Ripples and Levees – A Match Made in Sedimentological Heaven
Levees are positive topographic elements that bound channels, and classical depositional elements that are genetically linked with channelized, deep-marine turbidites. One of the most distinctive attributes of both modern and ancient deep-marine levees is the ubiquitous occurrence of current ripple cross-stratified sand (stone). Puzzlingly then is the comparative rarity of current ripples in the adjacent channel deposits, where at a minimum one would expect them to form during the latter low-energy stages of most/all sediment transport events, assuming at least moderate preservation. Current ripples, or simply ripples are low-energy unidirectional, angular bedforms. Like dunes, ripples become initiated from a bed-surface defect that alters the spatial distribution of sediment transport and deposition on the bed surface, which in turn allows for defect amplification and ultimately bedform growth. The inception of the defect is ultimately controlled by conditions in the very near-bed region. Specifically, a rapid upward decay of density (i.e. sediment concentration) is needed to spatially organize local bedload transport rates that generate the initial bed defects, which are the precursors to angular bedform development. This density stratification condition tends to occur when fluid shear is relatively low compared to particle settling velocity, and apparently coincides with flow conditions commonly developed over the backside of levees. Here, the flows feeding the levees are decoupled from the channelized flow through a variety of overbanking processes (overspill, flow stripping, inertia run-up), and are characterized by three important qualities that promote near-bed density stratification: 1) they are relatively fine and dilute, thereby containing less potential energy, and also allowing particles to easily settle; 2) they are sourced from the lower velocity (upper) regions of the channelized current; and 3) they are unconfined, and therefore prone to accelerated energy loss. These first two conditions indicate that overbank currents lack much of the suspension potential contained in the channelized flow, which becomes further exacerbated by lateral flow spreading due to the absence of confinement. As a consequence there is a propensity to develop exponential density profiles on the backside of levees, which in turn leads to the preferential development of low-energy current ripples compared to their channelized counterparts.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017