Redefinition of the Ancha Formation and Pliocene–Pleistocene deposition in the Santa Fe embayment, north-central New Mexico D. J. Koning, 14193 Henderson Dr., Rancho Cucamonga, CA 91739; S. D. Connell, N.M. Bureau of Geology and Mineral Resources, Albuquerque Office, New Mexico Institute of Mining and Technology, 2808 Central Ave., SE, Albuquerque, NM 87106; F. J. Pazzaglia, Lehigh University, Department of Earth and Environmental Sciences, 31 Williams Dr., Bethlehem, PA 18015 ; W. C. McIntosh, New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801 Abstract piedmont regions to the south and the gen- only about half of this unit is saturated erally degraded upland regions to the north. with ground water, it is as much as ten The Ancha Formation contains granite-bear- times more permeable than the more con- ing gravel, sand, and subordinate mud Introduction solidated and cemented Tesuque Forma- derived from the southwestern flank of the tion (Fleming, 1991; Frost et al., 1994). Sangre de Cristo Mountains and deposited on a streamflow-dominated piedmont. The Ancha Formation underlies the Santa Ground water generally flows west These strata compose a locally important, Fe embayment and is a distinct lithostrati- through this saturated zone from the San- albeit thin (less than 45 m [148 ft] of saturat- graphic unit in the synrift basin fill of the gre de Cristo Mountains, but local recharge ed thickness), aquifer for domestic water Rio Grande rift. We use the term Santa Fe also occurs (Frost et al., 1994). Aquifer tests wells south of Santa Fe, New Mexico. Spiegel embayment in the physiographic sense for from various consultant reports listed in and Baldwin (1963) originally defined a par- the southern arm of the Española Basin Koning and Hallett (2000) estimate tial type section for the Ancha Formation that extends south of the city of Santa Fe, hydraulic conductivity values from 2 to using a 49-m-thick (16-ft-thick) exposed New Mexico (Fig. 1). There, a west-sloping 130 ft/day (7.1 x 10-4 to 4.6 x 10-2 cm/s) in interval of weakly consolidated, subhorizon- piedmont along the southwestern flank of the Ancha Formation and from 1 to 20 tal, arkosic strata on the southwest slope of -4 -3 Cañada Ancha. However, new geologic the Sangre de Cristo Mountains overlies a ft/day (3.5 x 10 to 7.1 x 10 cm/s) in the mapping, sedimentologic field studies, and north-plunging syncline developed in pre- underlying Tesuque Formation. geochronologic data indicate that the Ancha Pliocene strata (Biehler, 1999; Grant, 1999; This paper redefines the Ancha Forma- Formation should be restricted to the upper Koning and Hallett, 2000). The Santa Fe tion type section in light of new mapping, 12 m (39 ft) of Spiegel and Baldwin’s type embayment is bounded by the granite- sedimentologic, and geochronologic data. section, with the underlying strata being cor- dominated Sangre de Cristo Mountains to We also designate four reference strati- relative to the upper Tesuque Formation. the east, Galisteo Creek to the south, the graphic sections and summarize sedimen- Because this 12-m (39-ft) interval is not well Cerrillos Hills to the southwest, basalt- tologic, surficial, and stratigraphic charac- exposed at the type section, we designate teristics. In closing, we discuss the age of four new reference sections that illustrate the capped mesas of the Cerros del Rio vol- textural variability of the Ancha Formation canic field to the northwest, and the dis- the Ancha Formation and possible tectonic and its stratigraphic relationship with other sected upland underlain by the Tesuque and climatic influences on its aggradation. rock units. Formation (Miocene) north of the Santa Fe Regional geology and previous work New 40Ar/39Ar data help constrain the age River, called here the Santa Fe uplands of the Ancha Formation. A bed of rhyolite (Figs. 1, 2). This piedmont ranges from The southern Española Basin contains sev- lapilli (1.48 ± 0.02 Ma) that is temporally cor- approximately 1,830 to 2,190 m (6,004– eral lithostratigraphic units relevant to the relative to one of the Cerro Toledo events is 7,185 ft) in elevation and has been incised Ancha Formation. The Tesuque Formation recognized near the top of the section south by drainages associated with the Santa Fe underlies most of the Ancha Formation of Santa Fe, New Mexico. A fluvial deposit and is a pinkish-tan, arkosic, silty sand- interpreted to be inset against the Ancha For- River, Galisteo Creek, and the Rio Grande. mation contains lapilli dated at 1.25 ± 0.06 The Ancha Formation was originally stone with minor conglomerate and silt- Ma. These dates indicate that deposition of proposed by Spiegel and Baldwin (1963, stone that is commonly exposed in the cen- the Ancha Formation generally ended dur- pp. 45–50) for arkosic gravel, sand, and tral and eastern