FIELD OBSERVATIONS AND STABLE ISOTOPIC ANALYSIS OF LATERALLY CONTINUOUS TRAVERTINE VEINS ASSOCIATED WITH FAULT ZONES: INSIGHT INTO ANCIENT FLUID TRAVEL

Storing CO₂ in subsurface geologic reservoirs is a proposed technique to reduce the amount of anthropogenic CO_2, a greenhouse gas that is accumulating in the atmosphere. By looking at different sites where bedrock is exposed with evidence of natural CO₂ leaks, we can understand how fluid flow in the subsurface behaves in relation to faults and fractures. Studying fluid flow in a natural system improves our understanding of the potential risks involved in the storage of anthropogenic CO₂ in subsurface reservoirs.

Two different outcrops of Mesozoic rocks associated with active CO₂ leaks and carbonate mineralization are investigated. The field locality for this work is in SE Utah, at Salt Wash Graben, adjacent to the Ten-Mile fault, a normal fault with hundreds of meters of offset. Field observations at this location allow an understanding of crosscutting relationships of calcium carbonate veins and travertine deposits associated with the Salt Wash fault zone. Maps of cross-cutting relationships in outcrop along with petrographic analysis of host rock and vein calcite are used to understand the timing of mineralization and its relationships to the host rock. Stable isotope analysis of carbon and oxygen in carbonates are used to understand the composition and variability in the fluid reservoir composition. Preliminary δ¹³C data on calcite veins range between 3.9 and 6.0 ‰ (PDB). When combined with new δ¹³C and δ¹⁸O data, these variations may indicate changes in fluid source, variations in temperature and depth of mineralization, or variable fractionation due to CO₂ degassing in the fault zone.