WATER IN THE DESERT: STRATIGRAPHIC ARCHITECTURE, AQUIFER HETEROGENEITY, AND ARSENIC IN THE JURASSIC NAVAJO SANDSTONE NEAR COMB RIDGE, SOUTHEASTERN UTAH

The Jurassic Navajo Sandstone is an important aquifer in Southern Utah and Northern Arizona, with localized arsenic levels in groundwater which exceed EPA standards. Previous work suggests arsenic concentrations may be elevated in interdune deposits and/or zones of hematitic staining. Aquifer partitioning by lower permeability bounding surfaces, may impact water quality and formation level hydraulic conductivity.

Here we characterize the stratigraphic architecture of the Navajo Sandstone along 11 km of the San Juan River to identify potential barriers or baffles to groundwater flow and possible facies-controlled sources of arsenic within the aquifer. Using the Hasiotis et al. (2021) system for defining bounding surfaces we collected high resolution photos, photo pans, and used Google Earth images and Lidar of the Navajo Sandstone surface exposures. Using these data we traced bounding surfaces, facies, soft sediment deformation, and other sedimentary structures (e.g., rhizoliths, desiccation cracks, bioturbation) throughout the field area. Thicknesses of units between bounding surfaces are constrained by a True Pulse 200x laser range finder, and USGS Lidar data. Ongoing research includes using a handheld XRF to determine absence/presence of arsenic within bounding surfaces, interdune deposits, and hematitic facies.

Compound and complex bounding surfaces extend for a minimum of 3.5 km in the study area and partition the 122 m thick Navajo Sandstone into 4 to 6 complex units (sequences). Five layers of interdune facies occur including carbonates, mudstones, and sand sheet facies. Two separate interdune carbonate deposits onlap dune foresets, providing minimum water depth estimates of 10 m. Sets of plane tabular cross bedded units 5 to 30 m thick are interpreted as transverse dune deposits. One- to three-meter-thick beds and bed sets of trough cross bedding are interpreted as parabolic or barchan dune deposits. Due to lower permeability, hematite staining is primarily preserved in the bottom of foresets and interdune facies.

This study has implications for both water quality and quantity within the Navajo Sandstone aquifer. Additionally, this work provides insights to the role of bounding surfaces in partitioning fluid flow within aquifers and potential oil and gas reservoirs.