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Abstract

Holocene rivers have a high degree of morphological variability, and many researchers see a continuum of channel forms that are transitional between end-member planforms such as braided, meandering, or straight. Individual rivers also show a high degree of longitudinal and vertical (through time) variability, as a result of changes in base level, climate, tectonics, tributary contribution, and/or valley slope. This high degree of variability in plan-view morphology of Holocene rivers is generally not reflected in interpretations of ancient fluvial deposits, which continue to be interpreted as meandering or braided. Many of these interpretations are suspect because of myths about modern rivers and because the characteristics described bear little relationship to plan-view morphology. Modern fluvial facies models are built around the nature and type of channel bars. Knowledge of the architecture of bar forms and their occurrence in Holocene river channels has been greatly enhanced in the past two decades with data gained from ground-penetrating radar (GPR) studies. In spite of our expanded data base for modern rivers and their deposits we still need additional data on width-to-thickness ratios and architecture of deposits of aggrading river systems.

A review of the literature suggests that data on channel bar deposits is lacking in most studies of ancient fluvial deposits. The few research efforts that describe paleochannel bars in 3D and 2D exposures of ancient fluvial deposits suggest that there was considerable variability in paleochannel form within a given formation. Our focus in the study of ancient fluvial deposits should shift from interpreting planforms to description and interpretation of preserved bar forms and how river systems evolved. In this regard many ancient fluvial sheet sandstones need to be re-examined.

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