Stepped Forced Regressive Valleys and Sequence Diachroneity
Recent flume models and Quaternary studies show that fluvially-eroded sequence boundaries may be highly diachronous and may not have the chronostratigraphic significance that has long been assumed. Early sequence stratigraphic models assumed that valleys were entirely excavated during sea-level falls and entirely filled during subsequent rises, such that valley-fill deposits were everywhere younger than the falling-stage deposits that they may overlie.
Cretaceous outcrop examples of compound incised valleys, including the Ferron Notom delta in Utah and the Mesa Rica sandstone in Colorado, show evidence of stepped, punctuated forced regressions. Cross-cutting valley fills are present in both formations indicating multiple valley scour events during regression. The Ferron also has older fluvial terraces that show greater tidal influence than younger valleys fills, which are more fluvial-dominated. Tidal influence in the valleys suggests that they are primarily controlled by downstream "buttress"shifts (i.e. sea-level). The vertical succession also suggests that valley fills record a change in the plan-view position of the valley system that is preserved in any given section. The succession records a progressive transition from estuarine-dominated outer- segments to fluvial-dominated inner- segments through time. This also suggests that valleys were formed by progressive sea-level fall, causing more distal valley segments to be replaced by proximal segments as the valley systems evolves.
As valleys extend over co-eval forced regressive shorelines, the resulting erosional discontinuity may have older earlier-formed fluvial terrace deposits above the surface in more proximal regions, and the same erosional surface may overlie younger, more recently formed deltaic and shoreface deposits in more distal regions.
Such stepped forced regressions are indicative of gradual, stepped base-level fall, such is commonly recorded in glacio-eustatic cycles, where glacial build up is slow. These outcrop examples also match observations seen in flume and Quaternary studies, and suggest that sequence boundaries may have far greater diachroneity than has been previously assumed.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California