46678-G8
Evaluating the History of Eolian and Interdune Fluid-Sediment Interactions and Mass Transfer in an Acid and Redox Influenced Diagenetic System: Mollies Nipple, GSENM
Research on the depositional and diagenetic history of the Navajo Sandstone in the Mollies Nipple (MN) area within Grand Staircase-Escalante National Monument in southern Utah has been very successful over the last year. To date, these studies have demonstrated the presence and distribution of hitherto unknown areas of jarosite cement within the sandstone. This unusual cement mineralogy suggests that in addition to regional reducing “bleaching” fluids, and oxidizing meteoric fluids, this area has been influenced by acidic (possibly hydrothermal) fluids. The diagenetic history of this area contrasts strongly with the history of the Navajo Sandstone elsewhere, and reveals complexities in reservoir heterogeneity and past fluid chemistry. The results from this research are being compared with other known iron-rich acid sulfate systems, to better understand the range of conditions where these types of mineral assemblages form.
Thus far, research efforts have included 1) field characterization and sampling of depositional and diagenetic facies; 2) analyses of spatial distribution of mineralogy using imaging spectroscopy; ; 3) laboratory analyses of sandstone mineralogy and geochemistry with infrared spectroscopy (both short-wave infrared reflectivity and thermal infrared emissivity), x-ray diffraction, and ICP-MS for major oxide and trace element concentrations; and 4) detailed petrography. Details about each of these approaches and the results from each are discussed below.
Field mapping of depositional and diagenetic facies in the MN region reveals the presence of relatively unaltered red sandstone, “bleached” sandstone, and multiple diverse alteration phases within both dune and interdune strata. The field relationships discredited the hypothesis that the jarosite-bearing strata were exclusively depositional in nature, and perhaps related to an ancient acid sulfate interdune playa. The brightly colored zones of jarosite-rich sandstone (based on results of the airborne spectral data) are exposed along the steep flanks of the MN butte, and crosscut eolian strata. The flanks of the butte are also armored by large (m-scale) boulders of planar laminated interdune deposits that also contain jarosite, but primarily as a weathering rind. A field-portable reflectance spectrometer was used to calibrate and test the results of the airborne spectral data and mineral distribution in the field. Evidence of localized but significant structural diagenesis, including fault gauge, zones of deformation bands, and brecciated sandstone cemented with iron oxide were documented.
Airborne hyperspectral analysis covering the visible- near infrared (VIS-NIR) wavelength region (0.35 – 2.5 microns; HyMap) and the thermal infrared (TIR) region (3.0-5.5 and 7.8-13.5 microns; SEBASS), both with spatial resolutions of at least 5 m per pixel, has identified the spatial distribution of cement mineralogy, and suggests that intersecting structures provided localized conduits for exotic fluids in this area. We continue to investigate the spatial relationships between iron oxide, clay, and sulfate cementation. In addition to jarosite, other sulfates including gypsum, alunite, angelsite, natrojarosite, and tschermigiitate have been identified from the thermal infrared airborne spectral data. The two types of spectral data cover different wavelength ranges that are sensitive to different mineralogies, so comparisons between the datasets show changes in both detrital and authigenic composition in the sandstone. Much of the work in this area is truly innovative, as these types of diagenetic sedimentary systems have not been studied in detail using VIS-NIR and TIR imaging spectroscopy. We are investigating how mineral and aerial mixtures affect spectral signatures, and the influence of surfacial weathering rinds in obscuring the true sandstone geochemistry.
Detailed laboratory analyses on representative samples from the MN field area reveal changes in mineralogy and geochemistry that reflect the composition of the fluids that have altered these rocks over time. Comparisons of laboratory spectra, x-ray diffraction, and major oxide ICP-MS analyses show the non-linear relationship between the abundance of Fe-bearing minerals and the depth of spectral absorption features, suggesting that the use of airborne spectral data to quantify the amount of Fe oxides will present challenges. We are continuing to conduct experiments that will allow us to better understand the controls on spectral features in these data. This data reveals at least four different types of Fe-bearing minerals in these samples (total weight percent Fe has been measured up to 28.5%). Fe-bearing phases include both oxides and sulfates, and we continue to investigate distribution and controls on Fe cycling in this system. The trace element data from MN rocks suggest that in addition to depositing Fe and S into the sandstone, fluids left unusually high concentrations of As (>2000 ppm in some areas), supporting the hypothesis that at least some of the alteration is due to hydrothermal fluids.
Petrographic work shows the spatial relationship between authigenic minerals and the location and textures of their occurrences, and will help to constrain the paragenetic relationships (temporal sequence) between mineral precipitation events. This data shows the extreme spatial heterogeneity in the amount of alteration that has occurred in these sandstones, and the intimate association with structural fabrics.
