DECIPHERING 3RD ORDER ACCOMMODATION TRENDS AMONG 4TH ORDER ICEHOUSE SEQUENCES: EXAMPLE FROM THE UPPER MISSOURIAN (PENNSYLVANIAN) OF NORTHERN OKLAHOMA

Deciphering the 3rd order organization of 4th order Icehouse sequences has been notoriously difficult: coastal onlap of 4th order sequences can display no clear order, the shelfward encroachment of basinal facies among 4th order MFSs can display no obvious trends, and shelf-wide subaerial exposure can characterize every sequence. Clearly, other indicators of evolving accommodation need to be leveraged in order to interpret the long term organization of Icehouse sequences.

Reconnaissance mapping of Upper Missourian deltaic and shallow marine strata in northern Oklahoma reveals that these 4th order sequences are organized into retrogradational, aggradational, and progradational sets of sequences based upon depositional margin trajectory and shelf versus basin thickness trends of individual sequences. Together, these sequence sets define the systems tracts of a 3rd order composite sequence.

The lower 3rd order unconformity and overlying lowstand sequence set is represented by incised valley fill and basin-wide coastal plain facies of the Chanute Formation. The overlying Avant (Iola) and Lower Wann (Wyandotte/Farley) sequences distinctly backstep and aggrade to the south (southern Osage County), leaving a starved, shallow basin to the north (northern Osage and Washington counties). These sequences compose the transgressive sequence set. The overlying Upper Wann (Plattsburg) sequence progrades into the basin, depositing two deltaic wedges near Bartlesville and Wann; however, the shallow basin persisted in the Dewey and Copan area. The overlying Barnsdall (Stanton) sequence displays strong progradation into this basin, filling the remaining relief with abruptly basinward stepping deltaic facies. The overlying South Bend and Iatan sequences are poorly characterized, but are dominated on outcrop by coastal plain facies with thin, shallow shelf marine maximum flooding horizons. These four sequences compose the highstand sequence set. The upper bounding 3rd order unconformity appears to coincide with the regional Tonganoxie incised valley system.

Cursory examination of Lower Missourian and Upper Desmoinesian sequences indicate similar composite sequence organization, indicating a consistent theme in the low order organization of Icehouse sequences.