nmgv24n3: redefinition of the ancha formation and pliocene … · 2006. 3. 27. · sangre de cristo...

August 2002, Volume 24, Number 3 NEW MEXICO GEOLOGY 75

AbstractThe Ancha Formation contains granite-bear-ing gravel, sand, and subordinate mudderived from the southwestern flank of theSangre de Cristo Mountains and depositedon a streamflow-dominated piedmont.These strata compose a locally important,albeit thin (less than 45 m [148 ft] of saturat-ed thickness), aquifer for domestic waterwells south of Santa Fe, New Mexico. Spiegeland Baldwin (1963) originally defined a par-tial type section for the Ancha Formationusing a 49-m-thick (16-ft-thick) exposedinterval of weakly consolidated, subhorizon-tal, arkosic strata on the southwest slope ofCañada Ancha. However, new geologicmapping, sedimentologic field studies, andgeochronologic data indicate that the AnchaFormation should be restricted to the upper12 m (39 ft) of Spiegel and Baldwin’s typesection, with the underlying strata being cor-relative to the upper Tesuque Formation.Because this 12-m (39-ft) interval is not wellexposed at the type section, we designatefour new reference sections that illustrate thetextural variability of the Ancha Formationand its stratigraphic relationship with otherrock units.

New 40Ar/39Ar data help constrain the ageof the Ancha Formation. A bed of rhyolitelapilli (1.48 ± 0.02 Ma) that is temporally cor-relative to one of the Cerro Toledo events isrecognized near the top of the section southof Santa Fe, New Mexico. A fluvial depositinterpreted to be inset against the Ancha For-mation contains lapilli dated at 1.25 ± 0.06Ma. These dates indicate that deposition ofthe Ancha Formation generally ended dur-ing early Pleistocene time. However, there isevidence suggesting that aggradation con-tinued into the middle or late Pleistocene inmountain-front canyons east of the Santa Feembayment. The age of the basal Ancha For-mation is diachronous and ranges from~2.7–3.5(?) Ma in the western Santa Feembayment to ~1.6 Ma in the eastern embay-ment near the Sangre de Cristo Mountains.

The Pliocene–early Pleistocene aggrada-tion that formed the Ancha Formation in theSanta Fe embayment occurred elsewhere inthe Española and Albuquerque Basins, sug-gesting a regional climatic influence on dep-osition in the uppermost Santa Fe Group. Arise in base level due to late Pliocene volcan-ism and tectonism may be responsible forthe significant differences of accommodationspace, as reflected in deposit thickness, of theSanta Fe embayment compared with the

piedmont regions to the south and the gen-erally degraded upland regions to the north.

IntroductionThe Ancha Formation underlies the SantaFe embayment and is a distinct lithostrati-graphic unit in the synrift basin fill of theRio Grande rift. We use the term Santa Feembayment in the physiographic sense forthe southern arm of the Española Basinthat extends south of the city of Santa Fe,New Mexico (Fig. 1). There, a west-slopingpiedmont along the southwestern flank ofthe Sangre de Cristo Mountains overlies anorth-plunging syncline developed in pre-Pliocene strata (Biehler, 1999; Grant, 1999;Koning and Hallett, 2000). The Santa Feembayment is bounded by the granite-dominated Sangre de Cristo Mountains tothe east, Galisteo Creek to the south, theCerrillos Hills to the southwest, basalt-capped mesas of the Cerros del Rio vol-canic field to the northwest, and the dis-sected upland underlain by the TesuqueFormation (Miocene) north of the Santa FeRiver, called here the Santa Fe uplands(Figs. 1, 2). This piedmont ranges fromapproximately 1,830 to 2,190 m (6,004–7,185 ft) in elevation and has been incisedby drainages associated with the Santa FeRiver, Galisteo Creek, and the Rio Grande.

The Ancha Formation was originallyproposed by Spiegel and Baldwin (1963,pp. 45–50) for arkosic gravel, sand, andsilt, inferred to be from late Pliocene toPleistocene in age, that lie with angularunconformity upon moderately tiltedTesuque Formation in the regional vicinityof Santa Fe. Spiegel and Baldwin (1963)included the Ancha Formation as theuppermost unit of the Santa Fe Group inthe Santa Fe area. The upper boundary ofthe Ancha Formation in most of the SantaFe embayment was defined by the Plainssurface, a formerly extensive construction-al surface preserved on interfluves (Spiegeland Baldwin, 1963; Koning and Hallett,2000).

