Linking Subsurface Paleofluid Flow Events to Tectonics and Basin Evolution using Geochronology: Examples from the Paradox Basin

Abstract

Subsurface fluids hosted in sedimentary basins can leave behind a series of superimposed geochemical and physical alterations in the rock record that reflect basin-scale fluid flow systems operating over long timescales. However, paleofluid systems are complex and often leave behind overprinting signatures on the rocks they affect. Understanding the evolution of fluid-rock systems is imperative for safe storage of low-carbon fuels and carbon dioxide, as well as water, mineral, and energy resource exploration and management. Geochronology is a critical tool we can use on fluid-rock interaction products to learn how these complex subsurface fluid-flow systems evolve in response to changing tectonic and climatic forcings. This study combines stable isotopes, whole- rock geochemistry, and geochemical modeling, with radioisotopic geochronology applied to a variety of fluid flow manifestations exposed in the Paradox Basin on the Colorado Plateau.Extensive regions of bleached, former redbeds reflect large-scale flow of reduced fluids that contain petroleum, CH4, or H2S. In this study, I show - using detailed mineralogical observations, whole-rock geochemistry, and an advective geochemical model - that flow of reduced fluids dissolved iron-oxide and K-feldspar, and precipitated quartz, calcite, pyrite, and abundant authigenic clay. Nearly all metals, high-field-strength (including Ti), and rare-earth elements were mobilized by reductive fluids and redistributed into iron concretions or clay-rich horizons. Metals such as copper, uranium, and vanadium are enriched in bleached sandstones following later stages of flow of oxidized fluids. Authigenic illite, copper sulfides, iron-, and manganese oxides are all fluid-rock interaction products that we utilized in this study to provide radioisotopic geochronological constraints on the timing of paleofluid flow events. I document a distinct phase of fault-related paleofluid flow and mineralization during the Eocene in the Paradox Basin. I combined K-Ar illite-age analysis (IAA) and Rb-Sr geochronology on fault gouge clay, with K-Ar IAA on authigenic clay associated with red bed bleaching and Re- 15 Os geochronology on copper sulfides. I produced statistically overlapping K-Ar and Rb-Sr dates of authigenic illite from six faults across the Paradox Basin fold-and-fault belt that range from 41.9 to 50.9 Ma. Fault-adjacent bornite by the Lisbon Valley fault has a Re-Os age of 47.5 ± 1.5 Ma. Authigenic illite that formed from the interaction between petroleum and oxidized Entrada sandstones, exposed in a paleo-oil reservoir near the Moab and Courthouse faults, has a K-Ar age of 41.1 ± 2.5 Ma. The Eocene is an enigmatic time interval for the Paradox Basin, as recent erosion removed the sedimentary record younger than ~75 Ma, therefore I cannot resolve the principal drivers behind this fluid flow episode. However, I show that episodes of paleofluid flow of reduced fluids can be triggered by erosion. I applied K-Ar IAA to another exposure of bleached sandstones containing high concentrations of authigenic illite near Salt Valley, and (U-Th)/He and K-Ar dating to Mn-oxides exposed along the Moab fault and Courthouse Syncline. I measured a Mn-oxide K-Ar age of 3.9 ± 0.2 Ma, and an authigenic illite K-Ar age of 3.60 ± 0.03 Ma, documenting two new records of flow of reduced fluids. In this study I show that rapid erosional exhumation of the Colorado Plateau 3-4 Ma formed steep topographic relief and high hydraulic gradients. Meteoric water, recharging at high elevation areas such as the Book Cliffs, transported reduced fluids towards discharge zones such as the Colorado River and fault zones. Dissolved gases such as methane were exsolved from the reduced fluids following erosional exhumation or as they flowed up-dip due to the decrease in pressure and subsequent decrease in methane solubility.