ABSTRACT: On the Origin and Subsequent Transgressive Modification of Incised Valleys

ALLEN, GEORGE P., Total Exploration Laboratory, Saint-Remy-les-Chevreuse, France, and HENRY W. POSAMENTIER, ARCO Oil and Gas Company, Plano, TX

The origin and evolution of incised valleys can be highly complex, occurring as a result of changes either in the upstream or downstream reaches of river systems. The primary downstream factor is base-level or relative sea-level fall at the river mouth; the primary upstream factors are tectonic uplift and possibly increased fluvial discharge and decreased sediment flux. An important distinction between upstream and downstream factors is that the effect of upstream factors approaches zero at the river mouths; in contrast, the effect of the downstream factor of base-level change diminishes in the upstream direction. The extent to which river systems incise owing to base-level fall depends on the gradient of surfaces exposed by the relative sea-level fall. In general, the greater the gra ient, the greater the depth of incision.

Incised valleys can be modified substantially by tidal processes during transgression. Many incised valleys can be filled by transgressive and regressive estuarine deposits directly overlying the incised valley floor. The absence of fluvial deposits on the incised valley floor suggests that tidal processes either have enhanced the erosion of the substrate begun earlier by fluvial processes or have been responsible for the incision. When transgression slows, these tidal erosive processes may remain focused in one area long enough to locally substantially deepen incised valleys. The extent of this deepening is directly proportional to wave energy as well as the duration of the stillstand that punctuates the transgression. This results in anomalous "holes" in the longitudinal profile of he valley floor.

Subsurface examples from the upper Albian Viking Formation, south-central Alberta, Canada, illustrate the irregular nature of incised valley longitudinal profiles as well as the occurrence of estuarine deposits immediately overlying the valley floor. Modern and Holocene examples of holes at estuary mouths support this concept of enhanced incised valley modification during stillstands that punctuate transgressive events.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)