WHEN UTAH WAS A BLUE STATE: THE SEDIMENTOLOGY AND DEPOSITIONAL HISTORY OF THE TIDALLY-INFLUENCED UPPER JURASSIC CURTIS FORMATION

The sedimentary record of east-central Utah during the Middle/Late Jurassic shows that arid conditions prevailed, punctuated by episodic incursions of the Sundance Sea from the north. It is some of these incursions that terminated the Entrada aeolian system, and deposited the overlying shallow-marine Curtis Formation in a tide-dominated, semi-enclosed basin. The shallow-marine system was neighboured by a coastal aeolian system belonging to the Moab Member of the Curtis Formation, and a fluvially-starved, sporadically-flooded, paralic belt of the Summerville Formation. These paralic deposits progressively blanketed the shallow-marine strata of the Curtis Formation as the sea regressed northward. This study deconstructs and analyses the continental to shallow-marine deposition during one transgressive-regressive cycle.

Our analysis sub-divides the Curtis Formation into three informal units: the lower, middle and upper Curtis. Allocyclically-driven, short-lived relative sea-level variations, along with episodes of uplift and deformation, dominated over autocyclic processes of the Curtis Sea as the lower Curtis was deposited. The system entered into tidal resonance following the onset of the major transgression that defines the base of the middle Curtis, as the flooded basin reached an optimal length that corresponded to an odd multiple of one quarter tidal wavelength. This high-energy resonant system overprinted the effects of allocyclic forcing within the middle Curtis. By contrast, the neighbouring coastal systems continued to record allocyclic signals preserved as five stacked aeolian sequences of the Moab Member of the Curtis Formation, and a cyclical pattern recognised within the supratidal Summerville Formation.

By documenting a resonant stage in a tide-influenced basin that overprints the otherwise dominant allocyclic processes, this study highlights the importance of assessing coeval depositional systems in order to build complete stratigraphic basin histories.

Keywords: allocyclic, autocyclic, Curtis Formation, stratigraphic surfaces, tidal resonance, aeolian sequences, Utah.