A basement of melanges: A personal account of the circumstances leading to the breakthrough in Franciscan research
Published:January 01, 1985
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Kenneth J. Hsü, 1985. "A basement of melanges: A personal account of the circumstances leading to the breakthrough in Franciscan research", Geologists and Ideas, Ellen T. Drake, William M. Jordan
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The Franciscan of the California Coast Ranges was considered a lithostratigraphic unit (a formation or group of formations) for more than a century. Normal stratigraphical methodology was practised, and few questioned the applicability of the laws of superposition, lateral continuity, and paleontological dating. Despite evidence to the contrary, researchers portrayed deformation of coherent strata. Metamorphic conditions were deduced on the basis of presumed field relations, and the geosynclinal theory of mountain building was the paradigm. A subjective choice of data allowed researchers to fit the Franciscan into a straight-jacket, and for many years it was described as a Late Jurassic Tithonian formation, deposited in a eugeosyncline on the western margin of the North American continent after the Nevadan Orogeny.
The classical model failed to take into account a number of key observations that challenged accepted beliefs. Micropaleontological evidence for the Cretaceous age of some Franciscan rocks was known but not taken seriously. Finally, however, the discovery of an ammonite of undoubted Cretaceous age in a Franciscan sandstone at the type locality led to a crisis. It became impossible to ignore mounting evidence that the Franciscan contains rocks coeval to the Great Valley Sequence, which ranges in age from Tithonian to Late Cretaceous, despite the fact that rocks of typical Franciscan lithology (e.g., ophiolites) underlie the Knoxville and should therefore be older than the Great Valley Sequence.
The application of the mélange concept and the assumption of underthrusting resolved the Franciscan-Knoxville Paradox, and resulted in a revolutionary change in our thinking on Franciscan geology. At about the same time, a new paradigm in geology, the new plate-tectonic theory, was introduced. With plate tectonics, crises could be overcome, contradictions eliminated, and a period of “normal science” could prevail.
Franciscan research went through five stages: 1) random observations; 2) synthesis; 3) crisis; 4) revolution, or overthrow of the old paradigm and establishment of the new; and 5) mopping up. A history of the process illustrates the theory of scientific revolution advocated by Thomas Kuhn.
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Geologists and Ideas
An unusually coherent, well-written volume. Prepared for DNAG by the History of Geology Division of GSA. Spotlights events, ideas, and people, and sheds light on the history of North American geology as a whole. With its many intellectual jewels on the evolution of scientific concepts, this book will provide many happy hours of entertainment and instruction for anyone interested in the history of science, especially that of the earth sciences. Thirty-four papers are organized into four categories: (1) The Evolution of Significant Ideas; (2) Contributions of Individuals; (3) Contributions of Organized Groups; and (4) Application of Significant Ideas. Excellent as a course-book or for additional reading for classes related to the history of geology or general science.