UTILITY OR FUTILITY? ASSESSING THE VALIDITY OF PALEOCURRENT ANALYSIS IN THE AMAZONIAN ROCK RECORD

A key factor in the evolution of the modern Amazon ecosystem, and one which is currently debated, is the character of the pre-Amazonian river system during the Paleogene and Neogene. It has been suggested that before and during the Miocene the predominant flow of water in what is now Amazonia was to the west, in a river of comparable scale to the modern Amazon River, with only a small section in the far east of the basin draining eastward to the Atlantic Ocean. The westward-flowing pre-Amazon river has been postulated to have terminated in an extensive "Pebas" lake or wetland near the foot of the Andes which in turn drained north into the Caribbean Sea. The flow of water in this river has previously been postulated to have reversed direction in the late Miocene, signalling the development of an Amazon River system similar to that seen today.

Some of the evidence for this reversal of proto-Amazon flow is based upon paleo-current analyses and detrital zircon provenance analyses. The use of paleocurrent analyses to extrapolate the behavior of a large, complex river system such as the modern Amazon or the pre-Amazonian river system has not been fully validated.

The accessible rock record of the proto-Amazon river system is sparse. However, that system as proposed is comparable to the modern Amazon River system apart from the overall flow direction: the modern system may therefore serve as a well-documented proxy for the ancient system. Here, we analyze the modern Amazon River system using hydrologic analysis of recently-released one-arcsecond SRTM data to assess the potential of reconstructing its behavior based on the rock record it would hypothetically leave. Within the modern Amazon River system water flows in all directions, with an eastward flow only predominant in the largest of its tributary rivers. We will present a detailed analysis of this system to address whether or not the eastward flow predominance could be recovered from its hypothetical rock record and whether the current evidence from the extant rock record is sufficient to draw the conclusion of westward flow that has been drawn up to this point.