New Mexico Geological Society Annual Spring Meeting — Abstracts

This field study presents the physical, geochemical, geochronological, and mineralogical properties of the informally named Pemada Canyon dike that is found 15 km east of Aztec, New Mexico. The geographical isolation of this dike merits investigation into its relationship to nearby igneous features and Cenozoic stress orientations in the San Juan basin. Surface exposures of this 3.8 km-long dike are found intruded into thick sequences of basin fill arenites, wackes, and mudstones of the Eocene San Jose Formation. It is 76 to 173 cm-thick, has measured outcrop strikes ranging from 355° to 012° (average strike 003°), and dips ranging from 83° to 90°. There are at least two left-stepping en echelon features in the dike. Bulk geochemical analysis of the dike indicates a low silica (37%), low alumina (9.5%), and high magnesium (12%) composition similar to some transitional minette/katungite dikes of the Navajo volcanic field (Nowell, 1994) but dissimilar to nearby dikes of the Dulce dike swarm (Lipman and Zimmerer, 2019). XRD analysis indicates a primary mineral composition of diopside, phlogopite, sanidine, and pigeonite with minor chamosite, hinting at more mineralogical affinity to the Navajo volcanic field than to the Dulce dike swarm. ⁴⁰Ar/³⁹Ar dating of dike groundmass suggests an early to middle Miocene age (20 to 12 Ma).

The Pemada Canyon dike’s accordance with regional fracture patterns in the San Jose Formation and subjacent formations, along with its Miocene age, give new insight into the history of stress orientations in the San Juan basin—it shows that the least compressive stress in the Miocene was east-west and horizontal. This study’s dataset allows further interpretation of the timing and development of documented north-south fracture sets within stratigraphy of the San Juan basin including the Dakota Formation and Mesaverde Group (e.g., Lorenz and Cooper, 2003), Mancos Shale/Niobrara Formation (e.g., Nelson and Sonnenberg, 2021), and Ojo Alamo Formation (e.g., Hobbs and Thacker, 2021). The age and composition of the Pemada Canyon dike suggests relation to late-stage Navajo volcanic field magmatism as opposed to magmatism from the Platoro caldera complex and associated Dulce dike swarm of the Southern Rocky Mountain volcanic field. This interpretation of the Pemada Canyon dike extends the eastern boundary of the Navajo volcanic field approximately 50 km to the east into the interior of the San Juan basin.