The Ancha Formation constitutes animportant hydrogeologic unit in the SantaFe embayment, where many communitieslocally rely on ground water. Although

only about half of this unit is saturatedwith ground water, it is as much as tentimes more permeable than the more con-solidated and cemented Tesuque Forma-tion (Fleming, 1991; Frost et al., 1994).Ground water generally flows westthrough this saturated zone from the San-gre de Cristo Mountains, but local rechargealso occurs (Frost et al., 1994). Aquifer testsfrom various consultant reports listed inKoning and Hallett (2000) estimatehydraulic conductivity values from 2 to130 ft/day (7.1 x 10-4 to 4.6 x 10-2 cm/s) inthe Ancha Formation and from 1 to 20ft/day (3.5 x 10-4 to 7.1 x 10-3 cm/s) in theunderlying Tesuque Formation.

This paper redefines the Ancha Forma-tion type section in light of new mapping,sedimentologic, and geochronologic data.We also designate four reference strati-graphic sections and summarize sedimen-tologic, surficial, and stratigraphic charac-teristics. In closing, we discuss the age ofthe Ancha Formation and possible tectonicand climatic influences on its aggradation.

Regional geology and previous workThe southern Española Basin contains sev-eral lithostratigraphic units relevant to theAncha Formation. The Tesuque Formationunderlies most of the Ancha Formationand is a pinkish-tan, arkosic, silty sand-stone with minor conglomerate and silt-stone that is commonly exposed in the cen-tral and eastern parts of the Española Basinnorth of Santa Fe. Galusha and Blick (1971)proposed the Chamita Formation for SantaFe Group strata of Miocene age that uncon-formably lie above the Tesuque Formationnorth of the Santa Fe embayment. TheChamita Formation is generally brown orgray sand with subordinate gravel ofmixed provenance; the gravel contains sig-nificant quartzite and other metamorphicclasts mixed together with granite, sedi-mentary, and volcanic clasts (Galusha andBlick, 1971; Tedford and Barghoorn, 1993).The Puye Formation is a coarse-grained,volcaniclastic, alluvial sequence on thewestern margin of the Española Basin thatwas shed primarily from the Tschicomavolcanic center during the Pliocene–earlyPleistocene (Manley, 1976b; Waresback,

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

76 NEW MEXICO GEOLOGY August 2002, Volume 24, Number 3

dated (Fig. 3). The lower part of theseexposures (units 1–7, Appendix 1) containsmore than 37 m (121 ft) of reddish-yellowto strong-brown, sandy pebble conglomer-ate to pebbly sandstone. Deposits are looseto moderately consolidated with localcementation by calcium carbonate. Bedsare subhorizontal. Gravel generally con-tains 93–99% granite, ≤ 1% quartzite, 5%amphibolite and gneiss, and < 1% volcanicclasts. Only unit 7 contains significant cob-bles and has as much as 3% quartzite. A 30-cm-thick (12-inch-thick) pumice bed (unit5) lies approximately 46 m (151 ft) abovethe floor of Cañada Ancha. This bed isinterpreted to be a primary fallout becauseit is composed of > 95% white, clast-sup-ported, lapilli-size pumice. In contrast,reworked pumice in overlying beds is sub-ordinate to arkosic sediment and generallycoarse ash in size. Pumice clasts in unit 5were dated at 8.48 ± 0.14 Ma using40Ar/39Ar methods (Table 1).

