New Mexico Geological Society Annual Spring Meeting — Abstracts

Geologic mapping astride Rio Grande rift-bounding normal faults near Abiquiu, NM, reveals evidence for structural deformation persisting from at least Eocene through Miocene, and specifically for reactivation of reverse faults of inferred Paleocene or Eocene age (probably Laramide) into Oligo-Miocene normal faults. The NNE-trending Valdez fault (new name) parallels the Canones fault, the trace of which lies 2 km east of the Valdez, in its hanging wall. Both faults dip steeply SE. A general pattern of sustained deformation is shown by repetition through time of angular unconformities in the ~60 km² region along the trace of these faults. The Eocene EI Rito Formation, Oligocene-lower Miocene Abiquiu Formation, and mid-late Miocene Lobato basalts all lie unconformably upon strata (Triassic through Miocene) that are progressively younger to the southwest. This repeated geometry suggests a roughly southward dip pervading in this area from at least Eocene through the Miocene. Furthermore, bedding orientation of Jurassic through Oligocene strata and thickness variations in Eocene and Oligocene strata indicate that Oligo-Miocene normal faults had reverse motion before the Oligocene. In the hanging wall of the Canones fault, inferred effects of fault-drag tilting are of opposite orientation within Jurassic and Oligocene strata. Lower Abiquiu strata dip 21° SE near the fault, shallowing to 5° SE farther away, indicating relative downward motion of the hanging wall. The Jurassic Entrada sandstone dips steeply toward the Canones fault near it (61 ° W), and shallows to 10° W farther away, indicating relative upward motion of the hanging wall. (EI Rito strata in places lie unconformably between these rocks but in places are absent, and their role is unclear.) Differences in thickness of Eocene and Oligocene strata across the two faults further suggest a change from contraction to extension. Eastward thickening of the Abiquiu Formation across the structures illustrates their normal motion during its deposition, and the EI Rito generally thickens westward across the faults, indicating reverse motion before or during its deposition.