ASSESSING MODELS OF STRATAL HIERARCHY IN SEQUENCE STRATIGRAPHY: COMPOSITE SEQUENCE FRAMEWORK OF MIDCONTINENT MISSOURIAN (PENNSYLVANIAN) HIGH - FREQUENCY SEQUENCES

Establishing the hierarchical organization of strata is a key task for sequence stratigraphy. The standard sequence hierarchy model assumes nested eustasy wherein short wavelength, low amplitude sea-level cycles are modulated by long wavelength, high amplitude accommodation cycles. However, Pennsylvanian “icehouse” high - frequency sequences (HFS) were deposited by high amplitude, short wavelength sea-level changes that are inferred to be of equal or greater magnitude than any longer term eustatic trends. Indeed, the standard criteria used to diagnose sequence hierarchy cannot be recognized in many Pennsylvanian sections, including the Missourian sequences of the Midcontinent.

To diagnose hierarchy among these icehouse sequences, new genetic models and recognition criteria need to be assessed. The focus of this study examines the product of convolving short term eustasy with longer term cycles of change in regional subsidence gradient. This model predicts that during periods of accelerated basinal and decelerated upper shelf subsidence (high gradient), the shelf margins of HFS back-step and aggrade, producing starved, amalgamated HFSs in the basin with coastal onlap occurring mid-shelf up-dip of the margins. As the system returns to the regional (low gradient) subsidence regime, the lowstands of HFSs fill the starved basin, and platform facies exhibit more tabular geometries and coastal onlap high on the shelf. Regional exposure and development of incised valley systems indicate periods of minimal regional subsidence.

Lateral and vertical lithofacies and sequence/systems tract thickness trends support this model of hierarchical organization for the Missourian of the Midcontinent. Three composite sequences are recognized (using Kansas formation names): Hepler – Swope, Galesburg – Dewey, Chanute – Stranger. Lowstand Sequence Sets of the respective Composite Sequences are represented by the Hepler, Ladore/Galesburg, and Chanute fluvial-deltaic deposits. Transgressive Sequence Sets are represented by the lower Shale Hill, Dennis, and Iola formations. Early Highstand Sequence Sets are represented by the Shale Hill/Hertha, Cherryvale, and Wyandotte/Plattsburg formations. Late Highstand Sequence Sets are represented by the Swope, Dewey, and Stanton/Stranger formations.