UNIT MOUTH BARS AND THEIR ROLE IN THE CONSTRUCTION OF RIVER-DOMINATED DELTAS

Unit bars are basic geomorphic elements that scale to the width of individual channel threads. In rivers, braid bars are composite features that form from the amalgamation of smaller-scale unit bars. Flow expansion at the bar head forms an upstream diffluence zone and converges downstream at the confluence. In steady-state flow, river bars migrate downstream or expand and migrate laterally, such as in meandering steams. In delta fronts, unit mouth bars initiate typically in several meters of water and several kilometers offshore, as a response to expansion of the inertial river plume. As this inertia-dominated unit mouth bar builds, it causes the plume to bifurcate and initiates upstream bar growth. In contrast to river bars, there is no downstream confluence zone and the bar front is a zone of net deposition characterized by large-scale avalanche foresets. Mouth bars grow primarily by downstream and upstream accretion, with only minimal lateral expansion. Channels typically become shallower and narrower as the bars grow and ultimately river flow shifts to another part of the delta. In a bifurcation-dominated delta, adjacent terminal distributary channels and their associated mouth bars will coalesce laterally, forming a bar assemblage. The along-strike distribution of bar assemblages may relate to the number of higher-order avulsion-dominated distributary channels, within the upstream delta plain. An early phase of inertial mouth bars builds a subaqueous sandy platform that produces significant bed and form friction. As the delta continues to build, friction-dominated mouth bars may overlie this shallow platform. Internally, inner mouth bars are built by low amplitude bed waves or dunes, depending on the bedform phase parameters (e.g. grain size, velocity and flow depths). Friction-dominated bars typically show a greater proportion of traction deposits (e.g. ripples, dunes and bedwaves). Inertial bar fronts are typically built by grain flows, although these may be reworked by waves or tides and may pass distally into hummocky cross-stratified or wave rippled sandstone beds. Friction-dominated bar fronts may show a greater proportion of downstream accreting dunes or other traction-type bedforms. The frictional bars may also show greater reworking by fair-weather waves and tides, depending on the overall ratio of marine versus fluvial energy.