Sequence Stratigraphic Analysis of Permian (Wolfcampian), Cyclic Ramp Strata (Chase Group) in the Midcontinent USA, and Comparison to Classic Pennsylvanian Cyclothems
S. J. Mazzullo, C. S. Teal, and C. A. Burtnett
Outcropping carbonate-siliciclastic ramp fades in the Chase Group (Wolfcampian) in Kansas and southern Nebraska are divided into 6 depositional sequences bounded by maximum lowstand red shales. Such sequences are therefore analogous to Pennsylvanian cyclothems in terms of their depositional response to eustasy. Whereas Pennsylvanian cyclothems compose stacked transgressive-regressive (T-R) carbonates separated by black shales believed to be maximum flooding surfaces (mfs), sequences in the Chase Group include 2 to 3 T-R carbonate packages without such "core" shales; fossiliferous, marginal- marine siliciclastics, deposited during incipient lowstands, separate T-R units within sequences. Hence, T-R units are higher-frequency cyclothems. Sequences and cyclothems in the Chase are regionally correlative, and display stacking patterns suggestive of deposition in fundamental response to known Late Paleozoic glacio-eustasy.
Condensed sections of light colored, calcitic, fossiliferous shale, interpreted as mfs, are recognized as regionally correlative events in half of the sequences, the remaining ones containing only conceptually defined mfs. Highest frequency, shallowing-upward cycles within cyclothems are not regionally correlative, and mappable 3-D facies changes across the outcrop belt clearly complicate cycle correlation. Erratic thicknesses of some cyclothems and component peritidal subfacies, the only local occurrence of some cycles on paleotopographic highs, poor correlation of cycles, and anomalously thick lowstand siliciclastics suggest the imprint of autogenic deposition, local variations in accommodation due to concurrent tectonism, and long-term, far-field tectonic subsiden e on sequence stratigraphic development in the Chase Group.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California