POSSIBLE IMPACT STRUCURE IN THE SANGRE DE CRISTO MOUNTAINS NEAR SANTA FE, NEW MEXICO: A PRELIMINARY REPORT

In 2005, probable shatter cones of cm-decimeter size were discovered by one of us (McElvain) in road cuts on the road from Santa Fe to the Santa Fe Ski Basin. They occur in Proterozoic biotite schist (Bauer et al., unpubl. prelim. geol. map, McClure Reservoir quad, NM Bur. of Geol.) as imperfectly formed clusters discordant to foliation, and as more perfectly formed cones in cross-cutting granite. The well preserved cones are very similar to features observed at Sudbury and the Gosses Bluff impact structures (HN). Nearby, a possible ejecta blanket up to 75 m thick is present over an area of at least 10 sq km, consisting of m-size clasts of Proterozoic crystalline rocks (some containing microcataclastites) in a soft (altered?) matrix. The breccia unconformably overlies the Proterozoic rocks (Read et al., unpubl. prelim. geol. map, Santa Fe quad). Over most of its extent, the breccia appears to be conformable with the overlying Pennsylvanian rocks (Sandia Fm.). Locally, pockets of Mississippian carbonates are preserved beneath Pennsylvanian strata and may contain interlayered portions of the breccia. The breccia is clearly post-Proterozoic and pre-Pennsylvanian, it may be Mississippian.

The possibility that the breccia is an ejecta blanket is intriguing, but alternative origins are being considered. The area has been involved in at least three major post-Proterozoic tectonic disturbances, each of which produced breccias: (1) Rise of the Ancestral Rockies (Mississippian-Pennsylvanian), (2) Laramide orogeny (late Cretaceous-early Tertiary) and (3) extension of the Rio Grande rift (Miocene to Quaternary). In addition, dissolution of Mississippian carbonates left residual breccias. The most likely alternative to an ejecta blanket would be a landslide or colluvial deposit, eroded off an Ancestral Rockies uplift. Less likely are breccias along major faults related to the Laramide orogeny or Rio Grande rift. Generation of these massive breccias from competent Proterozoic crystalline rocks by faulting at contacts with incompetent Paleozoic sedimentary rocks is unlikely. Clasts of crystalline rocks are unlikely components of a Mississippian karst. Detailed geologic mapping to resolve the nature of the breccia and the extent of the deposits is currently being complemented by a search for additional evidence of shock.