OPTICALLY STIMULATED LUMINESCENCE DATING OF YOUNG SPIT AND BAR DEPOSITS FROM THE RIO TAPAJóS, BRAZIL

The Rio Tapajós is a major river system draining the Amazon basin, running ~1930 km from the Mato Grosso plateau (14°25’ S) north to the confluence with the Amazon at Santarém (2°25’ S). In the last ~160 km the Tapajós widens to 6-14 km, deepens considerably, and forms a ria (flooded river valley). To the south, deposits of pristine quartz-rich sand line the banks of the ria. These sands are primarily sourced by Cretaceous bedrock that forms prominent bluffs as high as 90-120 m. Because of prevailing, equatorial tradewinds, the spits and bars exhibit an unusual pattern of upstream progradation. LandSat imagery indicates depositional systems have undergone only minor morphological changes in 4 decades. Because of an ~6 m, seasonal oscillation of river levels, the bars and spits are subaerially exposed in late summer and fall, but are subaqueous the rest of the year.

The drainage basin of the Tapajós is covered with dense rainforests on highly weathered, ancient shields, resulting in a clearwater river. Conversely, the mainstem of the Amazon has very high suspended sediment loads. Floodplain deposition along the Amazon has kept pace with Holocene rise in base level. Along the Tapajós, however, lack of sediment has resulted in a ria that is partially dammed along the Tapajós-Amazon confluence. The waters within these two disparate types of rivers maintain individual identity downstream of this confluence. A zone of mixing, very similar to the “meeting of the waters” at Manaus, occurs along the riverfront at Santarém.

Our focus in the study of the Rio Tapajós was to determine if the depositional age of the unusual spit and bar deposits could be determined by the use of optically stimulated luminescence (OSL) techniques. Sands from a number of spit and bar features from Cupari (3°43’ S) to just west of Santarém were sampled with a vertical push corer. OSL from sand-sized quartz was analyzed using a modified single-aliquot regenerative-dose (SAR) protocol. Luminescence test results indicate reliable intrinsic characteristics leading to depositional ages ~10 years. Despite the apparent decadal stability (from LandSat imagery), the OSL data indicate that continual reworking and redeposition occurs within the upper parts of the spits and bars. We will compare and contrast the OSL age data to the sedimentological aspects of spit and bar dynamics.