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The Mesa Rica Sandstone is an extensive fluvial sandstone sheet that is separated from underlying marine strata throughout northeastern New Mexico by a regionally smooth and laterally continuous sequence-bounding unconformity. The Mesa Rica Sandstone consists mostly of channel-fill elements, with lesser proportions of lateral-accretion elements, and almost everywhere is of the same thickness as the largest channel fills. This broad sandstone sheet reflects deposition on a coastal plain by frequent avulsion and minor lateral migration of straight and low-sinuosity streams under nearly stable base-level conditions, which are here attributed to rapid progradation of the adjacent Mesa Rica deltaic shoreline relative to change in relative sea level during maximum Kiowa-Skull Creek regression. Rapid deltaic shoreline progradation under such conditions caused seaward extension and gradient reduction in …