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 stratigra phy on the Amazon shelf and adjacent shoreline of Amapa, geological in vestigations were undertaken as part of A Multidisciplinary Amazon She lf SEDiment Study (AmasSeds). The design of the study provided results of a multifaceted nature: integrated with observations in other disci plines; focused on sedimentary processes; representative of fluctuatin g conditions on several time scales; and broadly distributed on the sh elf, including regions not investigated in the past. On short time sca les, 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 cove r the shelf as far northward as similar to 3.5 degrees N and allow str ong tides to propagate to shore, where tidal currents cause most of th e 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-si xth) leads to accretion of the northernmost Amapa shoreline and northw estward 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 se diment accumulation that alternate their predominance on time scales o f millennia, and lead to a two-stage progradation of coastal-plain and subaqueous-deltaic deposits. This muddy regressive sedimentation duri ng high sea level is replaced by formation of erosional sand layers du ring low sea level and transgressive conditions. Future research in th e study area should address important considerations that were delinea ted by the present study, including: mechanisms of shoreline accretion ; the Holocene history recorded in topset and coastal-plain strata; th e role in local sedimentation played by the large shoal extending from Cabo Norte; and the entrapment of Amazon sediment by the delta plain.