Depositional Facies and Geometry of an Exposed Megaflap: Pennsylvanian Honaker Trail Formation, Gypsum Valley Salt Wall, Paradox Basin, Colorado
Megaflaps are geometrically defined as steep-to-overturned, relatively-conformable, deep minibasin strata that extend long distances (several kilometers) up the margins of salt diapirs. The younger minibasin fill successively onlaps the megaflap, forming large-scale growth strata. Megaflaps have been penetrated during drilling in petroleum bearing salt basins (Gulf of Mexico and South Atlantic margins), due to misinterpretation of seismic along the poorly imaged salt-sediment interface. Pre-drill identification of megaflaps reduces the drilling hazards associated with encountering older, steeply-dipping strata, and impacts how trap, reservoir, and seal are risked. 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 and characterization of salt-flank traps.
On the southwestern flank of the Gypsum Valley Salt Wall, Paradox Basin, Colorado, outcrop exposures of the Pennsylvanian Honaker Trail Formation provide a 2 km strike-oriented view of a megaflap. The near vertical panel comprises a thin, black shale interval of the upper Paradox Formation, and a 200 m section of cyclic shallow-marine carbonates and siliciclastics of the Honaker Trail Formation. Stratigraphic analysis of the lower Honaker Trail Formation, documents conglomerates with shallow marine carbonates above abrupt depositional contacts. Near-vertical, upper Paradox and Honaker Trail strata are onlapped by a 200 m thick transitional wedge of steeply-dipping (70°), Permian, lower Cutler Formation siliciclastics. This panel is onlapped by progressively lower-dip strata of the Jurassic Entrada Formation to form a large-scale onlap wedge. The megaflap and onlap wedge were subsequently beveled at an angular unconformity, overlain by the Jurassic Summerville and Morrison formations that form the overlap wedge. Laterally, the megaflap gradually shallows to 50–60° to the southeast before being abruptly terminated by a large down-dropped block, whereas younger minibasin fill progressively buries the megaflap to the northwest.
AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015