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Supply Dominated Fluvial Sequences: Implications for Sequence Stratigraphy in Continental Settings


Methods of sequence stratigraphy rely on the location of shoreline position; however, fluvial architecture is dominated by sediment supply in noncontiguous updip regions. We present a model of nonmarine stratigraphy that illustrates fluvial spillover as the primary updip control on alluvial architecture. According to our model, underfilled basins initially undergo river aggradation, followed by spillover across low-relief topographic divides and into downdip basins. The transition to overfilled conditions promotes updip-propagating erosional surfaces and downdip integration through cascading spillovers. Fluvial spillover creates extensive erosion between basins that lead to the formation of headward-migrating sequence boundaries. We describe a well-studied example from the Rio Grande rift of North America that highlights fluvial spillover in an extensional setting. In this system, a stepwise pattern of fluvial spillover was responsible for more than 2500 km of lengthening into the Gulf of Mexico, which occurred from 18 Ma to 0.6 Ma. The resulting patterns of fluvial accumulation controlled reservoir distributions, where thicker updip fluvial deposits are more deformed relative to thinner and younger downdip successions. We consider fluvial spillover to be an underappreciated mode of drainage integration that relates basin geometry and orogenic proximity with rates of regional subsidence, discharge and climatic variability. Thus, we conclude that erosional and nonmarine components of sequence boundaries are indicative of sediment bypass, rather than to relative sea-level lowstands.