Imaging spectroscopy of jarosite cement in the Jurassic Navajo Sandstone

摘要

Imaging spectroscopy is a powerful tool for mapping surface mineralogy. Interactions of energy and surface materials on atomic and molecular levels result in specific absorption features that are diagnostic of mineralogy. Hyperspectral airborne sensors such as HyMap have sufficient spectral resolution to identify subtle features over narrow wavelength ranges. An anomalous zone of jarosite cement within the Jurassic Navajo Sandstone in southern Utah was identified with airborne hyperspectral data. To date no other locations of sulfate-rich cements such as this have been documented in the Navajo Sandstone. Here, we use multiple spectral analysis techniques over a micro to macro spatial continuum in order to focus our evaluation on the distribution and relative abundance of jarosite in this area. In this study, imaging spectroscopy is used to help determine the extent of the distribution of jarosite [KFe_3(SO_4)_2(OH)_6], at "Mollie's Nipple" (MN), a significant geomorphic landmark located within the Grand Staircase-Escalante National Monument, in the Jurassic Navajo Sandstone of south central Utah. The extent of the jarosite across the butte is identified in this study by mapping two diagnostic absorption features, referred to as the ~2.26μm and near-infrared ferric iron absorption features (~0.70-1.15μm), independently. This analysis shows that there is a single circular zone of abundant jarosite ~1km in diameter that includes both in-situ and weathered out accumulations. Jarosite formation requires acidic and sulfate-rich fluids, which are unusual for the Navajo Sandstone. Imaging and field spectroscopy provides the spectral resolution needed to map and analyze the mineralogic characteristics of this area; characteristics that may help constrain the conditions under which this atypical butte formed.