To the east of Cañada Ancha, depositsassigned to the informal coarse upper unitof the Tesuque Formation are generallyreddish-yellow to pink conglomerate andsandstone (Koning and Maldonado, 2001);these strata contain abundant granite withless than 1% quartzite, and paleocurrentdata indicate westward paleoflow from theSangre de Cristo Mountains. We assignunits 1–7 of the Cañada Ancha section tothe upper Tesuque Formation, principallybecause these units are lithologically simi-lar to the coarse upper unit of the TesuqueFormation described in Koning and Mal-donado (2001). Outcrops and clasts in hill-slope colluvium described between theseunits at the Cañada Ancha section andgood exposures of the coarse upper unit ofthe Tesuque Formation, located 3–4.5 km(2–3 mi) to the northeast, indicate physicalcontinuity between the upper TesuqueFormation and the Cañada Ancha section(Koning and Maldonado, 2001). In addi-tion, pumiceous beds that are similar tounits 3, 5, and 6 of the Cañada Ancha sec-tion extend northward along strike fromthe Cañada Ancha section into sandstoneand conglomerate strata of the coarseupper unit of the Tesuque Formation,where the average dip of the beds is 3° W(Fig. 2; Koning and Maldonado, 2001).Thus, mapping of these pumiceous bedssupports our interpretation that units 1–7correlate to the upper Tesuque Formationand are Miocene in age rather than corre-lating to the Ancha Formation. This is con-sistent with the 8.48 ± 0.14 Ma 40Ar/39Arage for unit 5 at the Cañada Ancha section.

Significant lithologic and provenancedifferences are recognized between units1–7 of the Cañada Ancha section and theMiocene Chamita Formation described byGalusha and Blick (1971) to the north. Inthe Cañada Ancha section, the preponder-ance of pink granitic clasts and paleocur-rent data indicate westward paleoflowfrom the Sangre de Cristo Mountains. In

with the thick Ancha Formation depositssouth of the Santa Fe River. Likewise,Miller et al. (1963) correlated ridge-cap-ping gravel deposits in the northeasternEspañola Basin to the Ancha Formation.However, Manley (1979a) recommendedabandoning this term for high-level graveldeposits in the northeastern EspañolaBasin, including the Oso, Entrañas, andTruchas surfaces. Manley (1976a, 1979b)interpreted that the Ancha Formationunderlies the Cerros del Rio basalts nearCañada Ancha, although Kelley (1978) didnot delineate it there.

MethodsExposures of late Cenozoic strata in or nearthe Santa Fe embayment (Fig. 1) wereexamined and described during the sum-mers of 1999 and 2000. In addition, weexamined outcrops along five stratigraphicsections. Sections were measured using aJacob staff and Brunton compass or Abneylevel. Descriptions of the type and refer-ence sections of the Ancha Formation andother Santa Fe Group deposits are in Con-nell et al. (in prep.); an abbreviateddescription of the Ancha Formation typesection at Cañada Ancha is in Appendix 1.The base or top of stratigraphic sectionswas surveyed using a hand-held, non-dif-ferentially corrected GPS unit, except incases where the section could be easilylocated on a topographic map (within anestimated ± 20 m [66 ft] horizontal dis-tance). Colors were described using Mun-sell (1992) notation. We described sedi-mentary structures and textures usingmethods discussed in Compton (1985),Dutro et al. (1989), and Ehlers and Blatt(1982). Sandstone composition was visual-ly estimated in the field and provisionallyclassified using the system of Folk (1974).Soil profiles were briefly described usingmethods and nomenclature of Birkeland(1999). We determined gravel clast compo-sition from sieved pebble concentrates(>9.52-mm diameter clasts) at selectedintervals or in situ at well-cemented out-crops. Paleocurrent directions wereobtained from clast imbrications alongwith crossbed and channel orientations.

Radioisotopic dating was performedusing the 40Ar/39Ar method at the NewMexico Geochronological Research Labo-ratory in Socorro, New Mexico. The dataare discussed and interpreted in Winick(1999) and Peters (2000). Older and lessprecise K/Ar and fission-track ages arefrom previously published studies (Bach-man and Mehnert, 1978; Manley andNaeser, 1977; Manley, 1976a).

Stratigraphy

Type section at Cañada AnchaThe Ancha Formation partial type sectionof Spiegel and Baldwin (1963) at CañadaAncha was re-measured, re-described, and

