Anomalous Vitrinite Reflectance Profiles in Sediments under Volcanics of Pacific Northwest--A Systematic Phenomenon

Neil S. Summer, Kenneth L. Verosub

Nearly all of the vitrinite reflectance data from sediments overlain by thick volcanic strata in Oregon and Washington have anomalous near-constant profiles in addition to showing higher levels of maturation than expected. Published data together with an extensive data set compiled from the state core repositories of Oregon and Washington indicate a systematic occurrence of the phenomenon. Vitrinite reflectance is a widely used indicator of thermal maturation, albeit with some drawbacks, and the extraordinary profiles may be the result of numerous factors. However, given the consistency of the data and the variety of sources, the widespread occurrence of this type of maturation anomaly can only be inferred to be a real systematic phenomenon. In addition, limited Rock-Eval data is consistent with the levels of maturation assessed by the vitrinite reflectance, although the slopes of the maturation profiles from the pyrolysis do not match those of the vitrinite reflectance. The vitrinite reflectance data cannot be interpreted using conventional Lopatin plots because the gradients are anomalously steep, and usually the stratigraphy of the basins is poorly known. Compounding the difficulty are thermal events from the active tectonic setting and fossil geothermal systems that have overprinted the vitrinite reflectance data. Because vitrinite reflectance is sensitive to the maximum temperatures attained in a stratum, these results indicate that the maturation of these volcanically lidded sediments occurred under unusual thermal conditions. Using a temperature/vi rinite reflectance relationship, one can determine a maximum temperature profile, and this together with stratigraphic data can make important contributions to understanding the geological histories of the volcanically lidded basins of the Pacific Northwest.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.