Initial Publication Date: December 3, 2017

Structurally controlled distributions and diagenetic conditions for carbonate cementation along the Moab Fault, UT

Keith Hodson, University of Washington
Juliet Crider, University of Washington
Katharine Huntington, University of Washington

Diagenesis within fault zones can alter fault rock mineralogy and localize cementation along the fault. These processes fundamentally alter the mechanical properties and permeability structure of the fault zone, making fluid-driven diagenesis an important component of fault zone deformation and associated fluid flow. Fault-hosted cements record the environmental conditions and source-fluid compositions during their formation. When placed in context with the deformation history of the fault zone, cements provide an important window into the relationships between structural deformation, fluid migration, and diagenesis at different stages of fault zone evolution.

Here, we use stable isotope geochemistry, including carbonate clumped isotope thermometry, to characterize the history of carbonate cementation and diagenesis associated with fluid migration along the Moab Fault zone in southeast Utah. Previous work in the area has produced a large body of literature characterizing the fault, including detailed studies at one fault-segment intersection zone describing fault-related deformation and cementation, and the relationships between them. At this locality, multiple episodes of cementation are closely tied to distinct deformation structures, including deformation bands and joints, suggesting that different structures hosted migrating fluids at different stages of fault zone growth and evolution.

We present new stable isotope data characterizing carbonate cements from along the entire northern Moab Fault system. These data allow us to further constrain the timing and conditions of fault-hosted cementation, and explore the spatial relationships between fault related deformation and the occurrence and composition of carbonate cements. Two main groups of cements were identified, each with distinct stable isotope compositions, clumped isotope precipitation temperatures, and textural characteristics. Earlier micritic cement has cool, Earth-surface precipitation temperatures (0 to 20C) and an apparent marine source fluid. Later crystalline cements have ranges of precipitation temperatures (18 to 90C) and reconstructed source fluid oxygen isotope compositions (-16 to -1 permil VSMOW) indicative of rock-buffered carbonate recrystallization during basin exhumation, following the main phase of deformation on the Moab Fault.

Spatial distributions of cement isotopic compositions and precipitation temperatures support distinct fault-scale permeability distributions during each phase of cementation. While early cements were observed at virtually all sampled locations along the fault, later cements were preferentially located along portions of the fault zone with greater structural complexity associated with fault segment interaction and secondary structures. This suggests that focused deformation associated with structural interaction was a primary control on the distribution of permeability along the fault zone.