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.
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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
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.