Sedimentology and Structure of Growth Faulted Flood-Dominated Mouth-Bars in the Cretaceous “Notom Delta”, Ferron Sandstone Member, Utah

En-echelon sets of mesoscale growth faults were mapped in the Cretaceous “Notom Delta” of the Ferron Sandstone Member, Utah. Cliff faces show cross sectional views, and the surfaces of the faults are also exposed on the cliff tops, allowing 3D reconstruction. Measured sections, photomosaics, digital mapping (RTK- GPS), GPR, and structural measurements were integrated to relate sedimentology, fault zone evolution and fault linkage.

Growth strata are exposed within a 10m thick upward coarsening facies succession, interpreted as a prograding prodelta and delta front. Pre-growth strata comprise interbedded prodelta shales and sandstones. Bioturbation levels are minimal and an abundance of graded beds, Bouma sequences, climbing ripples and soft sediment deformation suggest rapid deposition in a stressed environment. Growth strata are dominantly medium-grained upper plane bed stratified sandstones, up to 5 m thick, with rare humpback cross strata. These are interpreted as mouth bars formed in relatively shallow water depths at high flow velocities. We interpret the depositional environment as a highly fluvial, flood-dominated delta front setting.

Growth faults are listric with a maximum throw of several meters. As a system they define several subparallel belts that are concave basinward and sole/detach into prodelta shales. Strain within the detachment zone is accommodated by a combination of simple and pure shear deformation mechanisms. We interpret that rapid deposition of mouth-bars during floods traps underlying fluid mud, increases pore-fluid pressures and facilitates fault initiation. The mud flows basinward and towards fault tips, where sands are thinner, and forms diapiric ridges, deforming adjacent beds. The deformed beds are vertical to overturned. We interpret that oversteepening of adjacent strata, coupled with continued progradation of the mouth-bars, initiates the next fault basinward.