Concepts in Late Paleozoic Correlations: ABSTRACT

Abstract

Correlation of late Paleozoic strata presents two contrasting sets of concepts, one of intra-regional correlation and the other of inter-regional correlations. Correlation within a region, exemplified by a basin, segment of a geosyncline, or an epicontinental shelf, follows most of the classical procedures developed during the last 180 years. Where abundant well-log data or surface exposures permit the tracing of individual beds, development of three-dimensional facies analysis, key beds, and recognition of individual cycles within cyclical deposits provide accurate and detailed information for lithologic correlation. Where lithologic data are sparse, correlation based on fossil occurrence and abundance furnishes an additional type of correlation. Attempts to quantify fos il correlation include (1) presentation of the percentage of forms that two different localities have in common and (2) an analysis of the population in terms of the number of individuals. Individual guide fossils or sets of guide fossils with overlapping stratigraphic ranges also are widely used. Many of the most widely used guide fossils are pelagic, e.g., the ammonoids. However, some benthonic fossils also are excellent guide fossils, particularly along depositional strikes. Inter-regional correlations in the late Paleozoic, in contrast, are challenging in other aspects, because the tectonic and fossil history in one region may have virtually no similarity with nearby regions. Nevertheless, fossils still are the most reliable criteria for inter-regional correlation in late Paleozoic strata although the fossil yardstick as a time-stratigraphic scale commonly seems considerably less precise. One problem of particular concern is the development of faunal provinces and subprovinces in what appear to be isolated and semi-isolated regions segregated by late Paleozoic orogenic activity. Threshold levels of evolutionary adaptation and changes in physical environments seem to have enabled sporadic and irregular dispersal of different parts of these semi-isolated b otas, so that first appearances of one group in an adjacent region may have little relation to the first appearance of other groups. End_of_Article - Last_Page 479------------