale. These gray shales may represent two other sea-level drops of lesser magnitude than that which allowed deposition of the Haynies Limestone. 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 Haynies Limestone deposition the pycnocline may have broken up and water depth was shallow enough to allow benthic carbonate production and preservation. The black facies of the Larsh-Burroak Shale in this area is restricted to topographic lows, becoming gray over highs. The Larsh-Burroak Shale carries an abundant conodont fauna of several hundred to several thousand per kilogram, dominated by Streptognathodus pawhuskaensis. The Ervine Creek Limetone is the regressive limestone and contains triticitid fusulinids. The upper Calhoun Shale contains a gray blocky mudstone representing a paleosol and upper sequence boundary of the cyclothem.