1986; Turbeville, 1986; Griggs, 1964; Wares-back and Turbeville, 1990; Dethier, 1997;and Turbeville et al., 1989). An exposure inBayo Canyon on the west side of the RioGrande (Fig. 1) contains a ~5.3 Ma vitrictuff near the base of the Puye Formation;this tuff disconformably overlies TesuqueFormation strata containing an ash datedbetween 8.5 and 9.0 Ma and provides anage constraint for the base of the Puye For-mation (WoldeGabriel et al., 2001). ThePliocene Totavi Lentil of the Puye Forma-tion contains mostly rounded quartzitegravel from an ancestral Rio Grande (Man-ley, 1976b; Dethier, 1997; Griggs, 1964; andPurtymun, 1995). Waresback and Turbe-ville (1990) assign these gravels and relatedlacustrine deposits to the Totavi Forma-tion. The Tuerto formation is a gravel andsand piedmont deposit, approximately13–27 m (43–89 ft) thick, described byStearns (1953). The Tuerto formation local-ly overlies the Ancha Formation south ofGalisteo Creek and is locally buried bybasalt flows of the Cerros del Rio volcanicfield northwest of the Cerrillos Hills.North of and within the Santa Fe uplands,thin, 1–10-m (3–33-ft) thick gravelly ter-race deposits occupy many topographiclevels from the ridge tops down to the RioTesuque (QTg in Fig. 2). The Cerros del Riovolcanic field contains basalt and andesiteflows, tephra, and phreatomagmatic sedi-ment deposited during the late Pliocene tothe early Pleistocene (Aubele, 1978; Bach-man and Mehnert, 1978; Manley, 1976a;David Sawyer, pers. comm. 2001), butbasaltic volcanism was primarily activebetween 2.2 and 2.8 Ma (Sawyer et al.,2001; WoldeGabriel et al., 1996).

Spiegel and Baldwin (1963) mapped theAncha Formation throughout the Santa Feembayment, estimated its thickness at30–90 m (98–295 ft), and extended it north-ward into the Cañada Ancha drainage, atributary of the Rio Grande (Fig. 1). AtCañada Ancha, they described and meas-ured a partial type section for the AnchaFormation in a region of notably poorexposures. Subsequent geologic mappingin and near the Santa Fe embayment (Readet al., 1999 and 2000; Koning and Hallett,2000; Koning and Maldonado, 2001; Bach-man, 1975; Johnson, 1975; Booth, 1977; Kel-ley, 1978) and subsurface investigations(Frost et al., 1994; Grant Enterprises, Inc.,1998; Grant, 1999) indicate that the AnchaFormation of Spiegel and Baldwin (1963)can be mapped in a relatively consistentmanner throughout the Santa Fe embay-ment and are in general agreement with themapping of Spiegel and Baldwin (1963).

Delineation of Spiegel and Baldwin’s(1963) Ancha Formation north of the SantaFe embayment has been more problematic.Kelley (1978) and Galusha and Blick (1971)assigned Ancha Formation to surficialgravel deposits overlying erosion surfacesin and north of the Santa Fe uplandsbecause of a presumed temporal overlap


FIGURE 1—Shaded-relief map of the Santa Fe embayment, Hagan embay-ment, and Santo Domingo sub-basin of the Albuquerque Basin. Major geo-graphic and physiographic features include the Cerros del Rio volcanicfield, Santa Fe uplands, Sangre de Cristo Mountains, Ortiz Mountains, andCerrillos Hills. Abbreviations of stratigraphic sections (shown by short reddash) include: Cañada Ancha (CA), La Cienega (LC), Arroyo Hondo (AH),

and Galisteo #1 and #2 (G1 and G2, respectively). Red x and label denotesfield exposure H-136. Red circles denote tephra sample localities (Table 1).Black line encloses area shown in Figure 2. Base created from 30-m DEMdata from the U.S. Geological Survey National Elevation Database (NED).Black outline shows the map area for Figure 2.


FIGURE 2—Simplified geologic map and northeast-southwest cross-sec-tion of the Santa Fe embayment and vicinity (modified from Koning andMaldonado, 2001; Read et al., 1999 and 2000; Koning and Hallett, 2000;Borchert and Read, 2002; Dethier, 1997; Kelley, 1978; Johnson, 1975; Bach-man, 1975; Spiegel and Baldwin, 1963; Sawyer et al., 2001). Drainages andmajor highways shown for reference. Short vertical lines on the cross-sec-

tion denote drill-holes (refer to Koning and Hallett, 2000; Read et al., 1999).On map, v-patterns denote strata in upper Tesuque Formation that containsparse beds of poorly sorted, brownish to grayish, pumiceous sandstoneand pebble conglomerate. Abbreviations of stratigraphic sections (shownby short green dash) include: Cañada Ancha (CA), La Cienega (LC),Arroyo Hondo (AH), and Galisteo #1 and #2 (G1 and G2, respectively).


contrast, the Chamita Formation contains adiverse assemblage of clasts, includingabundant quartzite, which indicates deri-vation of sediment from the north andnortheast (Galusha and Blick, 1971; Ted-ford and Barghoorn, 1993). These observa-tions, in addition to map data (i.e. Galushaand Blick, 1971, and Koning and Maldona-do, 2001), do not support correlation ofunits 1–7 of the Cañada Ancha section withthe Chamita Formation of Galusha andBlick (1971).

