The Role of Climate in the Creation and Destruction of Continental Stratigraphic Records: An Example From the Northern Margin of the Sahara Desert
Christopher S. Swezey, 2003. "The Role of Climate in the Creation and Destruction of Continental Stratigraphic Records: An Example From the Northern Margin of the Sahara Desert", Climate Controls on Stratigraphy, C. Blaine Cecil, N. Terence Edgar
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The Quaternary stratigraphy of the Chott Rharsa Basin, which is located in southern Tunisia (North Africa) on the northern margin of the Sahara Desert, displays distinct patterns of sediment distribution and stratigraphic accumulation as functions of climatic and tectonic variables. The northern margin of the basin is characterized by terraces of fluvial–lacustrine origin and by alluvial fans that have prograded into the basin from uplifted areas to the north. The center of the basin is occupied by a continental sabkha that lies below sea level, and the southern margin of the basin consists of eolian, lacustrine, and sabkha deposits. The Quaternary stratigraphic record is thicker and spans a longer period of time (at least 160,000 years) on the northern margin of the basin, but available dates reveal that major changes in the stratigraphic record have occurred at a relatively low frequency (tens of thousands of years). In contrast, the Quaternary record on the southern margin of the basin is thinner and spans a shorter period of time (about 13,000 years), but major changes in the stratigraphic record have occurred at a relatively higher frequency (thousand-year intervals). In addition, the northern and southern margins of the basin are out of phase with respect to the timing of deposition. During times of humid climates, the alluvial and fluvial systems on the northern margin of the basin are more active, while eolian systems on the southern margin are less active (i.e., stabilized by vegetation). In contrast, during arid times the alluvial and fluvial systems are less active, eolian systems are more active (i.e., not stabilized), and older alluvial and fluvial deposits tend to be reworked into eolian deposits. Furthermore, the detailed record on the southern margin of the basin reveals a history of creation and destruction of eolian stratigraphic records via temporal and spatial movement of an eolian sequence boundary, with each rise and fall of the water table (and associated climate change). The resulting stratigraphic record is thus the net sum of the positions of sequence boundaries, as a function of climate and water table. Finally, the record from the Chott Rharsa Basin demonstrates that subsidence alone is not sufficient for the creation of a stratigraphic record, and that the role of climate in this matter is more important than tectonic activity.
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The role of climate as a primary control on stratigraphy is the cornerstone of this volume. The emphasis on climate is in distinct contrast to most previous studies, in which stratigraphic variability has been related to changes in sea level and in tectonic activity. Furthermore, the findings, derived from several years of detailed study of modern and ancient key geologic sections around the world, indicate that traditional depositional models generally do not fully explain the origin of fossil fuels. Although the results of the studies presented in this volume are intended to contribute to the disciplines of sedimentary geology and stratigraphy, the contributors recognize that their results may also contribute to a better understanding of global climate change. The theoretical background of climate control on sediment supply and stratigraphy is presented in the volume. With this background in place, detailed documentation and analysis of climate control on the lithologic variation of a single Middle Pennsylvanian.