SEQUENCE STRATIGRAPHY, GEOMETERY, AND FORMATION OF AN EXPOSED MEGAFLAP: PENNSYLVANIAN HONAKER TRAIL FORMATION, GYPSUM VALLEY SALT WALL, PARADOX BASIN, COLORADO

Megaflaps are halokinetic features that are increasingly targeted for hydrocarbon exploration, particulary in the deepwater Gulf of Mexico. Megaflaps are geometrically defined as structural panels of relatively conformable strata that extend long distances (as much as 5km) steeply to overturned, up the margins of salt diapirs.

In the Paradox Basin, the Upper Pennsylvanian Honaker Trail Formation forms part of a partially exposed megaflap on the southwestern flank of the Gypsum Valley primary salt wall and provides an excellent opportunity to enhance our understanding of diapir-proximal salt-sediment interaction. The megaflap comprises a 200m thick 5km wide steeply dipping (60 – 90°) stratal panel composed primarily of cyclic shallow marine carbonates and siliciclastics of the Pennsylvanian Honaker Trail Formation, but contains a basal thin (10’s m) black shale & dolomite cycle of the Pennsylvanian Paradox Formation, and an upper, thin (10’s m) cycle of siliciclastics of the Permian lower Cutler Formation. The megaflap/Gypsum Valley salt wall evaporite contact is not exposed, but is mostly in normal fault contact with the Jurassic Morrison Formation.

The megaflap is progressively onlapped by the Permian Cutler Formation through Jurassic Entrada Formation forming a large-scale growth stratal wedge referred to here as the “onlap wedge”. Both the megaflap and onlap wedge are erosionally beveled at the angular unconformity contact with the overlying Jurassic Summerville/ Morrison formations, which form the megaflap burying “overlap wedge”.

The exposed megaflap will be evaluated by measuring sections in order to develop a sequence stratigraphic framework. Preliminary stratigraphic analysis of this Pennsylvanian megaflap has revealed pronounced depositional contacts between supratidal carbonates and diapir derived debris flows. The findings of this study will provide assessment of the early formational history of megaflaps and add to the growing database of attributes of megaflap stratal packages.