Middle-Upper Miocene stratigraphy of the Velarde Graben, North-Central New Mexico: Tectonic and paleogeographic implications
D. J. Koning, S. B. Aby, and N. Dunbar
Geologic mapping and correlation of tephras in and near the Velarde graben supports the southward extension of the Cieneguilla member of Leininger (1982) into the graben, refines the stratigraphic relationships of various lithostratigraphic units, and has produced better estimates of vertical stratigraphic displacement on several graben faults. The Velarde graben is a 6-10 km-wide, northeast-trending extensional feature in the northern Española Basin of the Rio Grande rift. It coincides with a 19 km-long gravity low between the north tip of Black Mesa and the town of Hernandez to the southwest. In the general area of the Velarde graben, the Santa Fe Group is differentiated into seven lithostratigraphic units assigned to the Tesuque Formation. Some of these units were originally assigned to the Chamita Formation in previous studies. However, we propose abandoning the Chamita Formation, and reassigning its strata to the Tesuque Formation, because 1) strata assigned to the Tesuque Formation correlate into the Chamita type section, and 2) in the absence of the Ojo Caliente Sandstone Member it is not possible to map a contact between the Chamita and Tesuque formations with confidence. Vertical stratigraphic displacements along the border faults of the Velarde graben since 7.7-8.4 Ma range from as low as 65 m on the Rio de Truchas fault to 435 m on the Santa Clara fault. On the southern tip of Black Mesa, comparison of vertical slip rates for the Santa Clara fault over two time periods yields slightly higher vertical slip rate values for 3-8 Ma (48-56 m/Myr) compared to 0-3 Ma (35-48 m/Myr). On the gravity high separating the Velarde graben from another gravity low to the south, a vertical slip rate calculated for the Santa Clara fault gives 48-50 m/Myr for time after 9.9 Ma. Increasing slip rates on the Velarde graben faults in the late Miocene may have induced west-northwestward progradation of alluvial slope facies (lithosome A) derived from the Sangre de Cristo Mountains.
- Koning, D. J.; Aby, S. B.; Dunbar, N., 2004, Middle-Upper Miocene stratigraphy of the Velarde Graben, North-Central New Mexico: Tectonic and paleogeographic implications, in: Geology of the Taos Region, Brister, Brian S.; Bauer, Paul W.; Read, AdamS.; Lueth, Virgil W., New Mexico Geological Society, Guidebook, 55th Field Conference, pp. 359-373.