Abstract: Cyclic Variables Controlling Fluvial Sequence Development: Problems and Perspectives

Frank G. Ethridge, Lesli J. Wood, Stanley A. Schumm

The sequence stratigraphic model, with its initial emphasis on eustatically driven controls on sedimentary sequences, has generated considerable interest in the ultimate controls on alluvial successions. Arguments regarding controls on alluvial sequences have been ongoing for many decades. Proposed controls include global (eustatic) or local base-level fluctuation, climate, tectonics, and sediment supply. Since sediment supply is in general a function of one or more of the other three controls, the possible number can be reduced to three.

Most models and explanations for fluvial successions are too simplistic. They attempt to explain these successions on the basis of a single controlling factor. Additionally, the models commonly fail to take into account modern geomorphic concepts of thresholds and complex response, and ignore the fact that controls other than base-level fall can produce incision and that base-level lowering does not always result in incision and rejuvenation of a fluvial system. They also fail to realize that base-level fluctuations may have their greatest effect only in the lower reaches of a fluvial system and that the amounts of sediment produced by incision alone cannot account for the volume of sediment observed in stratigraphic sequences.

Tectonic activity, climate change and base-level change have all been identified as responsible factors for the development of ancient fluvial sequences which exhibit a similarity of characteristics. A recent series of experiments in large-scale flumes reaffirms the difficulty of distinguishing the various controls on fluvial deposits by demonstrating that similar type valley geometries and valley-fill successions can result from different external controls.

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