SM/ND CHEMOSTRATIGRAPHY OF EPEIRIC SEA CARBONATES: A PROXY FOR PALEODEPTH AND SEA-LEVEL CHANGE

Fanton et al. (2002) found that the fractionation of rare earth elements (as represented by the Sm/Nd ratio) in the acid soluble component of epeiric sea carbonates behaves as a chemostratigraphic indicator of relative paleodepth. High and low Sm/Nd ratios were interpreted to reflect deepening and shallowing of the water column, respectively. We attribute these stratigraphic changes in Sm/Nd ratios to aqueous fractionation of heavier REE from lighter REE. With increased transport distance from the paleo-shoreline (a trend that generally corresponds to increasing paleodepth) dissolved Sm is enriched over dissolved Nd in the seawater solution due to the preferential adsorption and sedimentation of the LREE by particulates. This aqueous signature is then imparted to the carbonate sediments on metaliferous coatings and in biogenic phosphate.

In this talk, we present the Sm/Nd chemostratigraphy of late Ordovician carbonates from the Williston Basin, whose sea level history is poorly constrained by traditional methods. Our chemostratigraphic paleodepth curve provides a sea level history for the area, and agrees with conodont paleoecology (distribution of deep vs. shallower water taxa), the position of formation contacts, and initial indications of shallowing associated with brining upward sequences. The data compiled for the Williston basin show that the Sm/Nd paleodepth proxy has the potential to yield a more detailed sea-level history than conventional facies analysis.

Fanton et al. (2002) GCA V. 66, p. 241-255.