STRATIGRAPHIC EVIDENCE OF CHANGES IN AMAZON SHELF SEDIMENTATION DURING THE LATE HOLOCENE

Late Holocene sedimentation on the Amazon shelf was investigated using a combination of seismic, sedimentological, and geochronological meth ods. Piston cores collected in the subaqueous delta region during A Mu ltidisciplinary Amazon Shelf Sediment Study (AmasSeds) penetrated a re gionally traceable (1.25 x 10(4) km(2)) seismic discontinuity, which c orresponds to a dense relict-mud reflector overlain by acoustically tr ansparent sediment. Distinct offsets in profiles of wet-bulk density, P-wave velocity, and Pb-210 activity are noted in cores at the reflect or horizon (unconformity), signifying a past change in sedimentation c onditions on the shelf. Bulk-sediment C-14 dating indicates net deposi tional conditions for the subaqueous delta from >1800 yr B.P. to 700 y r B.P. and a net erosional phase from about 700 yr B.P. to 100 yr ago. Sedimentological evidence of the erosional phase includes lag deposit s and burrows present at the unconformity surface in piston cores. The deposits above and below the unconformity are similar with respect to grain-size parameters and sedimentary structures, suggesting that env ironmental conditions during the two depositional periods were similar . According to C-14 dates of coastal deposits, the shoreline of Amapa State, Brazil served as a major depocenter of Amazon mud during the er osional phase on the shelf. Pb-210 geochronology indicates that the mo st recent phase of sediment accumulation in the study area commenced a bout 100 yr ago, resulting in rapid progradation of topset deposits an d burial of relict strata. In the present environment, oceanographic a nd meteorologic processes cause along-shelf variations in sediment dis persal and depositional patterns, and about 1400 km(2) of the erosiona l relict deposits are still exposed at the seabed. The short time scal es over which Amazon shelf strata respond to changes in the physical e nvironment preclude the influence of relative sea-level cyclicity on r ecent sedimentation patterns. During the late Holocene, sedimentation and strata formation on the Amazon shelf are linked to fluvial, oceano graphic and meteorologic processes that affect sediment supply, deposi tion and erosion.