New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA

  • Valentin Zuchuat University of Oslo
  • Arve R.N. Sleveland University of Oslo
  • Douglas A. Sprinkel Utah Geological Survey
  • Algirdas Rimkus University of Oslo
  • Alvar Braathen University of Oslo
  • Ivar Midtkandal University of Oslo
Keywords: Jurassic, Curtis Formation, tidal facies, Wanaka

Abstract

Based on a methodic sedimentological analysis, the Late Jurassic (Oxfordian) Curtis Formation unravels the intricate facies variability which occurs in a tide-dominated, fluvially starved, low-gradient, semi-enclosed epicontinental basin. This unit crops out in east-central Utah, between the eolian deposits of the underlying Middle Jurassic (Callovian) Entrada Sandstone, from which it is separated by the J-3 unconformity, and the conformable overlying supratidal Summerville Formation of Oxfordian age. A high-resolution sedimentary analysis of the succession led to the recognition of eight facies associations (FA) with six sub-facies associations. Based on the specific three-dimensional arrangement of these eight facies associations, it is proposed to separate the Curtis Formation into three sub-units: the lower, middle and upper Curtis. The J-3 unconformity defines the base of the lower Curtis, which consists of upper shoreface to beach deposits (FA 2), mud-domi­nated (FA 3a) and sand-dominated heterolithic subtidal flat (FA 3b), sand-rich sub- to supratidal flat (FA 4a) and correlative tidal channel infill (FA 4c). It is capped by the middle Curtis, which coincides with the sub- to intertidal channel-dune-flat complex of FA 5, and its lower boundary corresponds to a transgressive surface of regional extent, identified as the Major Transgressive Surface (MTS). This surface suggests a potential correlation between the middle and the upper Curtis and the neighboring Todilto Member of the Wanakah Forma­tion or Todilto Formation. The upper Curtis consists of the heterolithic upper sub- to intertidal flat (FA 6) and coastal dry eolian dunes belonging to the Moab Member of the Curtis Formation (FA 7), and it conformably overlies the middle Curtis.

The spatial distribution of these sub-units supports the distinction of three different sectors across the study area: sector 1 in the north, sector 2 in the south-southwest, and sector 3 in the east. In sector 1, the Curtis Formation is represented by its three sub-units, whereas sector 2 is dominated by the middle and upper Curtis, and sector 3 encompasses the extent of the Moab Member of the Curtis Formation.

This study also highlights the composite nature of the J-3 unconformity, which was impacted by various processes occurring before the Curtis Formation was deposited, as well as during the development of the lower and middle Curtis. Local collapse features within the lower and middle Curtis are linked to sand fluid over­pressure within a remobilized sandy substratum, potentially triggered by seismic activity. Furthermore, the occurrence of a sub-regional angular relationship between the middle Curtis and substratum implies that the area of study was impacted by a regional deformational event during the Late Jurassic, before the deposition of the middle Curtis.

View from the Lower South Desert Overlook, Capitol Reef National Park, displaying the earthy facies of the Callovian Entrada Sandstone (reddish-colored sandstone), overlain by the Oxfordian, tidally influenced Curtis Formation (light-colored sandstone). The two units are separated by the J-3 unconformity, which coincides here with the Major Transgressive Surface (MTS) at the base of the informal middle Curtis. Note the evidences of tidal ravinement at the base of the middle Curtis. The middle Curtis consists mainly very fine to fine-grained sandstone, and corresponds to a sub- to intertidal channel- dune-flat complex depositional setting. The middle Curtis gradually grades into the thinner-bedded, very fine-grained, sub- to intertidal heterolithic flat deposits of the upper Curtis, which is conformably overlain by the Summerville Formation. Note the geologist in the lower right quadrant of the photograph for scale. i
Published
2018-07-31
How to Cite
Zuchuat, V., Sleveland, A., Sprinkel, D., Rimkus, A., Braathen, A., and Midtkandal, I., 2018, New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA: Geology of the Intermountain West, v. 5, p. 131-165.