Abstract
Existing barrier island facies models are largely based on modern observations. This approach highlights the heterogeneous and dynamic nature of barrier island systems, but it overlooks processes tied to geologic time scales, such as multi-directional motion, erosion, and reworking, and their expressions as preserved strata. Accordingly, this study uses characteristic outcrop expressions from paralic strata of the Upper Cretaceous Straight Cliffs Formation in southern Utah to update models for barrier island motion and preservation to include geologic time-scale processes. Results indicate that the key distinguishing facies and architectural elements of preserved barrier island systems have very little to do with “island” morphology as observed in modern systems. Four facies associations are used to describe and characterize these barrier island architectural elements. Barrier islands occur in association with backbarrier fill (FA1) and internally contain lower and upper shoreface (FA2), proximal upper shoreface (FA3), and tidal channel facies (FA4). Three main architectural elements (barrier island shorefaces, shoreface-dominated inlet fill, and channel-dominated inlet fill) occur independently or in combination to create stacked barrier island deposits. Barrier island shorefaces record progradation, while shoreface-dominated inlet fill records lateral migration, and channel-dominated inlet fill records aggradation within the tidal inlet. Barrier islands are bound by lagoons or estuaries and are distinguished from other shoreface deposits by their internal facies and outcrop geometry, association with backbarrier facies, and position within transgressive successions. Tidal processes, in particular, tidal inlet migration and reworking of the upper shoreface, also distinguish barrier island successions. In sum, this study expands barrier island facies models and provides new recognition criteria to account for the complex geometries of time-transgressive, preserved barrier island deposits.
Highlights
Barrier islands comprise 10% of modern coastlines (Stutz and Pilkey, 2011), making them prominent coastal features
The barrier island deposits presented here are composed of four facies associations, which stack to create three barrier island architectural elements
Shoreface-dominated inlet fill is composed of high-energy, proximal upper shoreface deposits, recording the lateral migration of a barrier island through time
Summary
Barrier islands comprise 10% of modern coastlines (Stutz and Pilkey, 2011), making them prominent coastal features. They are home to growing coastal populations and expanding infrastructure (Zhang and Leatherman, 2011), but are threatened by increasing storm prevalence and magnitude, and rising sea-levels driven by global warming (Leatherman, 1983; Zhang et al, 2002; Moore et al, 2010; Masselink and van Heteren, 2014; Moore and Murray, 2018). There is an increasing drive to understand how barriers and barrier islands respond to forcing agents (e.g., sea-level rise and storms) in the context of local scale controls such as antecedent topography and geology (Cooper et al, 2018; Raff et al, 2018). Despite the strong emphasis on understanding modern barrier island stratigraphy and dynamics, little attention has been paid to barrier island preservation and dynamics over the longer-term geologic scale
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