The global distribution of large subaerial distributary fluvial systems

Abstract

Large subaerial distributary fluvial systems are cone-shaped, alluvial deposits that are ubiquitous features on Earth. These non-marine, non-lacustrine deposits have superficial similarities to small alluvial fans but are at a scale that is orders of magnitude larger. A variety of terms have been used to identify these features, including alluvial cone, inland delta, wet alluvial fan, humid alluvial fan, alluvial fan, subaerial fan, terminal fan, and megafan. The first systematic examination of these features was presented by Wilkinson et al. (1997). This thesis attempts to provide a more detailed characterization and classification of these features. Fourteen systems in South America, Africa, and Asia were examined using Shuttle photography and AVHRR imagery. This study included: (1) maps of each system and their global distribution, (2) morphometric analyses, (3) classifications by climate, geology, and hydrology, and (4) a comparison to other fluvial systems. These systems have areas 8,881 to 239,583 km², radii 130 to 806 km, and slopes 0.00006 to 0.00079. Active and former river channel patterns on these features suggest that they formed by the action of a single river avulsing across the feature. The area of these features is unrelated to the area of the upstream drainage basin, but is related to the size of the downstream basin. This sample of systems occurs in a variety of climates and in two different geologic settings - foreland basins and half graben. The rivers on all systems are orientated transverse to the trend of the adjacent mountain range or fault. A variety of channel patterns are found on these systems. Considering the definitions of alluvial fans, rivers, and deltas, these systems are best classified as a specific variety of alluvial fan. A new alluvial fan classification scheme more inclusive than that proposed by Stanistreet and McCarthy (1993) is needed. The adoption of a common term to describe these systems - such as megafan - would help to eliminate some of the definitional ambiguities between them and other alluvial fans. Knowledge of these features will enhance our ability to interpret analogous features in the rock record, including important petroleum and water aquifers (e.g., the Ogallala Formation).