The accumulation and preservation of fluvial and aeolian strata
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Sedimentary rocks are a record of the four-dimensional evolution of a planetary surface. Geomorphic features, including fluvial and aeolian dunes, river bars, and entire river channels move across Earth’s surface. As Earth’s surface evolves with this movement, sediment may accumulate in one location or be removed from another. In this way, through the unsteady processes of deposition and erosion, geomorphology is translated into stratigraphy, and time is lithified. Learning to read this record of Earth’s surface evolution is a fundamental goal of sedimentology. This dissertation covers three contributions to this effort. Though the chapters cover different types of sedimentary systems (fluvial and aeolian) at different scales (outcrop and seismic imaging), the fundamental driving questions are largely the same: which elements of the ancient sedimentary systems have been preserved? How is time represented? How do internal processes and external forcings control what makes it into the rock record? Chapter 2 is an examination of exhumed fluvial channel belts of the Cretaceous Cedar Mountain Formation, Utah, USA. Analysis of outcrop mapping and vertical and lateral sections confirm the preservation of complete barforms, river bed topography, and the formative channel-belt centerline. These features are representative of an aggradational fluvial system avulsing at rapid rates compared to lateral migration rates. Additionally, the preserved strata were constructed rapidly, indicating most of the time recorded in the channel belts is flattened onto erosional surfaces at the bases of each channel belt. Chapter 3 looks at fluvial channel belt preservation at the km scale, using seismic reflection volumes of the subsurface Gulf of Mexico offshore of the Brazos river delta, Texas, USA. Channel belts of two distinct preservation qualities are observed, and their stratigraphic arrangement confirms that a recently discovered source of relief in coastal zones aids in the preservation of channel belts. Chapter 4 discusses an outcrop study of the aeolian Page Sandstone, Arizona, USA. The Page is the record of multiple stacked, generally bypassing dune fields, only preserved because of a fluctuating water table. The lack of dune field aggradation is attributed to a lack of external processes forcing deposition. An exception is found within antecedent topographic depressions, which have preserved climbing cross-sets built by small dunes during the earliest phases of dune field development. Such deposits are absent from the rest of the Page, as they are inevitably reworked in the absence of available antecedent topography.