Climate Controls on Cyclic Sedimentation: Climatostratigraphy
C. Blaine Cecil, N. Terence Edgar, Thomas S. Ahlbrandt, 1994. "Climate Controls on Cyclic Sedimentation: Climatostratigraphy", Tectonic and Eustatic Controls on Sedimentary Cycles, John M. Dennison, Frank R. Ettensohn
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Cyclic sedimentation is generally attributed to tectonic and (or) eustatic controls. The stratigraphy of chemical and siliciclastic sedimentary rocks cannot, however, be explained on the basis of these physical processes alone. As an example, the Pennsylvanian System of the United States contains transgressive-regressive cycles (cyclothems) that appear to be eustatically driven; also, basins were tectonically active as subsidence was necessary to provide accommodation space. On a basin scale, however, stratigraphic repetition of chemical rocks (coal beds, paleosols, and marine and nonmarine limestone) and siliciclastic rocks is indicative of paleoclimatic cycles as well as baselevel change induced by tectonics and (or) eustasy. Such climate cycles (changes in rainfall patterns) are recorded in stratigraphic sequences by 1) changes in paleosediment flux, 2) laterally extensive paleosols, whose characteristics range from modern aridisols (arid-climate soils) to vertisols (seasonal-climate soils) to oxisols (everwet climate soils), and 3) paleobotanical changes that have long been attributed to changes in paleoclimate. On a continental scale, zonalcirculation paleoclimates are also recorded in addition to paleoclimate cycles.
The paleogeography of North America during the Pennsylvanian was such that the Appalachian basin was equatorial relative to the midcontinent and western United States. The resulting paleoclimate gradient from the Appalachian basin to the western United States (wetter to drier, respectively) is indicated by the development of coal beds in the Appalachian basin while evaporites were being deposited in the Paradox basin contemporaneously with alluvial fans and eolianites in the Rocky Mountain region. Paleoclimate, therefore, appears to be on a par with tectonic and eustatic changes as a primary control on sedimentation and stratigraphy. Thus, sedimentary sequences that exhibit the effects of climate as a primary control on sedimentation may be appropriately classified as climatostratigraphic units.
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The collected volume begins with a brief perspective by one of the conveners, followed by articles in order of increasing stratigraphic age. Eustatic sea-level changes and tectonic warpings of basins are competing mechanisms for explaining many stratigraphic patterns. The model for sea-level changes should be developed first for a basin, since it is allocyclic and leads to a series of time bands in the strata. The residual effects should then be modeled for tectonic patterns affecting the depositional processes. Doing the reverse limits time constraints on the tectonic warping models and will blur the resolution of detailed time surfaces in the strata. Case histories of situations with both tectonic warping and time surfaces marked by sea-level events will lead to improved interpretations of earth history.