Evaluating the Role of Tides in Building Delta Stratigraphy: Comparing Cretaceous Western Interior Seaway Deposits

Abstract:

Ancient delta deposits can provide valuable insight into the long-term evolution of the fluvial-tidal zone, but reconstructing the relative amount of tidal influence on an ancient coastline can be challenging. While characteristic sedimentary structures (such as flaser or herringbone bedding) can indicate the presence of tides, it is unclear whether these diagnostic features represent superficial reworking of largely fluvial deposits, or if they indicate that tides exerted a substantive, if not primary, control on the evolution of a given delta. The Ferron and Sego sandstones within the Cretaceous Mancos Shale are two similarly sized delta deposits that have been interpreted as fluvial-dominated and tidally influenced deltas, respectively. In order to evaluate the degree to which tides sculpted Sego deltas, we quantified stratigraphic organization of well-exposed Ferron and Sego sandstone outcrops in eastern and central Utah. Despite an abundance of quintessential tidal-sedimentary structures, the scale and geometry of bars and channels in both systems is similar suggesting that topset deposits of the tidally influenced Sego system were not appreciably different from those of fluvial Ferron deltas. Additionally, both deposits have even (compensational) stratigraphic organization at scales greater than one paleochannel depth and evidence of persistent (clustered) organization below the compensation scale; however, Sego deposits are significantly more persistent at sub-compensation scales. This suggests that, although tides controlled bedform- and bar-scale deposition in the Sego Sandstone, its network was generally similar to that of a fluvial delta. This raises the possibility that even classic tidally influenced delta deposits may be suitably modeled as fluvial deltas, and that truly tide-dominated delta stratigraphy may be difficult to identify.