Overlying units 1–7 at the CañadaAncha section is 6 m (20 ft) of loose andvery poorly exposed, light yellowish-brown, gravelly arkosic sand containing3–5% rounded quartzite cobbles and peb-bles (unit 8, Appendix 1). A 2-m-thick (7-ft-thick) pumice bed underlying similar sedi-ment at the same stratigraphic position asunit 8, exposed 200–250 m (656–820 ft)downstream of the Cañada Ancha section(Manley, 1976b, p. 22; K. Manley, pers.comm. 2002), returned a zircon fission-track age of 2.7 ± 0.4 Ma (Table 1; Manleyand Naeser, 1977). This pumice is strati-graphically higher than unit 5 and pro-vides a lower age constraint for thePliocene sediment exposed near the typesection.

Units 9–11 do not contain quartziteclasts but have scattered pebbles and cob-bles of granite and basalt within silty sandand sandy silt (Appendix 1). These stratapossess very thin, tabular, even-beddedbasaltic lapilli from the Cerros del Rio vol-canic field that are interpreted to representfallout phreatomagmatic tephra (Koningand Maldonado, 2001). A 14-m-thick (46-ft-thick) basalt flow caps the sediment at theCañada Ancha section (unit 12). Althoughthis has not been dated, mesa-cappingbasalt and basaltic andesite(?) flows nearthis site have yielded K/Ar and 40Ar/39Arages of 2.0–2.5 Ma (WoldeGabriel et al.,1996; Manley, 1976a).

Units 8–9 are similar to deposits map-ped to the north (Ta on the maps of Koningand Maldonado, 2001, and Dethier, 1997).These deposits are generally loose toweakly consolidated, brownish to yellow-ish, granitic sand and pebbles with 5–20%cobbles. Approximately half of the cobblesand 1–5% of the pebbles in these depositsare quartzite. Although the difference issubtle and not ubiquitous, these depositsmay be distinguished locally from theunderlying Tesuque Formation by theirslightly more brownish color, looser over-all consistence, relatively higher cobblecontent, and their relatively higher (by atleast a factor of 2) quartzite clast content.The contact between these units and sub-horizontal beds of the Tesuque Formationis a planar disconformity that is not obvi-ous in the coarse strata. These depositsunderlie the flows and phreatomagmaticdeposits of the Cerros del Rio volcanicfield and in places are interbedded withCerros del Rio phreatomagmatic deposits.

Extensive basaltic talus and colluviumgenerally cover these sediment units, butone good exposure demonstrating thisinterbedded relationship is observed 3.1km (2 mi) downstream along CañadaAncha, across from the mouth of CalabasaArroyo (H-136 in Fig. 1). Along the west-ern edge of the Santa Fe embayment to thesouth, the Ancha Formation is interbeddedwith fluvially reworked volcaniclasticdeposits, probably phreatomagmatic inorigin, of the Cerros del Rio volcanic field(see below). Thus, units 8 and 9 and thenearby correlative deposits of unit Ta(Koning and Maldonado, 2001; Dethier,1997) occur in the same stratigraphic posi-tion and are of similar age to the AnchaFormation. We therefore interpret thatunits 8 and 9 and Ta represent a northerntongue of the Ancha Formation. Perhapsthese units could correlate to one of thediscontinuous, high-level, gravel-bearingterrace deposits mapped to the east in theSanta Fe uplands (QTg in Fig. 2).

