Foraminiferal Biostratigraphy and Spectral Gamma Ray Profiling of Tununk Shale in Southeastern Utah

Frank F. Love

The Tununk shale was deposited during the Cenomanian-Turonian Greenhorn transgression in the Western Interior. Ammonities and planktonic foraminifera reveal that the Tununk shale, which is the lower Member of the Mancos Shale in Utah, is stratigraphically equivalent to the Frontier Formation in southwestern Wyoming and northeastern Utah, and to the Greenhorn Limestone and Carlile Shale in Colorado.

Six transgressive episodes are recognized in the Tununk shale. Benthic foraminifera, which are used to recognize the transgressions, also indicate that water depth ranged from inner to outer shelf during deposition of the Tununk shale. These transgressions are found to be regionally correlative with transgressions in the Frontier Formation and Greenhorn Limestone. During the maximum transgression recognized in these units, an interval of keeled planktonic foraminifera was identified. This interval, designated the Praeglobotruncana inornata interval, is used as a synchronous reference horizon to correlate the transgressions regionally.

A spectral gamma ray profile generated for the lower Tununk shale suggests that this method may be used to recognize transgressions and regressions. Where outcrops are relatively unweathered, changes in the ratio between thorium and uranium concentrations correlates to foraminiferal paleobathymetry.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.