Abstract

The basal marine sandstone of the Upper Jurassic Curtis Formation of northeastern Utah was deposited in a shallow, tide-dominated sea during marine transgression across the Entrada eolian sand sea. Entrada dunes within the study area are interpreted to have been over 100 m in height. Eolian paleotopographic "highs" along the Entrada-Curtis contact, however, are only up to 7 m high, clearly indicating that major portions of Entrada dunes were destroyed. Within the transgressive marine deposits are abundant massive sandstone beds up to 12 m thick that are interpreted as mass-flow deposits derived from the marine destruction of the large Entrada dunes. These massive sandstones are lenticular and locally contain clay and sandstone clasts, faint horizontal and contorted laminae, and dish structures. Some massive sandstones were emplaced with significant scour of underlying deposits, but others show nonerosional basal contacts that bury and perfectly preserve underlying marine megaripples and trains of megaripples. Most mass flows were probably initiated during marine storm events (surges) that undercut dunes and resulted in slumpage followed immediately by liquefaction and flowage. Liquefaction in water-saturated, basal portions of dunes probably also resulted in failure and flowage. Interpreted travel distances of a few kilometers over the gently sloping Curtis shelf suggest that liquefied flows evolved into high-density turbidity currents. Characteristics of the massive sandstone deposits, however, indicate that final deposition was from liquefied flows. The flows, therefore, apparently evolved from liquefied flows to high-density turbidity currents and then back to liquefied flows as momentum was lost. We suggest that slumping, liquefaction, and mass flowage of dune sands (which are very prone to liquefaction) commonly occur along the coastlines of some eolian sand seas and are significant mechanisms for the destruction of dunes during marine transgressions and for offshore transport of sand to the shelf. These mechanisms can also operate in flooded interdune areas during major storms. Available literature suggests that massive sandstones are abundant along eolian-marine contacts where dune topography has been largely destroyed and are relatively uncommon where dunes are preserved nearly intact. The primary factors that control the frequency of mass-flow events during marine transgression, and similarly whether dune topography is preserved or destroyed, are 1) the energy and nature of the transgressing marine environment, 2) the rate of transgression, and 3) the abundance of sand-stabilizing early cements.

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