17 Sequence Stratigraphy and Biostratigraphy of Pennsylvanian–Lower Permian Cyclothems in the North American Midcontinent I

Thursday, 12 April 2007
8:00 AM - 12:00 PM

LITHOSTRATIGRAPHY AND SEQUENCE STRATIGRAPHY OF THE SHAWNEE GROUP (VIRGILIAN, PENNSYLVANIAN) IN SOUTHEASTERN KANSAS

Paper number: 17-5
Program: THEME SESSIONS
Time: 9:20 AM – 9:40 AM

Authors

Abstract

Cyclothemic and megacyclothemic scale depositional sequences are readily identified in outcrops in southeastern Kansas. This study further differentiated these cyclothemic-scale depositional sequences of the Shawnee Group (Virgilian, Pennsylvanian). Outcrops were measured, sampled and described from various sites in a north-northeasterly, south-southwesterly trending outcrop belt situated in portions of Chautauqua, Elk, Greenwood and Woodson counties. These outcrop observations, paleontologic (primarily conodont) information, hand sample and thin section photomicrographic data, and 52 prepared cross-sections were used for final analysis. After all the available data was compiled, rock units at each location were organized within a specific sequence-stratigraphic framework (systems tract and sea level curve). A composite stratigraphic cross-section was prepared based on the interpolation of important sequence stratigraphic bounding surfaces and the interpretation of the depositional environment.

The Shawnee Group consists of six complete transgressive-regressive cycles of sedimentation forming four megacyclothems, as defined by Heckel in 1977. The Upper Lawrence (below the Toronto Limestone), Snyderville Shale, Kanwaka Shale, Tecumseh Shale, Calhoun Shale, Iowa Point Shale and the Severy Shale represent lowstand deposits. Four cycles had sufficient water depths at or near highstand necessary to form definite marine condensed sections. These condensed sections are represented by black shales, typically very phosphatic, containing an abundance and diversity of conodonts. Maximum flooding events represented by the phosphatic “core” shales include the Heebner Shale (Oread Limestone Formation,) the Queen Hill Shale (Lecompton Limestone Formation,) Larsh-Burroak (Deer Creek Limestone Formation,) and finally, the Holt Shale (Topeka Limestone Formation). The Holt Shale was previously believed to be absent in southeastern Kansas. These are highstand deposits formed during maximum transgression.