The Concept of Autocyclic and Allocyclic Controls on Sedimentation and Stratigraphy, Emphasizing the Climatic Variable
C. Blaine Cecil, 2003. "The Concept of Autocyclic and Allocyclic Controls on Sedimentation and Stratigraphy, Emphasizing the Climatic Variable", Climate Controls on Stratigraphy, C. Blaine Cecil, N. Terence Edgar
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The concepts of autocyclicity and allocyclicity (Beerbower, 1964) are extremely powerful tools in stratigraphic analysis. Unlike other approaches to genetic stratigraphy, autocyclicity and allocyclicity integrate sedimentary geochemical and physical processes through time and account for changes in both energy and materials in sedimentary systems. As used herein, energy refers to the physical energy by which sediment is transported and deposited whereas materials refers to transport and deposition of both solids and solutes. The term “cycle” in sedimentary geology refers to recurrent events (Glossary of Geology, 1997, p. 159), which may or may not be periodic. Given this definition of “cycle”, it follows that autocyclic and allocyclic events are recurrent, but they may or may not be periodic.
According to Beerbower (1964), autocycles are produced by processes within sedimentary systems. Responses to autocyclic processes tend to be local and may range from millimeter-scale ripple migration to regional-scale events such as delta switching. Autocyclic processes also include phenomena such as stream avulsion and meandering, and fluvial point-bar migration or lateral migration of beach-barrier bars. Unlike allocyclic processes, autocyclic processes tend to be instantaneous geologic events that are random in both time and space, and they contain few interregional feedback mechanisms. As a result, autocyclic processes are aperiodic. Because effects are local and mainly involve changes in energy, autocyclic processes generally result in changes in physical sedimentology and, more often than not, they do not appear to result in significant changes in chemical sedimentology.
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Climate Controls on Stratigraphy
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.