Abstract: Allocyclic Controls on Deposition of Continental Sediments: A Matter of Time

Keith W. Shanley, Pete McCabe

Studies of ancient continental successions suggest that sequence boundaries reflecting changes in stratigraphic base level are characterized by abrupt changes in alluvial architecture and changes in soil profile development. Over time periods ranging from five hundred thousand to several million years, changes in basin subsidence rates and the erosion of orogenic fronts coupled with changes in eustasy cause widespread changes in the architecture of continental facies. Over time periods of a few hundreds of thousands of years or less the importance of tectonic processes is diminished and changes in relative sea level and lake levels assume greater importance.

Studies in Pleistocene and Holocene continental deposits offer a different perspective. Temporal resolution in these studies is typically measured in hundreds to thousands of years and they stress the importance of autocyclic processes or very high frequency allocylic processes, such as short-term changes in climate, to explain changes in alluvial architecture, development of soil horizons, and the preservation of eolian strata. During a rise in stratigraphic

base level, regional erosion surfaces and changes in style of alluvial deposits can result from changes in sediment supply related to high-frequency climatic changes. The concept of graded equilibrium profiles is not valid over these short time periods.

Understanding the relationship between the sedimentary architecture of continental strata and autocyclic/allocyclic controls at varying temporal scales is important for prediction of sedimentary architecture within basins for petroleum exploration and production. Sequence boundaries that reflect changes in stratigraphic base level will normally be related to major changes in the architecture of the continental and associated marine strata. By contrast, regional erosional surfaces that reflect short-term changes in sediment supply will not be related to significant changes in the architecture of strata.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994