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Anatomy of Falling-Stage Deltas in the Turonian Ferron Sandstone of the Western Henry Mountains Syncline, Utah: Growth Faults, Slope Failures, and Mass Transport Complexes

Abstract

Recent work on the Upper Cretaceous (Turonian) Ferron Sandstone in the western Henry Mountains Syncline of south-central Utah has established its environment of deposition as a series of modest-sized, 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. Analysis of sandstone body stacking patterns in a 67 km-long, depositional strike-parallel (north-south) transect indicates that successive delta progradational events and their deposits were strongly forced by synsedimentary growth of a long wavelength (∼100 km, 50 m amplitude) fold structure. Herein, I document two discrete areas within this transect which 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 clinoform sets that offlap, downlap, and describe descending regressive trajectories with respective to basal and top datums. They are thus interpreted as the product of falling relative sea-level. 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 relative sea-level cycles. The north-south distribution of the stratal style described above seems to be focused on the flanks of the growth anticline, and so the numerous falling-stage systems tracts preserved within the FND likely owe their origin to synsedimentary structural growth, and the unstable fluid pressure regime that this growth imposed on the sea floor and shallow subsurface. The ancient deltas of the FND are small and yet because of this, entire delta systems can be examined in this well-exposed field area. The scaling relationships established here likely apply to larger deltas in the rock record.