Characterizing ancient avulsion stratigraphy and its significance.

摘要

The research presented in this dissertation was designed with the purpose of expanding and improving upon current field recognition and characterization criteria used for avulsion stratigraphy in order to further our knowledge of how this process behaves over basin-filling timescales. Avulsion, the process of a river leaving its established course in favor of a new location on the adjacent floodplain, is rarely observed on human timescales. However, the ancient rock record provides a rich database of ancient river avulsions and this offers insights into avulsion behavior over basin-filling timescales, resultant alluvial architecture or stacking arrangement of channel sand bodies, and this in turn aids in characterizing fluvial reservoir connectivity.; The integral portion of this study examines six ancient avulsion-dominated fluvial successions in the Rocky Mountain region and compares them to fourteen modern river analogs. Two categories of avulsion stratigraphy, transitional and stratigraphically abrupt, are defined based upon criteria that can be determined in either field outcrops, core descriptions or gamma-ray logs. The results suggest that autogenic (or intrinsic) controls, such as levee morphology, grain size distribution, and propensity for crevasse splay activity (all closely interrelated), play a larger role in the style of avulsion preserved in the rock record than the more commonly cited allogenic (or extrinsic) controls, such as tectonics and climate. The results also suggest that a particular style of avulsion dominates a formation throughout a particular basin. This particular conclusion allows one to characterize avulsion stratigraphy within a basin, or reservoir, and predict sand body connectivity.