LITHOLOGIC CHARACTERISTICS AND AGE RELATIONSHIPS OF THE ANCHA FORMATION DEPICTED ON THE GEOLOGIC MAP OF THE AGUA FRIA QUADRANGLE, SANTA FE COUNTY, NEW MEXICO

Recent 1:24,000-scale geologic mapping of the Agua Fria 7.5-minute quadrangle near Santa Fe indicates that the Ancha Formation (Spiegel and Baldwin, 1963) consists mostly of loose to weakly consolidated, stream-deposited, slightly pebbly sand to slightly cobbly, pebble gravel. The Ancha locally consists of sandy silt, silty very fine to medium sand, and pebbly sandstone that is weakly cemented by calcium carbonate. These sediments form thin (about 0.3-4 m) lenses and lenticular beds that are rich in Precambrian granite clasts and feldspar. The Ancha commonly is mantled by thin sheetwash deposits and is poorly exposed. Westward-flowing streams originating in the nearby Sangre de Cristo Range, graded to the Rio Grande, deposited the Ancha on northwest- to southwest-dipping Miocene sediments of the Tesuque Formation. The Ancha thins from south to north across the quadrangle. The Ancha is about 60 m thick near the southern boundary of the quadrangle, but thins to about 3–6 m in the northern part of the quadrangle where it forms pediment deposits. These pediment deposits project westward beneath the base of hydromagmatic deposits at the base of basaltic lava flows of the Cerros del Rio volcanic field (CdRvf). Elsewhere deposition of the Ancha Formation predates, was contemporaneous with, and postdates lava flows and related deposits of the CdRvf. The upper 30 m or more of the Ancha in the western part of the quadrangle contain varying amounts of predominately reworked basaltic tephra about 0.5–10 mm in diameter. The tephra was produced by hydromagmatic explosions contemporaneous with the eruption of extensive early phase (2.6–2.7 Ma) basaltic lavas of the CdRvf. Reworked rhyolitic pumice (~1.6 Ma) erupted from the nearby Jemez Mountains is locally present in the upper part of the Ancha. Buried soils in the Ancha have thin, eroded, clay-enriched (Bt) horizons and stage I to III, carbonate-enriched (Btk, Bk, and K) horizons about 15–60 cm thick. These soils record depositional hiatuses of at least a few thousand to several tens of thousands of years. Geomorphic relationships in and near the Agua Fria quadrangle suggest a few to several tens of meters of incision of the Ancha Formation by the Santa Fe River since about 2.6–2.7 Ma.