Deep Sea Drilling Project Leg 72: Southwest Atlantic paleocirculation and Rio Grande rise tectonics

Abstract

In order to investigate South Atlantic paleocirculation and the tectonic and sedimentary history of the Rio Grande rise, D/V Glomar Challenger occupied four sites during Deep Sea Drilling Project (DSDP) Leg 72: Site 515 in the Brazil basin, Site 516 near the crest of the Rio Grande rise, and Sites 517 and 518 on the lower western flank of the rise. Site 515 recovered sediments deposited by Antarctic Bottom Water (AABW) entering the Brazil basin through the Vema channel. We penetrated 617 m of rapidly accumulated, terrigenous and siliceous muds and mudstones before encountering an unconformity spanning 20 to 25 m.y. between the late Oligocene and early Eocene. The hiatus brackets the time of the proposed onset of AABW flow in the southwestern Pacific at ∼37 m.y.; however, evidence in the sediments at Site 515 suggests the presence of bottom-water circulation in the Brazil basin by early Eocene time. Sites 517 and 518 recovered calcareous sediments deposited under the influence of AABW and North Atlantic Deep Water (NADW) within the Vema channel. At both sites, an increase in the abundance of the AABW indicator species N. umbonifera during the late Pliocene implies that AABW extended to significantly shallower depths at that time than it does today. Detailed carbonate analyses at Site 518 may extend the late Pleistocene climatic signal through the entire Pleistocene and perhaps into the Miocene. At Site 516, near the crest of the Rio Grande rise, we penetrated 1,250 m of dominantly calcareous sediments and 21 m of basaltic basement rocks of Santonian-Coniacian age. With the exception of some minor hiatuses in the middle and late Miocene, sedimentation at this site was continuous for the past 80 m.y. Magneto-stratigraphic and biostratigraphic control is excellent, and the section may therefore be useful as a stratigraphic reference. The Cretaceous-Tertiary boundary was recovered intact and is directly comparable with the Italian Gubbio section. This interval may also provide some new evidence pertaining to the proposed association between mass extinctions and geochemical anomalies at the boundary. The igneous basement rocks recovered at Site 516 are T-type Mid-Ocean Ridge basalts (MORB), similar to those found on Iceland, and are unlike the alkalic basalts exposed on Tristan da Cunha. Their chemistry, together with the occurrence of shallow-water fossils in vein fillings, suggests that these basalts were erupted on an anomalously shallow part of the Mid-Atlantic Ridge. A Santonian-Coniacian age for basement is inferred from the age of the overlying sediments and is consistent with the identification of magnetic anomalies 33 and 34 on the rise to the east of the site. The basal sediments and underlying volcanic basement recovered at Site 516 may provide constraints on the subsidence history of the Rio Grande rise. However, this history may have been complicated by a middle Eocene episode of tectonic and volcanic activity, as suggested by ash layers, volcaniclastic sediments, and slump blocks in the mid-Eocene sediment section. The occurrence of reef debris in this interval implies that the uppermost part of the rise was very shallow and perhaps subaerial during this time.