Depositional Environments of the Dakota Sandstone and Adjacent Units in the San Juan Basin Utilizing Discriminant Analysis of Trace Elements in Shales

Abstract

Thirteen chemical and ten mineralogical variables were evaluated as possible indicators of depositional environments. Samples from 300 shale and coal outcrops of the Morrison Formation (Jurassic), and the Burro Canyon Formation, Dakota Sandstone, and Mancos Shale (all Cretaceous) were collected at 29 localities in and north of the San Juan Basin. Rock samples of marine and nonmarine origin were selected as control groups on the basis of their stratigraphic positions and physical and paleontological indicators of environment. Discriminant analyses were performed on the control group samples to establish criteria for estimating marine or nonmarine character in samples from uncertain depositional environments. Discriminant analysis using both trace element and mineral variables effectively identified all of the samples in the control groups as marine or nonmarine. Discriminant analysis using chemical variables alone correctly identified 91 percent of the control group samples. The most significant chemical variables for distinguishing between marine and nonmarine depositional environments are phosphorus, vanadium, iron, chromium, nickel, and zinc. The minerals illite, dolomite, mixed-layer clay, plagioclase, and percent HCl soluble minerals were also significant in characterizing marine and nonmarine environments. The discriminant classification of the shales in the Dakota and lower Mancos interval in addition provided a tool for stratigraphic correlation of the various sandstone and shale members of this interval. Discriminant analysis helped correlate the Paguate Sandstone Member of the Dakota Sandstone northward along the east side of the basin, westward to near Cortez, Colorado, and around the west side of the basin to near the San Juan River in New Mexico. The Twowells Sandstone Member of the Dakota Sandstone was correlated as a series of discontinuous sandstone lenses that extend into the Four Corners region. The numerous pinch-outs of the Paguate and Twowells Sandstone Members offer stratigraphic trap opportunities for future oil and gas exploration in the San Juan Basin.