A Seismic Reflection Study of the Rio Grande Rise

Abstract

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.

Submarine slide blocks and associated soft-sediment deformation in deep-water basins: A review

“…Data processing included resampling, spherical divergence corrections and zero-phase conversions undertaken prior to stacking, 3D pre-stack time migration using the Stolt algorithm and one-pass 3D migration. Published borehole data from Deep Sea Drilling Program (DSDP) Sites 356 (Kumar et al, 1997) and 515/516 (Barker, 1983;Barker et al, 1983;Gambôa et al, 1983), as well as the works of Bruhn and Walker (1997), Chang et al (1992), Meisling et al (2001), Viana et al (2003), Fiduk et al (2004) and França et al (2007), were used to identify and date seismicstratigraphic units with submarine slide blocks. Based on the dominant frequency of the interpreted seismic volume (40 Hz), vertical seismic resolution varies between 12 and 19 m in blockladen strata.…”

“…Coherence attributes convert a seismic volume of continuity (normal reflections) into a volume of discontinuity to emphasise faults and stratigraphic limits (Brown, 2004). Block height was measured directly on vertical seismic profiles after converting these for true depths using borehole information in Barker et al (1983) and França et al (2007). Remnant and rafted blocks were interpreted based on the identification of kinematic indicators described in this work.…”

Bi-modal deformation styles in confined mass-transport deposits: Examples from a salt minibasin in SE Brazil

“…To help our analysis, the 3D seismic volume was depth-converted for the Miocene-Holocene interval of interest using an estimated seismic velocity of 1500 m/s for the water column and 1800 m/s for buried strata, based on velocity data at DSDP Site 516 (Barker et al, 1983). The latter p-wave velocities and the dominant frequency of 40 Hz of the seismic volume indicate a vertical resolution of 10 m at the depth of the interpreted MTDs.…”

MTD distribution on a ‘passive’ continental margin: The Espírito Santo Basin (SE Brazil) during the Palaeogene

“…The data is zero-phased migrated with a 12.5 m grid line spacing (inline and crossline). Time-depth conversions were estimated using the seismic velocity profiles measured for Eocene-Oligocene strata at DSDP site 516 (Barker et al, 1983).…”