AN ANALOGY TO THE MAXIMUM FLOODING SURFACE OF A TRANSGRESSIVE SYSTEMS TRACT IN A REGRESSIVE SYSTEMS TRACT; PENNSYLVANIAN-PERMIAN CASPER FORMATION, SOUTHEASTERN WYOMING, USA

The concepts of sequence stratigraphy have been undergoing refinement since the seminal papers were presented in the mid 1980's. The initial concepts of systems tracts, sequence boundaries, parasequences and characteristics of each have been scrutinized for validity. Additionally, the initial model was built from a sequence developed along a passive siliciclastic shoreline, similar to the North American Eastern seaboard, and subsequent investigations have sought to determine the applicability to the various other types of shorelines and depositional environments that have existed.

In the original model, the transgressive systems tract usually contains the maximum flooding surface, a surface which separates a transgressive systems tract from a highstand systems tract. The surface is commonly characterized by a condensed section representing a period of slow deposition. This surface is the physical representation of the time at which the relative sea level halts its basinward migration. The regressive systems tract, by analogy, should contain some similar indication of when the relative sea level stops moving landward; however, the original model does not indicate that there is any such marker. This is more than likely a result of the physical characteristics of the depositional environments from which the siliciclastic sequence developed.

In the Pennsylvanian-Permian Casper Formation in southeastern Wyoming (USA), the strata preserve sediments deposited from a juxtaposed dune field and offshore carbonate platform. In the regressive phase of the sequence, dune cross-sets sometimes exhibit a progression upsection within the bedding from a low angle to higher and back to the lower angle. In previous studies of aeolian strata, relationships between angle of cross-sets and the moisture content of the sands forming them have been established that link wet sands with lower-angled beds. Thus this progression observed in the Casper cross-sets indicates a transition from wet sands to dry and back to wet sands again, a progression that is related to the turnaround time in the regressive phase of the relative sea level. As such, a marker analogous to the maximum flooding surface in the transgressive systems tract has now been identified.