A SEARCH FOR DETRITAL SHOCKED ZIRCONS IN THE ERODED SANTA FE IMPACT STRUCTURE, NEW MEXICO, USA

The presence of shatter cones and shocked quartz were used to confirm the newly identified, highly tectonized, Santa Fe impact structure near Santa Fe, New Mexico, USA. The age of the impact structure is poorly known, but is broadly constrained to have occurred in the interval from 1.7 Ga to the Mississippian. The structure is deeply eroded and highly deformed, with an estimated original size of 6-13 km (Fackelman et al., 2008). To provide more information about the distribution of shocked minerals and the intensity of shock metamorphism, a search was conducted for detrital shocked zircons at the Santa Fe impact structure. Recent studies in South Africa (Vredefort Dome), and Ontario Canada (Sudbury) have demonstrated that detrital shock zircons can survive post-impact metamorphism, uplift, erosion, and distal sedimentary transport.

Three sediment samples near the shatter cone outcrop were collected in April 2011 and analyzed for the presence of detrital shocked zircon. Techniques used include grain picking, UV light imaging (short and long wavelengths), and SEM imaging. Secondary electron and backscattered electron (BSE) images were used to search for shock features such as planar fractures. BSE images show zircon grain morphologies range from euhedral to subhedral. A total of 300 detrital zircon grains were examined, and 1.5% (3 grains) exhibit possible, but unconfirmed, shock features. One grain contains fractures that are parallel and form sets, but are not strictly planar, while other grains have planar parallel fractures to the c-axis. As opposed to the planar fractures identified in other studies such as the Vredefort Dome, the fractures are not deeply etched. The fractures, if impact related, might result from low-shock pressure deformation. A grain with a granular textured pyramid was found, but the cause of the granularization remains uncertain. The microstructures found are suggestive, but cannot yet be confirmed as having an impact origin. Preliminary results from this search are equivocal; detrital zircons with microstructures resembling both planar fractures and granular texture were identified, however the expressions of these features on the surfaces of grains identified thus far are insufficient to confirm an impact origin.