Chemical composition of strata of the Meade Peak Phosphatic Shale Member of the Permian Phosphoria Formation: Channel-composited and individual rock samples of measured section J and their relationship of measured sections A and B, central part of Rasmussen Ridge, Caribou County, Idaho

Abstract

This study, one in a series, reports bulk chemical composition of rock samples collected from a core, referred to as Measured Section J, drilled at a site that subsequently was developed into the Enoch Valley phosphate mine in southeastern Idaho. The core is continuous and cuts through the entire thickness of the Meade Peak Phosphatic Shale Member of the Phosphoria Formation. The steeply dipping Meade Peak extends from 182 to 495 feet below the ground surface, which is the greatest below-ground depth of rock that has been sampled to date. The core was drilled before the start of mining, and the rocks consequently have not been exposed to the atmosphere or surficial weathering processes or fractured as a result of mining as are the rocks from other described sections. Hence, this section of the Meade Peak in the core is the least altered section of this member sampled in this series of studies. The channel-sampled rocks from Section J form a set of contiguous intervals across the entire thickness of the Meade Peak. These channel samples characterize?in ascending order?the lower phosphate ore, interlayered middle waste shale, upper phosphate ore, and upper waste shale units of the member. The Section J channel-sample suite includes 3 composite samples of the uppermost 7 feet of the Grandeur Tongue of the Permian Park City Formation, a dolomitic unit that directly underlies the Meade Peak. It also includes an analysis of a 0.1 foot section of chert directly overlying the Meade Peak. The concentrations of the chemical elements in the channel samples are compared with those of Measured Sections Aand B that were obtained from the same mine. In addition to the channel samples, 85 rock samples were selected from the core to address specific geochemical questions that resulted from examination of the core. For example, several of these samples correspond to cored rock that had unusual concentrations of various elements that were determined using a hand-held, x-ray fluorescence instrument. Other individual samples consist of several samples taken within a short interval of the core that leads to an abrupt change in lithology. Overall, the rocks from Measured Section J exhibit the least alteration from interaction with ground water compared with rocks from any of the measured sections that have been described in our previous studies.