BENTONITE CORRELATION OF THE PIERRE SHALE OF SOUTH DAKOTA

Bentonites are common in the lower Pierre Shale of the Cretaceous Western Interior. These bentonites provide a distal record of volcanic activity during this time frame while also providing event-stratigraphic horizons useful for regional correlations. Stratigraphic correlation of the bentonites provides a high-resolution framework for regional tectono-stratigraphic interpretations in an active retro-arc foreland basin. In combination with the volcanic interpretation, this framework can provide a more comprehensive interpretation of basin activities.

Based on whole rock geochemistry, phenocryst composition and biotite geochemistry, individual bentonite horizons can be recognized and are useful for regional correlation. Whole rock geochemical analysis indicates that volcanism was active in a forearc, island arc and backarc setting during the deposition of the Lower Pierre Shale. Bentonites can be further distinguished based on the mineral compositions and biotite geochemistry, when available. Stratigraphic correlation of the bentonites provides a high-resolution framework for regional tectono-stratigraphic interpretations in an active retro-arc foreland basin.