Valley Widening and the Composite Nature of Valley Margin Sequence Boundaries: Evidence from the Neilson Wash Compound Valley Fill, Cretaceous Ferron Sandstone, Utah



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Incised valleys are at the center of a debate about whether sequence boundaries are chronostratigraphically significant. Current research has suggested that the sequence boundary that makes up the floor and walls of an incised valley is constantly being modified throughout the entire relative sea-level cycle. This would imply that sequence boundaries are strongly diachronous composite surfaces. New models of incised valley evolution show that incised valleys continue to widen as they are being filled which is in disagreement with existing models of valley evolution. The Cretaceous Ferron Sandstone in Neilson Wash, located west of Hanksville, Utah, contains a well exposed compound valley fill. The Ferron Sandstone in the study area was deposited as part of the Notom Delta that prograded into the Henry Mountains Basin from the southwest. From measured sections and digital photographs a detailed bedding diagram of the fluvial and facies architecture of the fill was completed. The data shows that the outcrop is one of a compound valley system made up of multiple cut and fill episodes. These terraces were formed by a series of relative sea-level fluctuations over a period of ~ 100,000 years. The oldest valley terraces shows tidal influence and are perched high in the system. Younger valley fill is coarser and shows mostly fluvial deposits. This is interpreted as a stepped forced regressive valley system. The 100,000-year duration of this cycle suggests relative sea-level changes are driven by glacioeustacy. Analysis of the oldest valley reveals that it widened as it was filled. Wheeler analysis of the compound valley fill reveals that the sequence boundary flooring the compound valley is a composite surface with terrace deposits preserved above it. The presence of terrace deposits above the sequence boundary is significant. During falling relative sea levels fluvial systems extend out over falling stage deposits along the ramp. These falling stage deposits may be younger than the terrace deposits and should lie below the evolving sequence boundary. This relationship, with older deposits above and younger deposits below the sequence boundary, suggests that sequence boundaries are not chronostratigraphically significant.



Incised, Valley, Sequence stratigraphy, Fluvial systems, Terrace, Ferron, Notom Delta, Compound valley, Glacioeustacy, Point bars, Valley widening, Sequence boundary