MORE DIRT ON THE FALLING STAGE - NEW DATA FROM THE CRETACEOUS FERRON SANDSTONE OF THE WESTERN HENRY MOUNTAINS SYNCLINE, UTAH

The Upper Cretaceous (Turonian) Ferron Sandstone in the western Henry Mountains Syncline of south-central Utah sheds new light on the architecture of and controls on falling stage deltas. The unit formed as a series of modest-sized (5-20 km wide), probably asymmetric, mixed-influence deltas (“Ferron Notom Delta”: FND) that dispersed sediment eastward from the rising, Sevier orogenic hinterland into the Western Cordilleran Foreland Basin. Herein, I document two discrete areas within the larger outcrop belt that provide superb three-dimensional exposure, in order to determine the details of stratal stacking patterns in the depositional dip direction, and thereby to assess the stratigraphic context of the FND. In the two study areas, dip transects expose facies representing from river mouth bar to distal delta front environments over distances of 2-4 km. Key stratal packages are clinothems that offlap, downlap, and describe descending regressive trajectories with respective to basal and top datums. Furthermore, facies preserved at the tops of deltaic progradational wedges become coarser-grained and more proximal in the down-depositional dip direction. These clinothems are interpreted as having formed during falling stage (drawdown) of sea level. The vertical extent of clinoforms suggests that deltas prograded into <30 m of water. Furthermore, these deltaic successions preserve abundant evidence of delta front slope failure, growth faulting, and incision and filling of deep (<15 m), slope gullies. Gully fills are composed of chaotic, intraformational breccia and/or massive sandstone, and constitute linear, “shoestring” sand bodies in the distal portions of individual paleodelta systems. They are interpreted to have been cut and filled during the late falling stage and lowstand of sea-level cycles. The north-south distribution of the stratal style described above seems to be focused on the flanks of a long wavelength growth anticline, and so the numerous falling-stage systems tracts preserved within the FND likely owe their origin to synsedimentary structural growth.