parts of the Española Basin ing early Pleistocene time. However, there is silt, inferred to be from late Pliocene to north of Santa Fe. Galusha and Blick (1971) evidence suggesting that aggradation con- Pleistocene in age, that lie with angular proposed the Chamita Formation for Santa tinued into the middle or late Pleistocene in unconformity upon moderately tilted Fe Group strata of Miocene age that uncon- mountain-front canyons east of the Santa Fe Tesuque Formation in the regional vicinity formably lie above the Tesuque Formation embayment. The age of the basal Ancha For- of Santa Fe. Spiegel and Baldwin (1963) north of the Santa Fe embayment. The mation is diachronous and ranges from Chamita Formation is generally brown or ~2.7–3.5(?) Ma in the western Santa Fe included the Ancha Formation as the embayment to ~1.6 Ma in the eastern embay- uppermost unit of the Santa Fe Group in gray sand with subordinate gravel of ment near the Sangre de Cristo Mountains. the Santa Fe area. The upper boundary of mixed provenance; the gravel contains sig- The Pliocene–early Pleistocene aggrada- the Ancha Formation in most of the Santa nificant quartzite and other metamorphic tion that formed the Ancha Formation in the Fe embayment was defined by the Plains clasts mixed together with granite, sedi- Santa Fe embayment occurred elsewhere in surface, a formerly extensive construction- mentary, and volcanic clasts (Galusha and the Española and Albuquerque Basins, sug- al surface preserved on interfluves (Spiegel Blick, 1971; Tedford and Barghoorn, 1993). gesting a regional climatic influence on dep- and Baldwin, 1963; Koning and Hallett, The Puye Formation is a coarse-grained, osition in the uppermost Santa Fe Group. A 2000). volcaniclastic, alluvial sequence on the rise in base level due to late Pliocene volcan- western margin of the Española Basin that ism and tectonism may be responsible for The Ancha Formation constitutes an the significant differences of accommodation important hydrogeologic unit in the Santa was shed primarily from the Tschicoma space, as reflected in deposit thickness, of the Fe embayment, where many communities volcanic center during the Pliocene–early Santa Fe embayment compared with the locally rely on ground water. Although Pleistocene (Manley, 1976b; Waresback, August 2002, Volume 24, Number 3 NEW MEXICO GEOLOGY 75 1986; Turbeville, 1986; Griggs, 1964; Wares- with the thick Ancha Formation deposits dated (Fig. 3). The lower part of these back and Turbeville, 1990; Dethier, 1997; south of the Santa Fe River. Likewise, exposures (units 1–7, Appendix 1) contains and Turbeville et al., 1989). An exposure in Miller et al. (1963) correlated ridge-cap- more than 37 m (121 ft) of reddish-yellow Bayo Canyon on the west side of the Rio ping gravel deposits in the northeastern to strong-brown, sandy pebble conglomer- Grande (Fig. 1) contains a ~5.3 Ma vitric Española Basin to the Ancha Formation. ate to pebbly sandstone. Deposits are loose tuff near the base of the Puye Formation; However, Manley (1979a) recommended to moderately consolidated with local this tuff disconformably overlies Tesuque abandoning this term for high-level gravel cementation by calcium carbonate. Beds Formation strata containing an ash dated deposits in the northeastern Española are subhorizontal. Gravel generally con- between 8.5 and 9.0 Ma and provides an Basin, including the Oso, Entrañas, and tains 93–99% granite, ≤ 1% quartzite, 5% age constraint for the base of the Puye For- Truchas surfaces. Manley (1976a, 1979b) amphibolite and gneiss, and < 1% volcanic mation (WoldeGabriel et al., 2001). The interpreted that the Ancha Formation clasts. Only unit 7 contains significant cob- Pliocene Totavi Lentil of the Puye Forma- underlies the Cerros del Rio basalts near bles and has as much as 3% quartzite. A 30- tion contains mostly rounded quartzite Cañada Ancha, although Kelley (1978) did cm-thick (12-inch-thick) pumice bed (unit gravel from an ancestral Rio Grande (Man- not delineate it there. 5) lies approximately 46 m (151 ft) above ley, 1976b; Dethier, 1997; Griggs, 1964; and the floor of Cañada Ancha. This bed is Purtymun, 1995). Waresback and Turbe- Methods interpreted to be a primary fallout because ville (1990) assign these gravels and related Exposures of late Cenozoic strata in or near it is composed of > 95% white, clast-sup- lacustrine deposits to the Totavi Forma- the Santa Fe embayment (Fig. 1) were ported, lapilli-size pumice. In contrast, tion. The Tuerto formation is a gravel and examined and described during the sum- reworked pumice in overlying beds is sub- sand piedmont deposit, approximately mers of 1999 and 2000. In addition, we ordinate to arkosic sediment and generally 13–27 m (43–89 ft) thick, described by examined outcrops along five stratigraphic coarse ash in size.