QUALITATIVE EVIDENCE FOR BLACK SHALE TYPES: JUSTIFICATION FROM GEOCHEMISTRY AND DIAGENETIC RELATIONSHIPS

Combined paleoenvironmental, sedimentological, and geochemical actions of oxygen-controlled environments are mainly affected by sea level fluctuations and paleoclimatic changes. Marine oxygen-limited (or deficient) environments are of extraordinary importance for several reasons: 1) most petroleum-source rocks are black shales, which presumably were deposited under oxygen-limited conditions, 2) anoxic events played a significant role in many of the mass extinction events of great consequence and 3) black shales are vital intra- and supra-regional stratigraphic marker horizons, most often correlated with marine transgressions. A separation of black shale types, based on statistical analyses, is therefore, of relevance.

During deposition, black shales record geochemical activities related to the provenance and paleoenvironment of deposition. Initial geochemical conditions are susceptible to subsequent alteration and paleoenvironmental overprinting by the aggressive onset of black shale diagenesis. Many chemical analyses only outwardly reveal geochemical relationships. Advanced methods using statistical analyses use an unbiased methodology to divulge black shale differences, which diagenetic activities have obliterated. Statistical assay, including cluster analysis and regressions of an extensive database collected by numerous workers, allows the development of a comprehensive model of qualitative bottom-water oxygenation relationships between black shale types

Relationships exposed in this study include a paleohistory of early diagenetic formation of pyrite. Statistical outcomes reveal this process is only observed in certain black shale "types", typically thought by many to be present in all black shales. This study demonstrates the wide variety of black shale depositional environments. Accordingly, all black shales cannot be assigned to anoxic, sulfidic environments without additional chemical analysis and interpretation. Statistical analyses clearly reveal relationships that otherwise would not be observed and add a qualitative component to black shale "typing".

Because numerous workers have collected such an extensive database, it is imperative to use collaborative efforts to synthesize viewpoints. To that end, a black shale/anoxia/chemostratigraphy database web site is presented.