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Stratigraphic Architectures of Deltas Formed During Forced and Normal Regressions, Ferron Sandstone (Turonian), Southeast Utah, USA

Abstract

Depositional-dip transects through fluvio-deltaic strata are key to sequence stratigraphic analysis. But many questions remain about the validity of these models, primarily because continuous exposures across the nonmarine-marine transition are sparse. We present the first detailed study of a 30-km depositional dip transect through nonmarine and marine facies of the fluvio-deltaic Ferron Sandstone on the southern limb of the Henry Mountains. Superb, continuous exposure allows us to examine relationships between facies, stacking patterns, and bounding surfaces in deltas formed during forced and normal regressions. The study interval defines a broad, tabular package of strata containing lower and upper members separated by a major, sequence-bounding unconformity (SB), which can be traced basinward to its correlative conformity. The lower member is dominated by delta front mudstones and comparatively thin delta front sandstones that are arranged into a series of eastward-dipping, offlapping clinothems defining a descending, regressive shoreline trajectory (falling stage deltas). Using the SB and a basal bentonite datum, we calculate a shelf gradient of 0.002 at the end of forced regression. The SB is a cryptic surface, without widespread incision, overlain by poorly developed floodplain facies or transgressive facies. Vertical separation between the SB, transgressive ravinement surface (TRS), and maximum flooding surface (MFS) is less than a few meters, and in many places the SB and TRS are merged. The upper member is arranged into a series of progradational to aggradational deltaic successions formed during normal regression. Sandstones transition down-dip from channel to mouth bar to delta front or shoreface. Shoreface facies increase upward, suggesting a change from river-dominated to wave-dominated deltas through time. By integrating observations across scales, we demonstrate relationships between facies and bounding surfaces under different conditions of base-level change. This information is useful for testing sequence stratigraphic principles and validating modeling studies, and it provides context for sequence stratigraphic analysis of similar deposits where strata are not as well exposed.