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Geometry and Depositional Facies of an Exposed Megaflap: Pennsylvanian Honaker Trail Formation, Gypsum Valley Salt Wall, Paradox Basin, Colorado


Megaflaps are geometrically defined as steep to overturned structural panels of relatively conformable strata that extend long distances (on the order of several kilometers) up the margins of salt diapirs. The younger minibasin fill form predictable stratal geometries that thin toward and successively onlap the megaflap developing large scale growth strata. Megaflaps are increasingly discovered post-drill in petroleum bearing salt basins (Gulf of Mexico, South Atlantic margins) due to misinterpretation of poorly imaged seismic along the salt sediment interface. Pre-drill identification of megaflaps reduces the drilling hazard associated with encountering older, steeply dipping strata. Therefore, the application of outcrop analogs that document the depositional facies, stratal geometries, and structural deformation of megaflaps is critical for successful pre-drill prediction. Outcrop exposures of the Pennsylvanian Honaker Trail Formation provide a 2km strike-oriented view of a megaflap on the southwestern flank of the Gypsum Valley salt wall, Paradox Basin, Colorado. The near vertical (80–90°) panel comprises a 30m black shale interval of the Pennsylvanian upper Paradox Formation, and a 200m section of cyclic shallow marine carbonates and siliciclastics of the Honaker Trail Formation. Preliminary stratigraphic analysis of the lower Honaker Trail Fm. documents conglomerates at abrupt depositional contacts with shallow marine carbonates. Conglomeratic facies thicken and thin (max 1–2m) along depositional strike and represent debris flows generated in response to early irregular salt inflation suggesting diapirism initiated during early Honaker Trail time. The near vertical upper Paradox and Honaker Trail strata are onlapped by a 200m thick transitional wedge of steeply dipping (70°) Permian lower Cutler Formation siliciclastics. This panel is progressively onlapped by the Jurassic Entrada Formation to form a large-scale growth package referred to here as the “onlap wedge.” The megaflap and onlap wedge were subsequently beveled at the angular unconformity with the overlying Jurassic Summerville and Morrison formations, which form the “overlap wedge”. Laterally the megaflap gradually shallows to 50–60° on the southern flank where it is abruptly terminated by a large graben, whereas younger minibasin fill progressively buries the megaflap on the northern flank.