Continental to Shallow Marine Transition in a Tide-Dominated, Low Accommodation basin - Controlling Factors and Depositional Architecture

Abstract

This study presented in this doctoral thesis addresses the development of the Upper Jurassic Curtis Formation, outcropping in east-central Utah. The study suggests a separation of the Curtis Formation in three geological sub-units, each characterised by specific constituents, and diagnostic sedimentary architectures: the lower-, the middle-, and the upper Curtis. The lower Curtis was deposited as the semi-enclosed Curtis sea was flooding the arid coastal Entrada plains and dunes. It is characterised by thin beds of alternating mudstone and sandstone. It reflects three short-lived episodes of relative sea-level oscillations. It is capped by the middle Curtis, which consists nearly entirely of sandstone. The well-defined beds of the lower Curtis are replaced over time by an amalgamation of dunes and tidal channel deposits, reflecting a much higher energy level in the sea than the conditions that prevailed during the deposition of the lower Curtis. This dramatic increase in energy is thought to be a consequence of the basin entering into tidal resonance, which amplified the tides to such a degree that the signatures of short-lived relative sea-level variations were completely erased. As the Curtis sea was regressing, the energy level decreased, and the middle Curtis was subsequently replaced by the finer sediments of the upper Curtis, while the aeolian dunes of the Moab Member of the Curtis Formation were migrating next to the sabkhas of the Summerville Formation. The effects of these relative sea-level variations, absent from the middle Curtis, kept being recorded by the neighbouring and contemporaneous units above-mentioned, highlighting the importance of considering adjacent sedimentary systems in order to obtain the most complete knowledge of a basin’s history.