Do River Channels Decrease in Width Downstream on Distributive Fluvial Systems? An Evaluation of Modern Megafans of the Chaco Plain, Andean Foreland Basin, South America

Abstract

Distributive Fluvial Systems (DFS) form the dominant geomorphic fluvial elements in modern continental sedimentary basins, thus we expect DFS deposits to comprise much of the fluvial rock record. Therefore, understanding geometries and distributions of channels and floodplains on modern DFS may provide valuable information on expected patterns of sediments in fluvial rocks. In our previous work, we hypothesized that rivers on DFS commonly decrease in size downfan. We are testing this hypothesis through evaluation of LANDSAT images of large DFS. In this preliminary study, we report on channel width for DFS in the Chaco Plain of the Andean Foreland Basin. We use ArcGIS to: (1) isolate regions that correspond to the active channel (e.g., exposed sand and water), (2) divide the active channel into equal segments of ~1000 m length, and (3) determine the area covered by the active channel in each segment. The mean active channel width is determined by dividing this area by the 1000m length for each stream segment. This work results in a graph that shows the change in channel width downfan from the apex every 1000m for each river system. For most rivers on DFS in the Chaco Plain, active channel width decreases downfan, thus supporting the hypothesis for this region. Several of the rivers (e.g., Rio Grande, Parapeti, and Pilcomayo) diminish in width to the point of termination in wetlands distal on the megafan. Though this method only shows widths of active channels and does not completely capture the channel-belt width, we expect the channel-belt width to remain relatively proportional to the width of the active channel. Thus, these results indicate that channel-belt sandstones should diminish in width distally in DFS deposits.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015