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Reflection seismic data provides a very clear picture picture of the resulting depositional Systems Tracts in the Zambezi River Basin. As the sea level fell, from water stacking on the poles during ice ages and from filling in gigantic basins like the Mediterranean Sea, the erosional base level fell, incised valleys were formed by rivers cutting new channels out to the sea, and large amounts of sediment were moved from the mountains and plains to the offshore. These are Lowstand Systems Tracts referenced above. As global warming melted the ice caps and the sea level rose, the incised valleys were filled with sediment, and a new set of fans and deltas covered previously deposited sediments during the Highstand Systems Tracts. Unconformity surfaces of non-deposition or erosion separate these Systems Tracts, and these boundaries can be recognized by truncation of eroded beds or onlapping pinchouts of sediments deposited on the unconformity surfaces or Sequence Boundaries. Figure 53A is one of the north-south lines from this study area (clicking on the on-line figure brings up a large expanded image, which can be evaluated in detail). Sequence Boundaries A through F and 1 through 25 were interpreted for this area. Red arrows show where new deposition onlaps on Sequence Boundaries. The intervals between some of the different Sequence Boundaries were colored to highlight packages of different geologic age rock.
Figure 53A. Offshore Mozambique seismic section V.
One way geologists study and come to understand the geological information on seismic reflection sections is to go "into the field" an identify analog patterns in geologic outcrops. Figure 53B juxtaposes an outcrop image taken in the Grand Canyon and interpreted by Ward Abbott, retired Chief Stratigrapher at Shell U.S. and then at Occidental Petroleum. One of the basic premises of modern geology is the concept of uniformitarianism. This philosophy assumes natural processes operating in the past are the same as those that can be observed to be operating in the present, i.e. the present is the key to the past.
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paradoxes
