AN INTRODUCTION TO THE GEOLOGICAL SIGNIFICANCE OF SEDIMENT TRANSPORT AND ACCUMULATION ON THE AMAZON CONTINENTAL-SHELF

In order to understand the formative processes and resulting stratigraphy on the Amazon shelf and adjacent shoreline of Amapa, geological investigations were undertaken as part of A Multidisciplinary Amazon Shelf SEDiment Study (AmasSeds). The design of the study provided results of a multifaceted nature: integrated with observations in other disciplines; focused on sedimentary processes; representative of fluctuating conditions on several time scales; and broadly distributed on the shelf, including regions not investigated in the past. On short time scales, most muddy sediment is transported northwest of the river mouth. From there, it moves seaward as fluid mud to cause rapid accumulation of foreset beds, and moves northwestward to prograde the northernmost Amapa shoreline and to supply sediment to the Guianas. Fluid muds cover the shelf as far northward as similar to 3.5 degrees N and allow strong tides to propagate to shore, where tidal currents cause most of the Amapa shoreline to undergo erosion today. Averaged over decades and centuries, about half of the Amazon sediment discharge accumulates on the adjacent shelf and another significant fraction (similar to one-sixth) leads to accretion of the northernmost Amapa shoreline and northwestward bypassing of sediment. The remaining sediment is hypothesized to be trapped in delta-plain deposits of the lower Amazon River system. The foreset region and shoreline represent the two common loci of sediment accumulation that alternate their predominance on time scales of millennia, and lead to a two-stage progradation of coastal-plain and subaqueous-deltaic deposits. This muddy regressive sedimentation during high sea level is replaced by formation of erosional sand layers during low sea level and transgressive conditions. Future research in the study area should address important considerations that were delineated by the present study, including: mechanisms of shoreline accretion; the Holocene history recorded in topset and coastal-plain strata; the role in local sedimentation played by the large shoal extending from Cabo Norte; and the entrapment of Amazon sediment by the delta plain.