Geochemical correlation in deltaic successions: a reality check

Whole-rock geochemical analysis can aid the stratigraphic correlation of faunally barren successions. Compared to petrographic techniques or heavy-mineral separations, such analysis is quick and objective, yet rarely has the reliability of the method been tested against unequivocal correlations. We have applied it to the Kenilworth Member of the Upper Cretaceous Blackhawk Formation, a well-exposed and easily correlated 100-m-thick unit in the deltaic successions of the Book Cliffs, Utah, United States. This is the first evaluation of geochemical analysis as an aid to sequence stratigraphic interpretation at the parasequence scale.

From the geochemistry alone, it is not possible with statistical confidence to separate individual parasequences or systems tracts nor the fluvial incised valley from the marine shoreface units. The magnitude of geochemical variation in a single sandstone bed over a 30 m lateral traverse is as great as vertically through an entire parasequence. Y, Nb, Rb, and Ti abundances are inversely correlated with grain size. Thus geochemical signatures follow sedimentological trends, and distal parts of a parasequence appear distinct from proximal parts. Y ratios compensate for this grain size effect but introduce confusing distortions for elements such as Zr that are not correlated with grain size. Contrary to some predictions, Zr is as common in the finest-grained units as the coarsest, and Sr is not correlated with Ca. In some sections Sr is also inversely correlated to grain size and cannot be used as an indicator for cement abundance.