Paleofluid system structure in the Monterrey Salient of the Sierra Madre oriental, northeastern Mexico
Zodarecky, Joshua William
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This project investigates the paleofluid system in a thinly tapering orogenic wedge, and assesses paleofluid system heterogeneity using geochemical analyses of veins and host rock. My goal is to determine the characteristics of paleofluids within a fold-dominated orogenic belt, and to use the spatial, structural and stratigraphic distribution of paleofluid types to assess whether the paleofluid system was compartmentalized within individual folds. Regional structural geometry was characterized by combining geologic maps, digital elevation data, aerial photo interpretation, and cross section balancing and restoration to create new cross sections though the Monterrey Salient. The chemistry, temperature and distribution of paleofluids were documented through petrographic, microthermometric and geochemical analysis of calcite and quartz veins and their corresponding host rock. Construction and analysis of the new balanced cross sections reveals that the basement is segmented into several horst and graben features likely attributed to the opening of the Gulf of Mexico. During detachment folding, evaporites at the base of the orogenic wedge migrated into anticline cores, sequentially lowering the regional level of adjacent synclines and causing them to become grounded at the basement. Basement structure appears to have influenced the vergence of detachment folds. Veins throughout the region formed from 130°C -- 250°C basinal brines, marine and meteoric waters in both open and closed fluid systems. The regional paleofluid system evolved through different stages of fluid distribution as deformation progressed. Initially, paleofluids were separated into upper and lower hydrostratigraphic units (UHU and LHU). At this time, high temperature and moderate to high salinity (15-30 wt% NaCl) fluids in the LHU were separated from lower temperature and lower salinity (0-15 wt% NaCl) fluids in the UHU by an impermeable shale layer. Isotopic analysis suggests that the fractures created during progressive deformation facilitated mixing between the UHU and LHU. Lower δ¹³C values in later stage veins in the LHU suggest that hydrocarbons migrated through the system. Overall, the regional stratigraphy, rather than individual structures, seems to have been the dominant factor controlling paleofluid distribution.