LONGITUDINAL VARIABILITY IN PREVEGETATION FLUVIAL SYSTEMS: MODELS AND EXAMPLES

Prevegetation fluvial deposit differ from their Post-Silurian equivalents in several aspects, mostly due to the effects of plant colonization on catchment areas, banks and floodplains. The two most obvious particularities of prevegetation fluvial deposits are the abundance of stacked, laterally continuous, few meters thick tabular sand bodies, and the scarcity of evidence of meandering channels. Despite that, prevegetation fluvial deposits show a great variability, and most, if not all, of the architectural elements that are found in younger systems have been recorded in pre-Silurian units.

Results of diffusion-based numerical modeling suggest that differences between the longitudinal profiles of vegetated and non-vegetated rivers can explain the different proportions of preserved architectural elements. The effect of vegetation on the stability of banks is directly related to the development of point-bars and to higher shear stresses on channels during floods, due to inhibition of bank erosion, which causes channel shallowing. In diffusion-based models, this leads to higher diffusion coefficient at the lower slope gradient reaches of the system. The combined effect of different diffusion coefficient and reduced sedimentation on floodplains leads to three different predicted prevegetation fluvial styles:

1- High slope gradient rivers with very low proportion of preserved overbank fines, characterized by shallow and broad channels. These deposits encompass most of the modeled successions;

2- Rivers with slope gradients equivalent to those of modern meandering systems, but without point bars or stable banks. These are preserved in relatively small proportions and are characterized by deeper channels, larger bedforms and finner-grained deposits than their upstream counterparts;

3- Very low slope gradient rivers with high proportion of floodplains, in which bank stabilization by mud could lead to meandering channels. These represent a very small proportion of the modeled successions.

Descriptions of Precambrian and Cambrian fluvial successions from Brazil, as well as compilation of published data from other units, confirm the existence of these three different styles and their relative frequencies of occurrence.