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At the 1982 meeting of the SEPM in Calgary, Robert H. Dott, Jr., of the University of Wisconsin gave a very thought-provoking presidential address on episodic sedimentation. He defined episodic sedimentation as punctuated or discontinuous deposition. He concluded that sediments are deposited episodically and are controlled by such factors as the local storms, floods, and tides. Considered by itself, the concept implies that one basin has no predictable relation to another. Thus, when applying the
episodic concept it follows that the best way to determine the distribution of sedimentary rocks within a basin is to understand facies relationships and the tectonic setting of the basin. This concept makes good sense and obviously applies to the vast majority of sediments. The weakness of the concept is its inability to explain the "rare event." For these rare events, the episodic-oriented geologist commonly calls on the 1,000 yr storm, the 500-ft (150-m) waves from meteorites, the blanket of dust that extinguishes life.
Our experience, based on seismic stratigraphic studies tied to well and outcrop sections, indicates that yes, sediments are deposited episodically, but they are packaged in genetically related depositional packages or sequences that are shifted back and forth in a predictable global cyclic pattern. We believe this global cyclic pattern is caused by rapidly fluctuating eustatic changes of sea level superimposed on more slowly changing tectonics. Each sequence is composed of all the rocks deposited during a complete cycle of sea level starting with the fall and progressing through the succeeding low, rise, and high before the next fall. We believe orderly cyclic sedimentation caused by eustatic sea level changes is a better explanation for many of the rare events. Deep-marine massive sa d fans and debris flows commonly ascribed to 1,000 yr storms or 500-ft (150-m) tidal waves may be explained better by rapidly falling sea level or sea level lows. Rapid rises of sea level and their associated condensed stratigraphic sections offer an alternative explanation for the massive faunal extinction and rare deposits associated with the Cretaceous-Tertiary or Eocene-Oligocene boundaries.
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