QUANTIFICATION OF LATERAL ACCRETION VERSES DOWNSTREAM TRANSLATION OF FLUVIAL MEANDER BENDS

Many studies have attempted to quantify geometric characteristics of meander bends (e.g., radius of curvature); however, geomorphological analyses do not generally distinguish the internal architecture of fluvial point bars. Although channel migration history and resultant scroll-bar patterns are used to qualitatively classify accretion style, a metric for quantifying lateral point bar accretion verses downstream translation is lacking. Such a metric is important, as recent studies have demonstrated that differing accretion styles are linked to varied processes, and ultimately, unique depositional products. We propose a new metric (θ) to quantify the degree of lateral accretion verses downstream translation, which considers the location of maximum curvature (bend apex) on the inner bend of a channel (C_max), in comparison with the vector accretion direction measured from accretion patterns. We perform field measurements of the Red Deer and South Saskatchewan rivers of southern Alberta, Canada, including grain size, channel geometry, sinuosity, bend apex and accretions patterns; we then determine the relationship between accretion style and sediment distribution.

Geometric results of the analysis are presented as a degree difference between the azimuth of a vector normal to the tangent of the inner bend apex (n), and the azimuth of a vector representation of the accretion pattern trajectory (t). Therefore, a small or negligible difference between the two azimuths indicates lateral accretion, where the accretion patterns are congruent with the inner channel bend apex. Large differences indicate downstream accretion, where the scroll pattern is oblique or perpendicular to the inner channel bend apex. The combination of these results with sediment distribution mapping around meander bends provides a predictive model for point bar sedimentation based on accretion direction. Downstream translating meander bends preferentially preserve deposits downstream of the bend apex (such as counter point bar), which are typically finer-grained than adjacent bar head deposits. These observations have significant implications for the characterization and interpretation of the depositional record, which we consider in Cretaceous-aged meander-belt strata of the McMurray Formation, Alberta.