Ancha Formation The Ancha Formation is predominantlysilty sand to pebbly sand with varyingamounts of gravel derived from the Sangrede Cristo Mountains. Common colorsrange from brownish yellow, yellowishbrown, and light yellowish brown to verypale brown (7.5–10YR hues). Strong-brown(7.5–10YR) clayey sand is subordinate andmostly found to the southwest. The sand issubangular to subrounded, poorly to wellsorted, and arkosic. The sand is predomi-nately fine grained in the southwest part ofthe embayment but is mostly medium tovery coarse grained to the north and eastnear the front of the Sangre de CristoMountains. Gravel is generally clast sup-ported, subrounded, and commonly con-tains 85–97% granite with sparse amphi-bolite, quartzite, and gneiss. Around theCerrillos Hills, sparse Oligocene intrusiveclasts derived from these hills are scatteredin the Ancha Formation within 2 km (1.2mi) from its contact with the Tuerto forma-tion. Bedding is tabular (particularly forsand and mud) or lenticular (particularlyfor gravels), and very thin to thick. Sedi-mentary structures are sparse and onlylocally recognized. Buried soils in theAncha Formation are generally not com-mon. Where exposed, these buriedintraformational soils generally containcalcic horizon(s) with stage II to III pedo-genic carbonate morphology that are over-lain by clayey Bt horizon(s). A veneer ofsheetwash or colluvium commonly coversthe Ancha Formation; good exposures tendto be limited to roadcuts and a few arroyowalls.

Muddy volcaniclastic deposits contain-ing altered and multicolored tephra andbasalt clasts are present as discontinuousor channel-shaped bodies as much as 4 m(13 ft) thick. These are locally recognized inthe western Santa Fe embayment along the

lower reaches of the Santa Fe River, ArroyoHondo, and La Cienega Creek (Figs. 2, 3).These volcaniclastic lithofacies probablyrepresent fluvially reworked phreatomag-matic deposits derived from the Cerros delRio volcanic field.

The Ancha Formation is weakly to non-deformed, and its lower contact is an angu-lar unconformity with tilted Miocene andPaleogene strata. The basal unconformityis locally well exposed in the bluffs northof Galisteo Creek and at the Arroyo Hondoreference section. Subsurface relief of thebasal Ancha contact where it overlies dis-tinctive Espinaso Formation near the west-ern margin of the embayment (Fig. 2; Kon-ing and Hallett, 2000) indicates that theunconformity is not everywhere a planar,beveled surface. Bedding is subhorizontalor dips less than 2° west-southwest. Whereexposed, the Ancha Formation is not obvi-ously cut by faults except for a locality900–950 m (2,953–3,117 ft) northeast of thetown of Lamy (Lisenbee, 1999).

Reference sections. We designate fourreference sections (Fig. 3) to supplementthe poorly exposed type section at CañadaAncha. These sections also illustrate thetextural variability of the Ancha Formationand its relationship with other rock units.Complete descriptive data is forthcoming(Connell et al., in prep.).

The Arroyo Hondo section (AH) is locat-ed on the north slope of Arroyo Hondo, 1.1km (0.7 mi) west of the exposed bedrock ofthe Sangre de Cristo Mountains (Figs. 1–3).The sediment is loose and composed pri-marily of lenticular-bedded, locally cross-stratified sand and gravel channel depositsthat unconformably overlie redder, tiltedsandstone beds of the Tesuque Formation.The gravel includes poorly sorted pebbles,cobbles, and boulders and is composed ofgranite with 1–3% amphibolite and gneiss.Fluvially recycled pumice is dispersed inthe sand and gravel in the lower 9.5 m (31ft) of the Ancha Formation. We correlatethis pumice to an outcrop of pumice lapillilocated 120 m (394 ft) to the southwest thatwas mapped by Read et al. (1999 and 2000)as the Guaje Pumice Bed (lower BandelierTuff). Near the top of the section is a well-developed soil possessing a calcic horizonwith stage III calcium carbonate morphol-ogy; this is overlain by 1.7 m (6 ft) of sandygravel (unit 6) whose top has likely beeneroded.

