Spectral gamma ray characterization of the Elko Formation, Nevada - a case study for a small lacustrine basin
Date
2015
Authors
McGowan, Erin M., author
Egenhoff, Sven O., advisor
Schutt, Derek, committee member
Cavdar, Gamze, committee member
Amerman, Robert, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Handheld gamma ray spectrometry is a cost-effective and time-efficient means of furthering understanding of lake facies and small-scale lake systems. Spectral and total gamma ray data were recorded every foot vertically through a succession in situ at four outcrops along a NNW-to-SSE transect representing the lower to middle Eocene Elko Formation in northeast Nevada, USA. The lacustrine Elko Formation consists of, from oldest to youngest, four major units: 1) a basal conglomerate, 2) an overlying carbonate, 3) a fine-grained organic-rich mudstone with intercalated carbonate mudstones, and 4) volcaniclastics. These units comprise fourteen sedimentological facies identifiable in outcrop. In this study, these fourteen facies have been reduced to eight that are discernable by spectral gamma ray (SGR) signals. Each recorded interval in the Elko Formation succession was assigned to one of these eight facies. These eight facies comprise five siliciclastic (plant-bearing mudstone, clay-dominated mudstone, microbial-mat-bearing mudstone, ash-bearing mudstone, and conglomerate) facies, two carbonate (calcareous mudstone and fossiliferous mudstone-wackestone) facies, and one volcanic tuff facies. In conjunction with SGR, outcrop observation, X-ray diffraction (XRD), thin section observation, and total organic carbon (TOC) analyses allowed a thorough understanding of facies composition and its SGR signal. The primary controls of SGR components [potassium (K), uranium (U), and thorium (Th)] reflect K-bearing volcanic minerals (feldspars and micas), U-enriched organic material, and clay abundance (illite and montmorillonite; potentially derived from volcanic ash), respectively. High radioactivity, with signals above 120 American Petroleum Institute (API) units, was demonstrated for five facies (plant-bearing mudstone, clay-dominated mudstone, microbial-mat-bearing mudstone, ash-bearing mudstone, and volcanic tuff) in contrast to the remaining three facies (calcareous mudstone, fossiliferous mudstone-packstone, and conglomerate) exhibiting low radioactivity of less than or equal to 120 API. Distribution of radioactive minerals across the outcrops was largely found to be not only a function of general lithologic composition, but also the paleogeographic locations of the outcrops within the lake basin, due to the differing contributions of organic debris and volcanic constituents. This distribution of radioactive minerals across each outcrop supports a recent depositional model of north-to-south diachronous deposition of the Elko Formation (Horner, 2015). The microbial-mat-bearing mudstone facies was a traceable, deep-lake sediment throughout the basin by which outcrops could be assigned to more proximal or more distal positions within the paleo-lake by applying Th/U ratios. The proximal outcrop data show a low Th/U ratio (below 2.5), as opposed to the distal outcrop data that have a high Th/U ratio (2.5–4). These data confirm that most likely Th/U ratios reflect the increase in the amount of clay with distance from the proximal outcrops. Thin section microscopy and scanning electron microscope (SEM) analyses allowed for recognition of eight diagenetic cements reflecting a strongly varying diagenetic history in the Elko Formation sedimentary rocks. One calcite, four stages of dolomite, two types of silica, and one zeolite cement were identified. The calcite, dolomite, and silica cements were typically formed sequentially as listed above, whereas the zeolites formed independently. Intraparticle, interparticle, matrix, shelter, and fracture porosity types are also present, each forming post-deposition, except for interparticle and matrix porosity within volcaniclastics. Calcite, dolomite,and silica cements were found succeeding shelter and fracture porosity, whereas the timing of zeolite cement in matrix porosity was unclear. Overall, cement phases and porosity were found to be minimal, and therefore, probably had only a minor influence on the overall gamma ray signal of the Elko Formation sedimentary rocks. SGR characterization of each of the eight facies across a proximal-to-distal transect of the Elko continental-lacustrine sedimentary basin reflects the strong influence that climate and tectonics have on depositional changes in a small-scale lake. Applying gamma ray techniques to the small-scale lake system of the Elko Formation was found to be a useful tool and provides a framework to apply to lacustrine studies as a predictive tool in future exploration.