Climatic Indicators in Margin Sediments off Northwest Africa
Published:January 01, 1982
Climatic variations, both in oceans and on continents, can be only partly recorded in marine sediments. Climatic factors more directly affect erosion and sedimentation on land, however local conditions may cause important interference of climatic changes in space and time. Seawater buffers these factors, making marine sediments more suitable to characterize regional, and sometimes global, climatic changes.
The following problems and results about climatic indicators off northwest Africa deal mainly with climatic changes on land. The present climatic zonation is given in Figure 1, and continental margin profiles are given in Figure 2.
Published information about African shelf and slope sequences is scanty because many drilling results are unavailable and sedimentation and/or recovery conditions were not ideal. For example, in Deep Sea Drilling Project Site 369, general sedimentation rates in the 500 m sequence, drilled from Aptian to Recent, are only about 5 m/million years. About 33% of the 100 million years are missing in hiatuses and, only about 33% of the sediments consist of terrigenous material. (Lancelot et al, 1978).
Further details from continental margin sequences off northwest Africa are given in other Initial Reports of the Deep Sea Drilling Project, especially in Leg 14 (Hayes et al, 1972); Leg 47A (von Rad et al, 1979); and Leg 50 (Lancelot et al, 1980). Only Sites 141 (Pliocene), 366, and 397 consist of thick pelagic and hemipelagic carbonate sediments (Figure 3), which display an almost continuous stratigraphic record with high-resolution.
Land climate is characterized today by the high aridity of the western Sahara, centered around 20 to 27°N, with transitions to more humid conditions to the north and south (Figure 1).
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Studies in Continental Margin Geology
"Studies in Continental Margin Geology" contains papers from a research conference co-sponsored by AAPG and the University of Texas Institute for Geophysics held in Galveston, Texas in 1981. Rapid advances in the understanding of continental margin geology were taking place during the time period, based on major improvements in the quality and availability of regional seismic surveys plus other fields such as organic geochemistry. For the first time it was becoming common to have a visual characterization of tectonic processes at significant depths below the surface. Twenty-seven papers are presented that deal with field investigations of continental margin structure and stratigraphy. The geographic areas of study are global in nature and many of the descriptive results are derived from modern seismic investigations in areas where that type of data had not previously been available in commercial publications. Fifteen of the papers focus on rifted margins and the other twelve concern convergent margins. Twelve papers are model investigations of a variety of margin environmental processes, related to subjects such as depositional environments, biostratigraphy, organic matter deposition, and oil and gas occurrences as a function of the plate tectonic setting. An additional nine papers model the thermal and mechanical tectonic processes involved in the structural development along continental margins.