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43855-AC2
The Paleohydrologic System Near a Salt Diapir and Weld in the La Popa Basin, Northeastern Mexico
Project Methods and Objectives
This project uses mesoscopic structural analysis, thin section petrography, isotope geochemistry, and fluid inclusion microthermometry to characterize the fluid-rock system near an exposed salt diapir and weld in the La Popa Basin of northeastern Mexico. Our research focuses on the fluids that moved through local and regional fracture networks. The geologic record of these fluids is today preserved in vein minerals such as calcite, quartz and barite that are found as fracture fillings throughout the area.
Project Results this Year
Over the past year we conducted two weeks of field work and collected nearly 200 vein and host rock samples from selected localities near the El Papalote diapir and along the weld zone. Field studies revealed an unexpected paucity of fractures, but identified consistent trends in the characteristics of the observed fracture networks. Veins are generally more abundant near the weld zone and in carbonate host rocks. They occur in bedding-parallel, cross-strike, and strike-parallel sets that often show evidence of multiple mineralization events. Irregularly distributed along the weld zone are multiple sites of massive mineralization, many of which comprise several hundred cubic meters of coarsely crystalline calcite and barite. Similar bodies have not been observed near the diapir. Although good indicators of vein timing have yet to be found, the general characteristics of the observed vein sets suggests that in the carbonate lentils near the weld zone there was an early, syn-diagenetic phase of fracturing, and at least one later phase of tectonic fracturing. Petrographic analysis of vein samples indicates that the vast majority of veins are comprised almost exclusively of calcite.
Our geochemical analyses thus far have been limited to stable isotopic work. With the exception of the Muerto Formation, preliminary stable isotopic analyses of calcite show that host rocks and veins exhibit similar d18O and d13C values, suggesting that most of the vein-forming fluids were locally derived. The following d18O (vSMOW) and d13C (PDB) values have been determined, and are considered signature, average values for the related formations: Potrerillos Fm., d18O = 19.05 ‰, d13C = -4.43 ‰, Viento Fm., d18O = 16.72 ‰, d13C = -3.43 ‰, El Toro lentil, d18O = 21.83 ‰, d13C = 1.65 ‰, Parras Fm., d18O = 19.23 ‰, d13C = -3.68 ‰, Indidura Fm., d18O = 26.86 ‰, d13C = 3.29 ‰. In contrast, d18O values in the Muerto Fm. range from 16.95 - 20.30‰, and values from nonsystematic fracture meshes in or adjacent to the weld show d18O of 15.44 ‰, which as of yet, is the lowest value observed in the region and ones that are not indicative of any adjacent formation. This suggests that paleofluids in the immediate vicinity of the weld were not locally derived, and instead may have been a mixture of far-traveled fluids originating from multiple sources.
Project Impact
Although funding for the project began in the fall of 2006, a graduate student was not recruited until the summer of 2007. This has limited our progress during the first project year, and made it difficult to assess the scientific impact of the study. This will be more easily assessed once we present some of our early results at an upcoming La Popa Basin consortium meeting. In contrast, the project has made a significant impact on the currently support MS student and the PI. The student, Adam Smith, began working on the project as an undergraduate in March of 2007. His work on the project has been outstanding, and his involvement as an undergraduate motivated him to pursue graduate studies in geology. There is a high probability that he will pursue employment in the petroleum industry when he graduates with his M.S. in 2009. The impact on my career has likewise been positive. I have developed important new professional collaborations and plan to use this project as background research for a larger NSF proposal in 2008.
