AN EVALUATION OF RECOGNITION CRITERIA FOR ANCIENT BRAIDED VS. MEANDERING STREAMS

Many approaches have been used to identify river pattern in ancient fluvial deposits. In order to evaluate several of these approaches we have collected data from modern meandering and braided rivers, including the Niobrara and North Loup rivers, Nebraska, braided streams, and the Calamus River, Nebraska and Mississippi River (south of Cairo, Illinois), meandering streams. Data collected includes bedform size and migration direction, grain-size distributions and mudstone sequestering, channel aspect ratio, bar geometries and migration extent, and the distribution and geometry of channel shapes.

These data suggest that the recognition of channel pattern in ancient deposits is equivocal, at best, based on spread of paleocurrent indicators, bar attachment geometry and overall grain-size distributions. In addition, both braided and meandering streams may easily produce sand bodies with large (>50:1) channel body aspect ratios. A far better basis for discrimination is the recognition of free bars vs. forced bars. Free bars, such as alternate bars and mid-channel bars, are those that form due to instabilities in the flow field. These features often migrate quickly with respect to channel position, can locally influence the flow field forcing other bars to deposit or get eroded, and individual bars may not migrate very far downstream before they vanish due to along length changes in the flow field. Forced bars, on the other hand, are those that are forced to deposit due to changes in the geometry of the river planform, such as channel curvature, widening and divergence. As such their position does not change until the channel geometry changes. As a result, forced bars, including point bars, tend to be more long-lived features than free bars. Bar types can be recognized in the ancient by the distance of bar migration, scaled by bar clinoform width, before termination of the barform by either other bars or by channel abandonment. While all types of bars can coexist in either type of stream pattern, dominance of free bars strongly suggests braided streams and forced bars signify meandering stream pattern.