Floodplain sand sheet

Position: Floodplain

 

Form

Flat, tabular laterally extensive sheets in non-levee settings with massive, often poorly sorted facies. Show little lateral variation in thickness, mean grain size or internal structure. Surface expression generally conforms to the underlying floodplain. Differentiated from splays by their shape, extensive area, and lack of distal thinning.

 

Process description

Associated with rapid sediment charged bedload deposition on the floodplain during extreme flood events. Requires competent overbank flows for bedload materials to be deposited on the floodplain in sheet like forms that cover the entire surface. Deposited as planar, homogenous sequences. Common in sandy ephemeral streams. Often formed downstream of transitions from confined to unconfined flows and associated with a break in slope (as on alluvial fans). Sand sheets build the floodplain vertically.