EUSTATIC SEA-LEVEL FLUCTUATION IN THE HIGH-STAND SYSTEMS TRACT (HST) OF A PENNSYLVANIAN TRANSGRESSIVE-REGRESSIVE SEQUENCE (CYCLOTHEM) IN MIDCONTINENT NORTH AMERICA

The Larsh-Burroak Shale represents the high-stand systems tract (HST) of the Deer Creek cyclothem in Mid-continent North America. The lower part consists of about 36 cm of non-sandy, black, fissile, phosphatic shale, overlain by 10 cm of medium gray shale, 6 cm of black shale, and 18 cm of light gray shale. In Iowa and Nebraska, at the northern end of the outcrop belt, the light gray shale is replaced by a lime wackestone to packstone (Haynies Limestone). Above the Haynies Limestone and its shale equivalent lies a 5 cm dark gray shale, a 15 cm medium gray shale, a 3 cm dark gray shale, a 15 cm medium gray shale, another 3 cm dark gray shale, and at the top a 15 cm light gray shale. The dark gray and black shales are interpreted to represent deposition in deeper, more offshore, anoxic to dysoxic waters during transgressive events. The medium and light gray shales represent deposition in shallower, more oxic conditions, with the Haynies Limestone being deposited above the photic zone during regressive events. In southern Kansas, the thin dark gray to black shales above the lower black fissile facies, disappear. They are inferred to have merged with the top of the black shale at the southern edge of the upper shelf, where the water was deep enough that minor sea-level fluctuations did not affect bottom sediment. The Larsh-Burroak Shale is interpreted as having been deposited on a shallow, low gradient, sloping shelf, with deeper water to the south in Kansas and Oklahoma and the shoreline to the north of the outcrop belt in Iowa and Nebraska. It has a very complex architecture with several minor sea-level fluctuations being superimposed upon the major sea-level high-stand and earliest regression.