Overprinted allocyclic processes by tidal resonance in an epicontinental basin: the Upper Jurassic Curtis Formation, east‐central Utah, USA

Abstract

Modern, tide‐dominated and tide‐influenced coastlines are characterized by a range of environments, including deltas, estuaries and lagoons. However, some tide‐dominated basins and related sedimentary units in the rock record, such as the semi‐enclosed, shallow, Utah–Idaho Trough foreland basin of the Jurassic Curtis Sea, do not correspond to any of these modern systems. Persistent aridity caused the characteristic severe starvation of perennial fluvial input throughout this basin, in which the informal lower, middle and upper Curtis, as well as the underlying Entrada Sandstone, and the overlying Summerville Formation were deposited. Wave energy was efficiently dissipated by the shallow basin's elongated morphology (approximately 800 × 150 km), as its semi‐enclosed morphology further protected the system from significant wave impact. Consequently, the semi‐enclosed, shallow‐marine system was dominated by amplified tidal forces, resulting in a complex distribution of heterolithic deposits. Allocyclic forcing strongly impacted upon the system's intrinsic autocyclic processes as the lower Curtis was deposited. Short‐lived relative sea‐level variations, along with uplift and deformation episodes, resulted in the accumulation of three parasequences, each separated by traceable flooding and ravinement surfaces. The subsequent transgression, which defines the base of the middle Curtis, allowed for the shallow‐marine part of the system to enter into tidal resonance as a consequence of the flooded basin reaching the optimal configuration of approximately 800 km in length, corresponding to an odd multiple of the quarter of the tidal wavelength given an average minimum water depth of 20–25 m. This resonant system overprinted the effects of allocyclic forcing and related traceable stratigraphic surfaces. However, the contemporaneous and neighbouring coastal dune field sedimentary rocks of the Moab Member of the Curtis Formation, characterized by five stacked aeolian sequences, as well as the supratidal deposits of the Summerville Formation, lingered to record allocyclic signals, as the Curtis Sea regressed. This study shows that a tide‐dominated basin can enter into tidal resonance as it reaches its optimal morphological configuration, leading to the overprinting of otherwise dominant allocyclic processes by autocyclic behaviour. It is only by considering the sedimentological relationships of neighbouring and contemporaneous depositional systems that a full understanding of the dynamic stratigraphic history of a basin alternatively dominated by autocyclic and allocyclic processes can be achieved.