Erosion and Progradation in the Deep Sea—Examples from the Western South Atlantic
L.A.P. Gamboal, P. Ganey, R.T. Buffler, 1983. "Erosion and Progradation in the Deep Sea—Examples from the Western South Atlantic", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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The effects of strong circulation for the generation of erosional surfaces in the deep sea are observed in the region of the Rio Grande Gap, Western South Atlantic. Conspicuous unconformities are observed on multichannel seismic lines shot by the University of Texas Institute of Geophysics (UTIG) research ship R/V Fred Moore in July 1979, while surveying the Rio Grande Gap and the Brazil basin for future locations of DSDP sites.
The Rio Grande Gap, a low basement area with an average width of 150 km (93 mi), is located between the Rio Grande Rise and the basement high to the west (Figure 1). The Rio Grande Gap is the major connection between the Argentine basin to the south and the Brazil basin to the north. The 600-km (372-mi) long Vema Channel stretches along the western limit of the Gap (Figure 1). This channel allows significant quantities of northward-flowing Antarctic Bottom Water (AABW) to enter the Brazil basin. Abroad terrace extends from the Vema Channel to the base of the Rio Grande Rise. Regional seismic studies of this part of the South Atlantic (for example, Le Pichon et al, 1971; Gamboa, 1981) have revealed a complex depositional history in the area, mainly related to the onset and fluctuations of the Antarctic Bottom Water circulation. The purpose of this paper is to present some examples of these erosional and depositional events identified on seismic lines across the Rio Grande Gap and the southern portion of the Brazil basin.
Analyses of UTIG multichannel seismic data in the Rio Grande Gap allow us to distinguish four major seismic sequences within the sedimentary cover of the region. The sediments in the Rio Grande Gap are about 1.2 km (.7 mi) thick and the sequences are designated by letters A to D from the base to the top (Figures 2 and 3). Sequence A lies on a strong reflector inferred to be top of oceanic crust. In general, the basement reflector is fairly smooth, but in places considerable relief is observed, which appears to indicate offset by faulting. Sequence A is characterized by weak (relatively low amplitude) but continuous subparallel internal reflectors. The lower part of this sequence onlaps and fills the relief on the basement. The upper limit of Sequence A is defined by a prominent regional unconformity (unconformity A) which truncates this sequence at several places. This unconformity is a fairly smooth and level surface and probably marks a major change in the bottom-water circulation through this area.
Sequence B is acoustically transparent, showing only few discontinuous reflectors. Sequence B thins and pinches out locally beneath the axis of the Vema Channel. The upper boundary of Sequence B is a prominent regional reflector, unconformity B. Sequence B probably represents a regime of restricted sedimentation controlled by deep sea currents, which began to affect the Rio Grande Gap area. Initiation of these currents probably eroded sequence A to produce unconformity A. Sequence C forms the major part of the terrace to the east of the Vema Channel. This sequence is characterized by dipping (prograding) and contorted internal reflectors (Figures 2 and 3). In cross section Sequence C is somewhat similar in geometry to an alluvial terrace (Figures 1, 2 and 3). This sequence represents a striking change in the sedimentation pattern in the Rio Grande Gap area. Its dipping layers indicate a progradation of sediments transported along the bottom, which filled the gap and formed the terrace to the east of the Vema Channel.