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Title:The sedimentology of bedforms to barforms within tidally-influenced fluvial zones (TIFZx): lower Columbia River, OR/WA, USA, and the lower Chehalis River, WA, USA
Author(s):Prokocki, Eric Waschle
Director of Research:Best, James L.
Doctoral Committee Chair(s):Best, James L.
Doctoral Committee Member(s):Parker, Gary; Anders, Alison; Dalrymple, Robert; Feldman, Howard
Department / Program:Geology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:The tidally-influenced fluvial zone (TIFZ), or ‘fluvial-tidal’ transition, is an environment characterized by unidirectional fluvial, bidirectional tidal, and oceanic oscillatory currents. TIFZ environments (i.e., deltas to estuaries) occur everywhere rivers meet ocean basins, and thus are important to understand from a geomorphological, morphodynamic and sedimentological perspective. However, most of the few existing studies of modern TIFZs have investigated a single region within a TIFZ, or relied heavily upon sediment cores and/or trenches. A large percentage of current knowledge of TIFZs thus comes from ancient outcrop studies, where the boundary conditions of the environments being studied are unknown. Therefore, most sedimentological models of TIFZs are built upon interpretations and qualitative discussions, and not via direct observations. There is thus a need to systematically investigate modern TIFZs using geophysical data collection techniques beyond sediment cores and/or trenches, in order to refine current sedimentological models with observations made within the context of known boundary conditions. The aim of this research is therefore to integrate a number of geophysical datasets collected within two modern TIFZ environments: (a) the single-threaded lower Chehalis River, Washington, USA, and (b) the multi-threaded lower Columbia River (LCR), Oregon/Washington, USA, in order to characterize the morphology, sedimentology, and internal sedimentary architecture of the bedforms and barforms in these systems. These results will be used to further the current understanding, and sedimentological models, of single- and multi- threaded TIFZs. New data are presented defining the large-scale TIFZ hydraulic environments along the lower Chehalis River using Power Spectral Density (PSD) and wavelet transform analyses of water elevation time series data. These findings were used to provide a large-scale hydraulic context for sedimentological results/interpretations gained from (a) Ground Penetrating Radar (GPR), (b) Parametric echosounding (PES) sub-bottom profiling, (c) shallow (≤ 5m in depth) vibracores, and (d) time series of aerial imagery of point-bar deposits across the lower Chehalis River TIFZ. These results show that point-bars located within the fully-fluvial to mixed tidal-fluvial regimes undergo relatively rapid laterally-oriented migration (> 1.5 myr-1), and possess an internal alluvial architecture consisting of lateral-accretion packages composed of continuous vertically-stacked sets of parallel strata. However, point-bars located within the tidally-dominated, fluvially-influenced regime experience relatively slow laterally-oriented migration (< 1.5 myr-1), and possess an internal alluvial architecture consisting of lateral-accretion packages composed of discontinuous- or chaotically- bedded vertically-stacked sets of strata. These vertically-stacked discontinuous strata are the product of bidirectional tidal-current reworking of initially deposited parallel strata. The large-scale geomorphic/morphodynamic evolution of barforms within the multi-threaded LCR TIFZ from the mid-Holocene to present was investigated via shallow vibracores, deep sediment cores (≥ 10m depth), and optically stimulated luminescence (OSL) dating. This study demonstrates that the large-scale geomorphic state of the mid-Holocene to present-day LCR TIFZ is that of an ‘entrenched’ tidally-modified fluvial-deltaic environment, and not a pure estuarine basin or a late-Holocene ‘bay-head delta’ prograding into an unfilled estuary. This new LCR TIFZ geomorphic/morphodynamic model was then utilized as the context to examine the morphological variations of dunes within the LCR TIFZ thalweg, and the sedimentology and internal alluvial architecture of mid-channel barforms. Analysis of National Oceanic and Atmospheric Administration (NOAA) multibeam echo sounding (MBES) surveys of the main LCR navigation channel bed, reveals that in the mixed tidal-fluvial regime, dune morphology drastically changes from tidally-reversing 2-2.5D bedforms possessing heights < 1m, to 3D dunes that are downstream-migrating with heights between ~ 2-3m. This change in dune morphology marks the upstream-most point of intense flood tidal-current (landward-oriented) bedload transport, and the switch to strong downstream-oriented bedload transport. Lastly, the sedimentology/internal alluvial architecture of LCR TIFZ mid-channel bars were investigated using GPR, PES and coring. These results show that LCR mid-channel bars formed within the TIFZ fluvially-dominated, tidally-influenced hydraulic regime, possess an internal alluvial architecture and sedimentology indistinguishable from that of fully-fluvial braid-bars. Here, bars are mainly constructed from vertically-stacked (~ 10m thick) dune to unit-bar scale trough-cross and cross-bed sets. Thus, tidal hydraulic processes such as twice daily bar submergence/emergence, and ‘slackwater’ intervals, play no pivotal role in modifying the small- to large- scale sedimentology of these bars. In addition, within the LCR TIFZ mixed tidal-fluvial hydraulic regime, the internal alluvial architecture of mid-channel bars transition to bar cores, which are ~ 2-3m thick, consisting of vertically-stacked trough- and planar- cross-sets overlain by low-angle dipping (generally < 5°) longitudinally- and laterally- oriented accretion sets up to 5m in thickness. These low-angle dipping accretion-sets are interpreted as the product of the reworking of stacked dune to unit-bar scale cross-sets into stacked-beds of ripple-scale cross-laminae via the combined effects of flood-tidal currents and intrabasinal wind-waves. Vibracores taken from two mixed tidal-fluvial mid-channel bars have vertical grain size trends displaying coarsening-upwards sequences towards the bar-tops, where the bar-top medium sands become very-well sorted. This coarsening-upwards in bars in the mixed tidal-fluvial regime is reasoned the product of the winnowing of clays to fine sands via tidal-currents in conjunction with intrabasinal wind-waves, which cause the preferential preservation of coarser sediments. Importantly, these results illuminate the importance of intrabasinal wind-waves in playing a major role in the sedimentological products of the TIFZ, which is typically ignored in previous models of the fluvial-tidal transition.
Issue Date:2017-07-14
Rights Information:Copyright 2017 Eric Prokocki
Date Available in IDEALS:2018-03-02
Date Deposited:2017-08

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