DIAGENETIC CONDITIONS AND SOURCE FLUID COMPOSITIONS ASSOCIATED WITH MULTIPLE EPISODES OF CEMENTATION ALONG THE MOAB FAULT, UT

Faults impose strong effects on rock permeability, potentially acting as fluid conduits, barriers, or some combination of the two. To better understand the complex relationships between faults, fluids and diagenesis, we use clumped isotope thermometry, as well as stable isotopes of carbon and oxygen, to characterize the calcite cementation and diagenetic histories associated with fluid flow along the Moab Fault, Utah. Our data indicate that distinct episodes of cementation are tied to specific stages of deformation along the fault system, with implications for fault zone permeability and fluid-driven diagenesis at different stages of its deformation history.

Our previous work at a single segment intersection zone on the Moab Fault identified multiple episodes of cementation that are closely tied to distinct deformation structures, including deformation bands and joints. We present a new, broader dataset characterizing calcite cements from along the entire northern Moab Fault system. Broad trends are similar across these two scales, with well-correlated δ¹³C and δ¹⁸O values of -8 to 1 ‰ and -23 to -6 ‰ (VPDB), respectively. Clumped isotope temperatures and textural observations distinguish the cementation episodes, including earlier micritic cement with Earth-surface precipitation temperatures (~0 to 25°C) and later crystalline cement with warmer precipitation temperatures (~20 to 100°C). While earlier cements formed from a marine source fluid, source fluids for later cements have a wider range of reconstructed δ¹⁸O values: between -18 and -1 ‰ (VSMOW). We show that the large range of reconstructed source-fluid δ¹⁸O values can be attributed to rock-buffered recrystallization at variable temperature. Following the main episode of faulting under peak burial conditions, small volumes of fluids continued to migrate along the fault, driving dissolution and precipitation of existing calcite cements at decreasing ambient temperatures as the basin was exhumed.