Paleo-Valley Depths: Comparison of Pennsylvanian Icehouse and Cretaceous Greenhouse Examples

Stephen Flint¹ and A. Guy Plint^2
1 University of Liverpool, Liverpool, United Kingdom
² Department of Earth Sciences, University of Western Ontario, London, ON

The depth of valley incision at sequence boundaries is a function of several controlling factors including magnitude of relative sea level fall, gradient of coastal plain/shelf and nature of substrate. Valley incision depth also varies longitudinally and may increase at confluences. Despite these complex and partly inter-related controls, studies of Upper Cretaceous valley systems in the Western Interior Basin of North America by many workers have shown that valleys within 4th order sequences (duration of approximately 0.1 to 0.5 My) exhibit remarkably similar depths of incision, averaging approximately 20 meters. This figure applies to valley systems incised into shoreface and coal-bearing coastal plain deposits from New Mexico and Arizona, through Utah and Wyoming, into western Canada. Similar datasets exist for paleo-valleys of Pennsylvanian age across Northwest Europe and in the Appalachian Basin of eastern North America. Valleys incised into delta mouth bar and coal-bearing coastal plain strata within 4th order sequences show average depths of incision of 30-40 m. We argue that this systematic difference in incision depth between otherwise similar valley systems of similar duration is a function of icehouse versus greenhouse climate. The frequency of the sea level cycles is similar (possibly the eccentricity component of the Milankovitch band) but in icehouse times the absolute amplitude and rate of eustatic change was greater (resulting in greater incision depths) than during the warmer Cretaceous greenhouse period. One exploration implication is that the deeper icehouse valleys are more likely to be imaged in conventional seismic data.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005