TRACE ELEMENT GEOCHEMISTRY OF SHALE : IMPLICATIONS ON THE DEPOSITIONAL ENVIRONMENTS

The shale unit selected in this study is interbedded with bentonite horizons, laterally persistent, and belongs to the Upper Cretaceous Frontier Formation, north-central Wyoming. The geochemical data involving Peay (lower Frontier) and Torchlight (upper Frontier)shale samples provided significant clues as to the precise identification of the depositional environments of these fine-grained sediments. A multi-element plot shows similar trace element concentrations for both Peay and Torchlight shale samples. Ni, Ce, and Zn are significantly abundant in both Peay and Torchlight shale. Relatively higher concentrations of La in the Peay shale compared with the Torchlight shale reflect less terrigenous quartz input into the depositional basin and perhaps signify deposition in a quiet, less turbulent, offshore marine setting. Consistent Zr, Ni, Y, and Nb concentrations in all the shale samples analyzed suggest a similar source for the Peay and Torchlight shale, and the overall trace-element behavior (with the exception of La) is controlled by the rate of sedimentation, terrigenous input, and subsequent deepening of the depositional basin.