STRATIGRAPHY OF THE ANCHA AND TUERTO FORMATIONS (UPPER SANTA FE GROUP), SANTA FE AND HAGAN EMBAYMENTS, NORTH-CENTRAL NEW MEXICO

Field studies and 40Ar/39Ar dating of generally non-deformed upper Santa Fe Group strata in the Santa Fe and Hagan embayments indicate that revisions of the Ancha and Tuerto fm stratigraphic concepts are needed. The Ancha Fm was originally defined along the headwaters of Cañada Ancha (Spiegel and Baldwin, 1963); however, this type section is lithologically similar to the Pojoaque Mbr (Tesuque Fm) and contains an 8.48±0.14 Ma tephra -- a date relatively close to Tesuque Fm deposition. Although the type section was miscorrelated, the Ancha Fm has been mapped in a consistent manner throughout the Santa Fe embayment. Therefore, we retain this term and propose a new type section near Lamy, New Mexico. The Ancha Fm is a ~15-90(?) m thick succession of granite-bearing gravel, sand, and subordinate mud of a piedmont apron derived from the southern flank of the Sangre de Cristo Mts. Cerro Toledo rhyolite lapilli (1.48±0.02 Ma) is interbedded near the top. An interpreted terrace deposit containing upper Bandelier lapilli (1.25±0.06 Ma) is inset against the Ancha Fm. The base of the Ancha Fm lies below Cerros del Rio basalt flows (2.3-2.8 Ma). Thus, the Ancha Fm was generally deposited from the Pliocene to early Pleistocene.

Stratigraphic relationships along Galisteo Creek indicate that the Ancha and Tuerto fms are generally correlative. The Tuerto fm is commonly thinner (~13-27 m thick) than the Ancha Fm and contains Ortiz porphyry, subordinate hornfels, and rare granite gravel. A correlative limestone-bearing unit is recognized along the eastern slope of the Sandia Mountains. South of Cienega Creek, the exposed base of the Tuerto and Ancha fms is commonly well cemented with sparry calcite. Surface soils are typically partially stripped and exhibit Stage III carbonate morphology. These two formations are temporally correlative to the Puye Fm and to the uppermost Sierra Ladrones and Arroyo Ojito fms; the corresponding regional aggradation may perhaps be attributed to paleoclimatic factors.