Meander-Bend Migration Style Controls Bar Type and Resulting Heterogeneity

Abstract

Many studies have qualitatively identified the various ways in which fluvial meander-bends migrate (e.g., expansion, rotation, translation); however, few have offered quantitative metrics to classify these processes. Migration style imparts a strong control on the resulting bar architecture, including the relative proportion of point bar verses counter-point bar deposits, and ultimately heterogeneity. In order to evaluate the impact of migration style on bar architecture, we analyze time-lapse Landsat images to document 32 years of meander migration on the Mamore River, Bolivia. Mapped channel centerlines provide curvature values, while migration magnitude is measured between successive centerlines. We apply a “bar type index” to mapped channel centrelines to classify each meander bend into zones based on the local relationship between curvature and migration. This metric differentiates between channel segments where a point bar or a counter-point bar will develop and gives an indication of how well-developed the point bar / counter point will be. Individual meander-bends are defined between channel centerline inflection points and the amounts of expansion, translation, and rotation are measured.

Our results quantify the distribution and preservation of bar zones in the context of meander-bend migration style. We find that pure expansion of a meander-bend is rare, and it often takes place in conjunction with rotation. Translation is common and preferentially preserves the downstream zones of bars, which results in a higher proportion of counter-point bar deposits. We compare the results of this study with sediment cores along a series of point bar to counter-point bar transitions on the Peace River, Alberta to provide lithologic context. We suggest that meander-bends that are dominated by translation result in preferential preservation of fine-grained, heterogeneous counter-point bar deposits. Utilizing a simple numerical model of channel meandering, we run a forward model to provide a longer-term evolution of a channel belt and compare quantitative results with the Mamore River dataset. This new approach to analyzing and modeling meander-bend development has significant implications for predicting heterogeneity in analogous fluvial meander-belt reservoirs.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018