Terrigenous sediment on Ceara Rise: a Cenozoic record of South American orogeny and erosion

Ceara Rise, located east the Amazon River mouth, is covered with a thick bl anket of pelagic carbonate and hemipelagic terrigenous sediment. The terrig enous component has been extracted from 57 bulk sediment samples at Ocean D rilling Program (ODP) Sites 925 and 929 on Ceara Rise to obtain a Cenozoic record of riverine discharge from northern South America. From the early Eo cene to early Miocene (55-20 Ma), terrigenous accumulation was dominated by moderate amounts of generally large-grained, gray to green sediment especi ally depleted in elements that are enriched in post-Archaean shale (e.g. Cs , Th, Yb). However, pulsed inputs of relatively small-grained, gray to gree n terrigenous sediment less depleted in the above elements occurred in the late Eocene and Oligocene. The accumulation of terrigenous sediment decreas ed significantly until 16.5 Ma. In the middle Miocene (16.5-13 Ma), terrige nous accumulation was dominated by small amounts of small-grained, tan sedi ment notably depleted in Na and heavy rare earth elements. The accumulation rate of terrigenous sediment increased markedly from the latest Miocene (1 0 Ma) to the present day, a change characterized by deposition of gray-gree n sediment enriched in elements that an enriched in post-Archaean shale. Ob served changes in terrigenous sediment at Ceara Rise record tectonism and e rosion in northern South America. The Brazil and Guyana shields supplied se diment to the eastern South American margin until the middle Miocene (20-16 .5 Ma) when a period of thrusting, shortening and uplift changed the source region, probably first to highly weathered and proximal Phanerozoic sedime nts. By the late Miocene (9 Ma), there was a transcontinental connection be tween the Andes and eastern South America. Weathering products derived from the Andes have increasingly dominated terrigenous deposition at Ceara Rise since the Late Miocene and especially since the late Pliocene. (C) 2001 El sevier Science B.V. All rights reserved.