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CONNELL, S.D., CATHER, S.M., MCINTOSH, W.C., DUNBAR, N., New Mexico Bureau of Mines and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801
KONING, D.J., 14193 Henderson Dr., Rancho Cucamonga, CA 91739
TEDFORD, R.H., American Museum of Natural History, New York, NY 10024

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, respectively, above the subjacent Espinaso Fm. A >700-m thick succession of conglomerate, sandstone, and minor mudstone, informally 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 ~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 (~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.

Connell, S.D., Cather, S.M., McIntosh, W.C., Dunbar, N., Koning, D.J., and Tedford, R.H., 2001, Stratigraphy of lower Santa Fe Group deposits in the Hagan embayment and near Zia Pueblo, New Mexico: Implications for Oligo-Miocene development of the Albuquerque basin [abstract]: New Mexico Geology, v. 23, n. 2, p. 60-61.

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