New Mexico Geological Society Annual Spring Meeting — Abstracts

Shatter cones, equal in size to those of the Sudbury (Ontario) and Vredefort (South Africa) impact sites, are exposed in road cuts along NM 475 in the Sangre de Cristo Mountains, near Santa Fe, New Mexico. Breccias in Proterozoic crystalline basement and MississippianPennsylvanian carbonates are being examined to determine whether their distribution is compatible with the development stages that have been worked out for large impact structures: Excavation stage: Ejecta blanket, fall-back breccia, in situ breccias of crater wall and floor. Breccia dikes and pseudotachylites, injected into the crater wall and floor subsequent to the shock wave responsible for shatter cones.

Enlargement stage: Landslide blocks and megabreccias (clasts >1 m), from collapse of crater wall.

Outcrops along NM 475, at progressively deeper structural levels, can be interpreted as a traverse from crater margin to subfloor. Distances are in miles, (tentative interpretations in bold). 0.0 Santa Fe, intersection of NM 475 and NM 590.

4.1 Curve: Matrix-supported megabreccia, crystalline clasts (>1 m), along a fault zone, overlain by unbrecciated Mississippian (?) carbonates. Near here: Small fractures (mm-wide) in carbonate clasts contain granite fragments with multiple fracture sets in quartz (possible planar deformation features, PDFs): (ejecta or wall/floor breccias). Paleozoic- Proterozoic contact appears to be depositional in most places.

4.3 Cross bridge: Elongated (max. 10+ m) steeply dipping brecciated granite clasts and m-size clasts of sheared mafic schist. Down creek 50 m: Fault-bounded breccia tower with decimeter-to-m clasts of angular granite and sparse rounded mafic schist. Clasts are supported by intracataclastic matrix: (collapse or wall/floor megabreccia).

5.9 Shatter cones, cm-to-m size, in mafic schist, granite gneiss, and granite pegmatite; best developed in granite gneiss: (subfloor).

PDFs (if confirmed) and shatter cones are widely accepted as impact criteria but tectonic interpretations of breccias cannot be ruled out. Dimensions of the proposed impact structure remain unknown but several tectonic events have undoubtedly distorted its geometry. Resolution of age question depends on interpretation of breccias at the contact between Paleozoic rocks and brecciated crystalline basement.