New Mexico Geological Society Annual Spring Meeting — Abstracts

High-resolution aeromagnetic data recently collected over the southern Espanola basin show expressions of faults, igneous rocks, Precambrian crystalline rocks, and anthropogenic structures. A characteristic, rough-textured, aeromagnectic map pattern between the Cerillos Hills and Lamy reveals the distribution of the volcaniclastic Espinaso Formation under a thin cover of Ancha Formation. The rough aeromagnetic pattern ends at an east-west curvilinear boundary on the north that probably represents an abrupt downward ramp of the Espinaso Formation to the north below the younger, nonmagnetic Tesuque and Ancha Formations, the primary aquifers in the Santa Fe area. This boundary provides a southern limit of the thick clastic aquifer deposits. The rough aeromagnetic pattern is also limited on the eastern side near the middle of the Eldorado housing subdivision, indicating the buried eastern limit of the Espinaso Fonnation. High-amplitude aeromagnetic anomalies that correspond to magnetic Precambrian granite east of Eldorado extend into the eastern part of the subdivision where the granite can be inferred at depth. Delimiting the Precambrian granite under cover may help locate overlying Madera Limestone, a potential aquifer.

The aeromagnetic data also reveal an intriguing, triangular pattern located where the Espinaso Formation is mostly covered west of Eldorado. Where the cover is not present at the extreme southern extent of the triangular pattern, linear anomalies comprising the triangle correspond either to mapped faults or erosional boundaries of the Espinaso Formation. Preliminary interpretation of the anomalies using these relations suggest a NNW-trending, left-stepping en echelon pattern of down-to-the-east normal faults that probably formed during west-to northwest-directed extension. The pattern is compatible with minor, post-Oligocene, left-lateral movement on the adjoining Canoncito fault system. Other geologic evidence suggests right-lateral movement occurred on the Canoncito fault system during Laramide to post-Eocene time, and perhaps even earlier.