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Abstract

As rooted vascular plants were not a significant factor in controlling bank stability or surface runoff before the late Silurian, it is not surprising that many modern humid and temperate climate river models are not represented in the Precambrian or early Paleozoic sedimentary record. Many pre-vegetation systems have features that are more closely allied to modern ephemeral and dryland systems, although direct comparison remains elusive. To date, wandering gravel-bed rivers, low-sinuosity sandy braided systems with alternate bars, and fine-grained meandering and anabranching systems have not been positively identified. Precambrian gravel-bed braided systems are common, and have architecture similar to that of modern systems, except that scour hollows are typically absent or difficult to identify. Point-bars in cobble-grade gravel-bed meandering systems are wider and have lower inclination than modern systems. Sandy meandering systems can be identified using the directional relations between foreset orientation and point-bar inclination. Sandy braided systems are dominated by composite barforms with predominantly downstream accretionary elements. Sandy ephemeral channelized upper-flow-regime elements and unconfined sheetflood deposits are common, but they lack many of the associated fine-grained components seen in modern systems.

Most known pre-vegetation fluvial systems were preserved within rifts, along continental margins, or in foreland-basin settings. Nearly all known Precambrian and early Paleozoic river deposits are dominated by channel and sheet-channel facies, and have high lateral continuity and high net-to-gross ratios. Thick overbank and interfluvial deposits are underrepresented, or where identified, are suspect and may represent channel abandonment. Direct comparison with younger systems requires detailed architectural analysis, with specific attention to the relative inclination and directional variability of foresets and first-order to sixth-order surfaces. Unfortunately many studies of modern systems lack this attention to detail, especially of the small-scale morphology of submerged in-channel features, hence direct evaluation of the paleohydraulic characteristics, sinuosity, and slope of older systems remains enigmatic.

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