DOI:10.1306/13331517M983513

Copyright 2012 by The American Association of Petroleum Geologists.

Gamma-ray Cross Sections—Correlation of Steptoean Strata in the Eastern Great Basin Using Outcrop Gamma-ray Profiles

Kevin R. Evans¹

¹Department of Geography, Geology and Planning, Missouri State University, Springfield, Missouri, U.S.A.

The strata of the Steptoean Stage (Figure 1) are exposed in several mountain ranges across the eastern Great Basin of western Utah and eastern Nevada (Figure 2). Palmer (1962, 1965) described the trilobite faunas of this interval, and many subsequent studies have focused on the faunal provincialism, paleoenvironments, and biochronostratigraphy. The Steptoean positive isotope carbon excursion (SPICE) event spans this interval, with a ¹³C maximum occurring in the Dunderbergia Zone, near the middle of the succession (Saltzman et al., 1998).

Figure 1. Names and lithology of lithostratigraphic units associated with and adjacent to the Steptoean Stage in the eastern Great Basin. See Figure 2 for the abbreviations of locations across the top row. Steptoean interval is indicated by light-gray overlay.

Figure 2. Locations of stratigraphic sections and gamma-ray profiles in the eastern Great Basin. The light-green area indicates the principal study area.

The interpreted sea level changes during this interval have been somewhat controversial (e.g., Palmer, 1965; Lochman-Balk, 1970). Evans (1997) described the depositional environments of the Steptoean and has interpreted that these strata record two major sea level falls during the Dunderbergia and early Elvinia Chrons and a major sea level rise during the late Elvinia Chron. Saltzman et al. (2004) considered that the SPICE maximum coincided with a sea level fall in the latest Dunderbergia Chron, which they considered to be the Sauk II-III unconformity. This chapter provides an overview of the depositional environments of the Steptoean Stage, together with a series of gamma-ray profiles that help illustrate the sequence stratigraphy. The gamma-ray profiles make it possible to correlate measured stratigraphic sections with one another and to correlate them with subsurface well logs (see Evans et al., 2012).