This is a summary of principal findings made by ODP Leg 113 investigators concerning the latest Cretaceous-Cenozoic climatic, cryospheric, and oceanographic history, and biogeographic developments of the Weddell Sea region, Ant arctica. During Leg 113, 22 holes were drilled at 9 sites that sampled 4 contrasting environments: open-ocean pelagic sedimentation on Maud Rise (Sites 689 and 690

Drilling at Site 516 on the northern shoulder of the main Rio Grande Rise has improved our understanding of the tectonic evolution and subsidence history of the Rise and of the entire Rio Grande-Walvis seamount and aseismic ridge system. Evidence from this site indicates that basalts at the bottom of Hole 516F were produced at the ridge crest and that the ridge crest was subaerial. I attribute the anomalous elevation of the Rise to an eastward ridge crest jump to the western end of the Rise, 91 Ma, and, recognizing a southward progression of such eastward jumps, I suggest a model for the Rio Grande-Walvis system involving a slow westward component of drift of the ridge crest off a hot-spot swell. This drift caused off-axis volcanism that in the Cretaceous succeeded but in the Cenozoic failed to capture the ridge crest. The probable mechanism of capture involved counteraction of the ridge push force within the lithosphere by a swell push force. This would make capture more likely in lithosphere produced by fast spreading, perhaps explaining the change of mode at the end of the Cretaceous. Within the pelagic carbonate sedimentary succession at Site 516, the partly volcaniclastic, turbiditic middle Eocene Unit 4 contains volcanic ash beds (yielding a 47.4 ± 0.7 Ma K-Ar age from fresh alkalic biotite) and a 15-m-thick basal slide. Reflection profiles show it was produced by sliding from and by subaerial erosion of a large tilted and uplifted guyot upslope from the site. Data from the site suggest that a single short off-axis event affected the entire crestal region of the Rise. Perhaps the same midplate hot spot that produced an 80-50 Ma volcanic episode in the Serra Geral of Brazil was responsible, but that was not the present Tristan hot spot. The data from Site 516 have been incorporated into a detailed model for the subsidence history of the main body of the Rise. The model uses an "oceanic" thermal isostatic model, but accounts for the effect of subaerial subsidence and incorporates the changes of the middle Eocene event. A detailed sediment compaction model is developed, and a smooth eustatic sea level correction is applied. The effects of "basement compaction" and use of local rather than regional isostatic compensation are assessed each at about 50 m. The computed paleodepth at Site 516 ranges from sea level 84.0 Ma through a Paleocene 1250 m maximum and middle Eocene 600 m minimum to 1313 m today. The "tectonic" depth curves for both Sites 516 and 357 are compared with paleoecologic depth estimates. In general, these paleoecologic estimates lie deeper, probably because of the difficulties of applying accurate subsidence and compaction corrections to the comparison sites.

Seismic reflection profiles have been interpreted in combination with deep-sea drilling data to examine the sedimentary evolution of the Rio Grande Rise.Restricted and unevenly distributed seismic reflection coverage (particularly multichannel) and limited well control confined most of our interest to the northern flank of the main western elevated block of the Rise, near to DSDP Site 516. The basement of much of the Rise above approximately 3000 m present depth has the "dipping reflector" character of some continental margins, produced by interbedded lavas and sediments formed directly above sea level. The overlying Late Cretaceous and early Tertiary sediments are mainly pelagic, but lap onto originally subaerial basement in places. A major middle Eocene tectonic event (involving uplift, tilting, faulting, and probably local volcanism) resulted in subaerial erosion, submarine slumping, and turbidite deposition. Submarine slides associated with the early stages of this tectonism probably caused the chaotic midsection reflector sequence identified with the middle Eocene Unit 4 at Site 516. Middle Eocene tectonism also produced the central graben of the Rise, which contains rotated fault blocks, and the broad guyot between the graben and Site 516. The guyot shows thick sequences dipping away from the graben and truncated, presumably by subaerial erosion. Prograded biogenic debris, swept off the top by bottom currents after resubmergence in the Oligocene, extended the guyot's top farther. Biogenic debris from the same source probably formed the initial load of the density currents that have dissected the steeper northern slopes of the Rise. Interpretation of reflection profiles from all around the Rise suggests erosion between depths of 2000 and 3600 m, even where the creation of density currents upslope is unlikely; a component of contour current erosion by North Atlantic Deep Water may be involved. The Vema Terrace, which in the west separates the Rio Grande Rise from the Vema Channel, shows signs of sedimentation from turbidity currents originating on the Rise. The Terrace and the Vema Channel extend around the north flank of the Rise also; their positions are controlled by the topography of the east-west Rio Grande Fracture Zone. If the eastward flow of Antarctic Bottom Water north of the Rise is inferred, the direct transport of debris from the Rise to the Brazil Basin by turbidity currents becomes a more complex process.

No abstract

The sedimentary evolution of the Rio Grande Gap and southern portion of the Brazil Basin has been reconstructed by the analysis of single-channel and multichannel seismic reflection profiles. Conspicuous erosional and depositional episodes identified from the seismic records are dated by cores recovered from Site 515 located to the north of the Vema Channel. A major erosional event, marked by a 22-Ma hiatus, occurred between the early Eocene and middle Oligocene. This erosional event is tentatively associated with the initiation of the flow of Antarctic Bottom Water (AABW) through the Rio Grande Gap and into the Brazil Basin. After AABW started to flow, the net sediment accumulation in the Rio Grande Gap and southern part of the Brazil Basin was small until the latest Oligocene, when large amounts of sediment began to accumulate in these areas. The Vema Channel was formed in the latest Oligocene when these sediments began to infill the Rio Grande Gap. This high influx of sediments occurred concomitantly with a major low stand of sea level. The lowered sea level allowed large quantities of terrigenous sediments to reach the lower continental rise and abyssal plain of the Argentine Basin and from there to be transported northward by AABW. Sediments that bypassed the Rio Grande Gap, mainly through the Vema Channel, formed a large fanlike deposit at the southern portion of the Brazil Basin.