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 the adjacent Mesa Rica coastal plain. Overextension and resultant loss of power in Mesa Rica rivers crossing this coastal plain forced both stream straightening and storage of nontransportable coarser sediment in coastal-plain channels. Because base level was stable, Mesa Rica channels could aggrade only minimally, and were forced to spread excess sediments mostly laterally by avulsion. Frequent avulsion and minor lateral migration of Mesa Rica channels under these conditions caused both regional scouring of a smooth sequence-bounding unconformity and deposition of a sheet sandstone with minimal channel stacking. The Mesa Rica Sandstone and the sequence boundary that underlies it represent a distinctive genetic association of single-story fluvial sheet sandstone and smooth sequence-boundary morphology that is probably not unique to the Lower Cretaceous of northeastern New Mexico. Such unincised sequence boundaries are also implied in conceptual models and flume experiments of previous authors.