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.
Highlights
The complex aspect of tide-dominated environments was reported as early as first century AD, when Pliny the Elder, in his Historia Naturalis, wondered whether such areas “invaded twice each day and night by the overflowing waves of the ocean...are to be looked upon as belonging to the land, or whether as forming portion of the sea?”
The main objective of this study is to develop a detailed data-driven classification of heterolithic facies and facies associations present on a tidally influenced siliciclastic-dominated shelf of regional extent, represented by the Curtis Formation of Early Oxfordian age (Kreisa and Moiola, 1986; Caputo and Pryor, 1991; Wilcox and Currie, 2008; Ogg and others, 2016)
Composition, and structure(s) are the key to interpreting the processes and conditions under which these sediments were deposited. These facies have been organized in eight main facies associations (FA 1 to FA 8) with six sub-facies associations (FA 1a, FA 1b, FA 3a, FA 3b, FA 4a, and FA 4b), which are summarized in table 2 and carefully described below
Summary
The complex aspect of tide-dominated environments was reported as early as first century AD, when Pliny the Elder, in his Historia Naturalis, wondered whether such areas “invaded twice each day and night by the overflowing waves of the ocean...are to be looked upon as belonging to the land, or whether as forming portion of the sea?” (translation from Bostock and Riley, 1855). It has been shown that the essence of tidal deposits resides within their typical three-dimensional intricate assemblage of heterolithic facies, which distribution is dictated by a fine equilibrium of sediment input, basinal hydrodynamic forces, as well as avulsing and migrating channels (Davis and Dalrymple, 2012; references therein). This complexity is further enhanced by temporal and cyclical variations of tidal currents in a basin, which unevenly impact the erosion-transport-deposition mechanisms of the different grain classes within the system (Kvale, 2012; Wang, 2012; references therein). Despite highly complex depositional scenarios, these conditions may produce substantial volumes of reservoir-grade sandstone, and represent potentially viable aquifers, CO2-injection targets, or petroleum reservoirs (Martinius and others, 2005; Halland and others, 2014)
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