COMPLEX FLUVIAL RESPONSE TO LOW GRADIENTS AT MAXIMUM REGRESSION - A GENETIC LINK BETWEEN SMOOTH SEQUENCE-BOUNDARY MORPHOLOGY AND ARCHITECTURE OF OVERLYING SHEET SANDSTONE

The Mesa Rica Sandstone is an extensive fluvial sandstone sheet that i s separated from underlying marine strata throughout northeastern New Mexico by a regionally smooth and laterally continuous sequence-houndi ng unconformity. The Mesa Rica Sandstone consists mostly of channel-fi ll elements, with lesser proportions of lateral-accretion elements, an d almost everywhere is of the same thickness as the largest channel fi lls, 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 delta ic shoreline relative to change in relative sea level during maximum K iowa-Skull Creek regression, Rapid deltaic shoreline progradation unde r such conditions caused seaward extension and gradient reduction in t he adjacent Mesa Rica coastal plain. Overextension and resultant loss of power in Mesa Rica rivers crossing this coastal plain forced both s tream straightening and storage of nontransportable coarser sediment i n coastal-plain channels, Because base level was stable, Mesa Rica cha nnels could aggrade only minimally, and were forced to spread excess s ediments mostly laterally by avulsion. Frequent avulsion and minor lat eral migration of Mesa Rica channels under these conditions caused bot h regional scouring of a smooth sequence-bounding unconformity and dep osition of a sheet sandstone with minimal channel stacking. The Mesa R ica Sandstone and the sequence boundary that underlies it represent a distinctive genetic association of single story fluvial sheet sandston e and smooth sequence-boundary morphology that is probably not unique to the Lower Cretaceous of northeastern New Mexico, Such unincised seq uence boundaries are also implied in conceptual models and flume exper iments of previous authors.