CARBONATE DEPOSITIONAL HISTORY REFINED BY GAMMA RADIATION ANALYSIS: MIDDLE TO LOWER UPPER CAMBRIAN HIGHLAND PEAK FORMATION, EASTERN NEVADA

Gamma ray profiling of the Middle to lower Upper Cambrian Highland Peak Formation corroborates previously identified environmental changes as well as reveals subtle environmental changes not previously identified by sedimentologic analyses. Gamma ray profiling is used to measure the natural radioactivity of sedimentary rocks in stratigraphic successions. In the past, gamma ray and spectral gamma ray logs/profiles were used primarily for identification and correlation of subsurface siliciclastic successions for petroleum exploration. They are now increasingly used for correlation and interpretation of carbonate stratigraphic successions in outcrop. Use of gamma ray profiling on lower Paleozoic carbonate strata across the Great Basin is becoming an important technique for corroborating and refining traditional stratigraphic and sedimentologic analyses.

The Highland Peak Formation in eastern Nevada contains over 1000 meters of shallow-water cyclic carbonates deposited along the passive margin of western Laurentia. This diverse assemblage of shallow water limestone, dolostone, and siliciclastic facies represents deposition on a flat-topped fully aggraded carbonate platform during the Middle Cambrian, and a low-angle ramp during the early Late Cambrian. Limited faunal data indicates that the Highland Peak Formation spans, from oldest to youngest, the upper Glossopleura, Ehmaniella, Bolaspidella, and somewhere within the Cedaria or Crepicephalus trilobite zones.

Measurement of natural gamma radiation emitted from sediment within the Highland Peak Formation near Panaca, Nevada was performed using a hand-held standard gamma ray scintillometer. Readings were taken every 1.5 meters along the 942 meters of previously measured stratigraphic section. Gamma ray analysis, when combined with analysis of cycle stratigraphy, sequence stratigraphy, Fischer plots, cycle grouping correlation, cycle architecture, and facies proportions within cycles, corroborates and refines identification and interpretation of carbonate stratigraphic successions and their depositional history. This gamma ray analysis provides essential new data to improve correlation of Cambrian strata regionally and globally.