NEW INSIGHTS INTO POINT-BAR ALLUVIAL ARCHITECTURE ACROSS THE FLUVIAL-TIDAL TRANSITION: LOWER CHEHALIS RIVER, WA, USA (Invited Presentation)

When interpreting ancient tidally-influenced fluvial zones (TIFZs), it is crucial to identify the sedimentological products of purely tidal- and fluvial- influenced estuarine/deltaic barforms. However, unambiguously discriminating unique bar-scale tidal- and fully- fluvial signals in the rock record has proven difficult. This challenge stems from the inability to quantify the longitudinally varying (upstream to downstream) ratio of fluvial/tidal energy in ancient estuarine/deltaic systems, where the initial and boundary conditions are not well-constrained. This drives the necessity for investigations of variations in alluvial architectural of modern barforms across the fluvial-tidal transition in systems where the fluvial/tidal energy ratio can be directly quantified, or more easily inferred. These modern studies can then be used as analogues to ancient systems, and help reveal the fundamental sediment transport/depositional processes operating within the TIFZ. This paper presents the findings from a Ground Penetrating Radar (GPR) and Parametric Echosounder (PES) dataset collected from point-bar deposits across the TIFZ of the single-threaded meso-tidally influenced lower Chehalis River, WA, USA.

These results reveal point-bars within the fluvially-dominated, tidally-influenced to mixed tidal-fluvial hydraulic regimes, which undergo relatively rapid rates of migration (> 1.0 myr^-1), are governed by downstream-oriented currents that produce an internal alluvial architecture consisting of vertically- and laterally- continuous accretion-sets similar to fluvial point-bars. Alternatively, within the downstream tidally-dominated, fluvially-influenced hydraulic regime, where meander bend migration rates approach < 1.0 myr^-1, accretion-sets are discontinuous, or chaotic, and do not maintain vertical- and lateral- continuity. This type of bar architecture is the product of infrequently deposited accretion-sets exposed to tidal-current reworking for long time intervals due to slow bend migration rates. From these findings, a new fluvial-tidal point-bar depositional model is proposed, which takes into account variations in: i) hydraulics, ii) meander bend migration rates, and iii) grain size, across the TIFZ.