COMPLEX FLUVIAL SYSTEMS PRODUCE COMPLEX SEQUENCES: INSIGHTS FROM MODERN FLOODPLAINS AND IMPLICATIONS FOR TERRESTRIAL PALEOENVIRONMENTAL RECONSTRUCTION

Inference of the depositional environment of ancient fluvial systems presently relies in large part on the simplifying assumption that tropical settings are characterized by channelized flow within river banks stabilized by dense, seasonally-invariant vegetation whereas arid conditions give rise to unchannelized flow as a result of poorly stabilized channel banks. Nevertheless, this assumption belies significant complexity observed in a number of carefully studied modern systems–complexity that should be appropriately incorporated into models of ancient fluvial deposition.

At 150,000 km^2, the Pantanal of west-central Brazil is today’s largest tropical wetland. Despite the dense flora of the Pantanal, this active sedimentary basin is characterized by a dynamic landscape of regular channel unconfinement, sedimentation-induced avulsion, and annual flood pulses driven by seasonal rainfall. Indeed, floodplain-wide unchannelized flow occurs seasonally in wet periods, while in dry seasons, water only flows in river channels. The sediment input in combination with the extreme seasonality of flooding in the Pantanal has produced an annually dynamic, and decadally evolving, fluvial setting.

Here we analyze a multi-decade series of low-orbit satellite images of the Pantanal in order to both estimate the time-scale of stream and channel modification and to model the patterns of deposition for comparison with ancient depositional sequences which may reflect similar conditions. We find that a simple equivalence between evidence of ancient unchannelized flow and an extra-tropical depositional setting is untenable. A series of additional and critical factors must also be considered: these include seasonality of precipitation, concordant seasonal turnover of sediment-stabilizing flora, and the frequency of channel avulsion as a function of the balance between accommodation space and sediment input of the fluvial system. Accounting for this extended picture of fluvial dynamism allows for fresh interpretation of the terrestrial geologic record during critical intervals when the subtle signals of shifting climate and increasing seasonality might otherwise be difficult to detect.