Attributes of the Fisher Valley Megaflap and Comparison to the Gypsum Valley Megaflap, Paradox Basin, Utah and Colorado: Implications for Controls on Megaflap Formation

Megaflaps are panels of steeply dipping to overturned strata that extend kilometers up the side of a salt diapir or equivalent weld. They often form important components of salt-diapir related hydrocarbon systems, though little is known about their geologic characteristics or how they form. In this study, I collected structural, stratigraphic, and sedimentologic data of the Fisher Valley megaflap and compared these attributes to the previous studied Gypsum Valley megaflap to test the differential sediment loading model proposed for initiation of megaflap formation. This model predicts that the sediment source proximal megaflap of Fisher Valley should have started rotation earlier, contain a coarser-grained, siliciclastic-dominated, thicker stratal succession, as well as have wider lateral extent than the sediment source distal megaflap of Gypsum Valley. The Fisher Valley and Gypsum Valley megaflaps formed in the Late Paleozoic in the Ancestral Rocky Mountains system of the Paradox Basin of Utah and Colorado. They are both formed by strata of the Pennsylvanian Honaker Trail Formation and the Permian Lower Cutler Formation, although the Fisher Valley megaflap is about twice as close to the sediment source (20 km) from the Uncompahgre Uplift as Gypsum Valley (40 km). Thicker packages of sediment were deposited on the northeastern side of the Fisher Valley salt wall than the southwestern side, beginning in Honaker Trail time and continuing through Cutler Formation deposition, whereas significant differential sediment loading at Gypsum Valley takes place during Upper Cutler deposition. The timing of megaflap rotation was determined by the stratigraphic position of a significant angular unconformity across the megaflap. At both Fisher Valley and Gypsum Valley this is documented at the Mid-Cutler unconformity, which means that megaflap rotation started at approximately the same time at both megaflaps. Fisher Valley megaflap rotation concluded in the Triassic during the late Moenkopi or Chinle formation deposition, based on the continued shallowing of dips. The Fisher Valley megaflap is composed of mostly of coarse-grained siliciclastics and minor carbonates deposited in three depositional facies associations (FA): Fan Delta FA, Shallow Marine Carbonate FA and Braided Fluvial FA, each of which has several lithofacies. The stratigraphic cyclicity typically seen in the Honaker Trail and Lower Cutler formations in other parts of the Paradox Basin, including Gypsum Valley megaflap is not observed in the Fisher Valley megaflap. However, there are distinct depositional episodes defined by two marine flooding events of the Shallow Marine Carbonate FA that delineate three major phases of fan delta development in the Fan Delta FA before nonmarine Braided River FA progradation occurs. Fisher Valley and Gypsum Valley facies differ in several ways: proportion of siliciclastics to carbonates, coarse-ness of clastics, and lateral facies changes. Carbonates at Fisher Valley occur only twice during megaflap deposition, while carbonates at Gypsum Valley occur consistently throughout the megaflap panel. Siliciclastics at Fisher Valley include a large proportion of conglomerates and conglomeratic sandstones, while coarse-grained siliciclastics and clastics as a whole are not common in Gypsum Valley. Lastly, while most Fisher Valley lithofacies are distributed throughout the megaflap in a facies mosaic, Gypsum Valley lithofacies show well defined stratigraphic cyclicity and little lateral variation. Despite all these differences in facies between the two megaflaps their overall thickness is roughly the same; Fisher Valley is 190 m thick and Gypsum Valley is 200 m thick. The documented coeval timing of megaflap rotation suggests that rotation probably was not driven by regional differences in sediment loading. At Fisher Valley, the differential sediment load may not have been great enough during Honaker Trail deposition to initiate drape-fold rotation, but a critical tipping point may have been reached during deposition of the Upper Cutler Formation. Another possibility is that differential loading may have been generated by differential erosional stripping off and thinning of the diapir overburden at the Mid-Cutler Unconformity. What this outcrop analogue showed is that proximity to sediment source can have a large impact on megaflap attributes such as depositional facies, stratigraphy, and structure and that differential load-induced megaflap rotation may be achieved in several different ways besides depositional accumulation thickness variation. Future work should target testing these alternative mechanisms