Funny Things Meanders Do: A Summary of the Diversity of Meander Processes and Morphology and Implications for Reservoir Geometry and Quality within Channel Belts

Holbrook, John

The generation of point bars by lateral accretion during lateral expansion and downstream translation of river meanders are well described processes, and a primary means for generation of sandy bar reservoirs within fluvial strata. More recently, the concept of "counter" point bars has been stressed to explain meanders with concave growth ridges that tend to form mud-rich bar systems with low reservoir quality. Detailed mapping within the Missouri River system reveals a wide diversity of meander style. These styles include translation, expansion, and counter point bars, and three additional variations: wavering meanders, spin-out meanders, and meander pile ups. Wavering meanders superficially resemble point-bar meanders but did not grow by lateral accretion. Instead, these form by repeated accretion of mid-channel bars to the channel bank in otherwise braided reaches. The result is an amalgamated set of small fusiform bars that collectively form a larger arced meander form. Since the mid-channel bars can accrete to either the outer or inner bank, the meander can grow in both outward and inward directions, and may thus "waver." The resulting reservoir is an amalgamated set of smaller channel bars separated by shallow channel fills with excellent reservoir connectivity and quality within and between these meander lobe deposits. Also within the braided section occur highly contorted "spin-out" meanders. These occur locally and randomly in both time and space and record short sections of braided river that are temporarily compressed into single-thread channels. The energy within the system exceeds the stability of sustained meandering. The loop is characterized by rapid point-bar accretion that generates complexly compound forms. After this brief "spin out", the meander is quickly cut off, and the river returns to its normal braided pattern. These meanders form large high-quality sand-bar reservoirs with unusual shapes, and well-defined bounding channel fills. Meander pile-ups occur where tips of normal expanding and/or translating meanders encounter bedrock constrictions. Here, bedload transport is locally concentrated. Sedimentation forces accelerated bar growth in the area around the constriction. Likewise, downstream bar translation is stopped. Meanders "pile up" on the floodplain preserving multiple and abundant crosscutting meanders with hair-pin form. Reservoirs are sandy but narrow and long and dissected by channel fills.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013