61st Annual Report on Research 2016

Carbonate cement, vein fill, and concretions along fault zones hold a record of geologic fluids and fluid migration pathways. Our work applies the new carbonate clumped-isotope paleothermometer along with more traditional stable isotope and petrologic analyses to describe the source fluids and map the distribution of ancient fluid temperature in sandstones in exhumed fault zones in the Paradox Basin, southeast Utah. By making observations at various scales along several fault zones in the basin, we are testing hypotheses regarding the origin of diagenetic fluids and the feedbacks between fault zone development and the transmission and compartmentalization of fluids.

Field work.Building on the earlier field work in the Moab Fault zone, Ph.D. candidate Keith Hodson returned to the Paradox Basin in April 2016. While the initial field excursions were focused on a particular fault intersection zone (Courthouse Junction), this new work takes a broader view of the fault zone, mapping and documenting selected sites with structure-from-motion photogrammetry and sampling carbonate cements along the entire Moab Fault and adjacent fault zones. Samples were collected for petrographic analysis in thin section and for bulk- and clumped-stable isotope analyses.

The first phase of our investigation found that spatial distribution and relative ages of carbonate cements are related to the distribution and ages of fault zone structures (Hodson et al., 2016). Guided by these findings, the observation and sampling plan was designed to test the hypotheses that 1) fault zone permeability is primarily controlled by structural complexity at fault segment boundaries; and 2) cements precipitated in these permeable zones record ambient subsurface conditions during exhumation of the rocks.

Analyses.Thin section observations were made under standard cross polarized light and cathodoluminescence (CL), in order to describe the sedimentological and structural context of carbonate cements . As at Courthouse Junction, both luminescent and non-luminescent cements are observed, suggesting two fluid sources with distinct trace element compositions. Coarsely crystalline luminescent cement commonly fills fractures and joints, infiltrating pore space in the surrounding host rock. Non-luminescent carbonate is typically finely crystalline and associated with deformation bands. Cross cutting relationships between the carbonates are rare, but where observed, the luminescent (joint-hosted) cements are typically younger than the non-luminescent (deformation-band hosted) cements.

We have measured bulk carbon and oxygen stable isotope compositions on samples along the length of the Moab Fault zone. Consistent with our prior findings, luminescent and non-luminescent carbonate cements have distinct δ13C and δ18O values. A subset of samples were analyzed for clumped isotope thermometry and reveal that temperature ranges for luminescent and non-luminescent carbonate are distinct: luminescent cements show warmer temperatures of precipitation than non-luminescent cements. Along the length of the fault, “cool” cements are ubiquitous in small volumes, whereas “warm” cements are found only at fault intersection zones, but in greater abundance.

Findings.Hodson et al. (2016) unraveled the temporal and spatial connections between deformation structures and the carbonate cements they host. “Cool”cements are hosted by deformation bands. The deformation bands formed at shallow depths very early in the burial history, preconditioning the rock for fracturing and associated increases in permeability. “Warm” cements are closely associated with jointing, capitalizing on increases in permeability associated with fracturing during faulting and subsequent exhumation. Carbonate clumped isotope temperatures allowed us to associate structural and diagenetic features with burial history, revealing that structural controls on fluid distribution are established early in the evolution of the host rock and fault zone, before the onset of major displacement.

Our ongoing work considers the broader spatial distribution of the fluids. Early analyses suggest that the distribution of “warm” cements is correlated with fault zone complexity but not with displacement maxima along fault segments or proxies for shale-gouge ratio along the fault.