New Mexico Geological Society Annual Spring Meeting — Abstracts

Geologic studies and ⁴⁰Ar/³⁹Ar dating of deposits near Arroyo de la Vega de los Tanos (Tanos Arroyo), Hagan embayment, New Mexico, demonstrate late Oligocene onset of Santa Fe Group deposition in the Albuquerque basin. These volcanic-bearing deposits are locally derived from the adjacent Ortiz Mountains on the footwall of the La Bajada fault. Sparse quartzite and hornfels pebbles are locally present upsection. The base of this succession, informally called the Tanos fm, disconformably overlies the Oligocene Espinaso Fm at Espinaso Ridge, is ~253 m thick, and is subdivided into basal piedmont conglomerate, middle mudstone, and upper tabular sandstone members. A 25.41±0.32 Ma basaltic flow and 11.65±0.38 Ma ash are 9-29(?) m and ~970 m (estimate), respectively, above the subjacent Espinaso Fm. A >700-m thick succession of conglomerate, sandstone, and minor mudstone, infonnally named the Blackshare fm, conformably overlies the Tanos fm and is mapped at the lowest conglomerate bed. The Tuerto fm overlies these deposits with angular unconformity.

The interval represented by the Tanos Arroyo section is similar in age to the Zia Fm, exposed ~30-50 km to the west, allowing for comparisons of early rift sedimentation across the Albuquerque basin. Ash layers, geochemically correlated to ca. 10.8-11.3 Ma units of the Trapper Creek tephra, are ~40 m below the top of the Zia Fm (Cerro Conejo Mbr) in the Bernalillo NW quadrangle. The basal Zia Fm is late Arikareean in age (ca. 19 Ma) along the western basin margin where it unconformably overlies Eocene Galisteo Fm and Cretaceous strata. Thus, the base of the exposed Zia Fm sections may be up to 6 m.y. younger than the basal Tanos Arroyo section. Zia sections thicken eastward from ~420 m along the Ceja del Rio Puerco, to over 900 m in the Tamara #1-Y well, where the Zia Fm overlies probable subjacent volcanic-bearing sediments. These stratigraphic relationships suggest that the Albuquerque basin and Hagan embayments probably began as east-tilted fault-block basins.