The La Cienega (LC) section includes 28m (92 ft) of Ancha Formation (Fig. 3). Thelower 16 m (52 ft; units 2–6) is composed ofsand, silt, and clay. One 3.7-m-thick (12-ft-thick) interval (unit 4) contains local thin tomedium, wavy, well-cemented beds. Thishorizontally bedded sediment is not corre-lated to the Tesuque Formation because anearby outcrop of Tesuque Formation,located 0.6 km (0.37 mi) to the northwest,has indurated gravel beds dipping 26°southeast. The upper 13 m (43 ft) of theAncha Formation contains sandy pebbles


and cobbles composed of granite with5–7% rounded quartzite and 1–2% amphi-bolite and gneiss. Locally within the upperAncha Formation are 2–4-m-thick (7–13-ft-thick), non-laterally extensive, channel(?)deposits of reworked volcaniclastic sedi-ment composed of massive muddy sandwith 10–50% altered tephra and volcanicpebbles. Above the Ancha Formation,across a covered contact, lies approximate-ly 3 m (10 ft) of Quaternary sand and grav-el terrace deposits (Qao1 of Koning andHallett, 2000).

Two sections, Galisteo #1 and #2, weredescribed along Galisteo Creek, where theAncha Formation clearly overlies tiltedPaleogene and Cretaceous strata with dis-tinct angular unconformity (Fig. 3, G1 andG2). The basal 10–13 m (33–43 ft) is a well-cemented pebbly sandstone and subordi-nate pebble to cobble conglomerate. Clastsare composed primarily of granite withminor limestone, schist, fine-grained toporphyritic volcanic and intrusive rocks,sandstone, siltstone, and quartz. The por-phyritic intrusive and volcanic rocks areprobably derived from the Ortiz Moun-tains and related Oligocene igneous cen-ters. At Galisteo #2, the upper 7 m (23 ft) ofstrata is weakly cemented and poorlyexposed. At section Galisteo #1, granite-bearing conglomerate and sandstone of theAncha Formation is overlain by 5 m (16 ft)of Tuerto formation containing sand withporphyritic andesite- and monzonite-bear-ing gravel. Paleocurrent directions meas-ured from trough cross-stratification in thelower Ancha Formation range from 270° to280°.

Members. The texture of the Ancha For-mation varies across the Santa Fe embay-ment, and this allows the differentiation oftwo informal members. The fine alluvialmember (fa) is located south of I–25 in thewestern part of the Santa Fe embayment(Fig. 2). There, the Ancha Formation is gen-erally composed of clayey to silty sand,with minor pebble lenses and coarse- tovery coarse grained sand beds. This mem-ber is represented by units 2 through 6 ofthe lower La Cienega reference section(Fig. 3). Sediment coarsens to gravellysand in the eastern part of the Santa Feembayment and is informally called thecoarse alluvial member (ca; Fig. 2), repre-sented by the Arroyo Hondo reference sec-tion and unit 7 of the La Cienega referencesection (Fig. 3).

Between the Santa Fe River and ArroyoHondo drainages, the coarse alluvial mem-ber extends to the western margin of theSanta Fe embayment and consists mostlyof sandy gravel with subequal proportionsof pebbles to cobbles. Boulders composeapproximately 2–5% of the total sedimentvolume in the west and as much as 20% inthe east. The westward extension of thecoarse alluvial member north of the finealluvial member (Fig. 2) may representdeposition by an ancestral Santa Fe River.

Tesuque Formations is difficult because oflithologic similarities between the twounits. However, analysis of drill-hole dataand surface geologic observations suggestthat differentiating these formations maybe possible locally based on slight texturalchanges, with the Tesuque Formationbeing slightly finer, and on pronouncedlocal color changes, such as observed in theArroyo Hondo reference section. In theupper reach of Gallina Arroyo, seismicrefraction studies suggest that the AnchaFormation is approximately 30–90 m(98–295 ft) thick (S. Biehler, pers. comm.1999). Ten to 11 km (6–7 mi) to the south,however, exposures along the bluffs northof Galisteo Creek near Lamy, New Mexico,are approximately 25–45 m (82–148 ft)thick (map data from Johnson, 1975, andLisenbee, 1999). The difference betweenthese two thickness estimates suggests anorthward thickening of the Ancha Forma-tion or may reflect the uncertainty in thegeophysical modeling. Between the Cerril-los Hills and the Sangre de Cristo Moun-tains, the thickness of the Ancha Formationis relatively easy to constrain using welldata because it overlies light- to dark-gray,lithic-rich sandstone and conglomerate ofthe Espinaso Formation; reddish-brown,clay-rich sandstone and mudstone of theGalisteo Formation; or limestone of theMadera Formation (Koning and Hallett,2000; Grant Enterprises, Inc., 1998). In thesouthwestern Santa Fe embayment, east ofthe Cerrillos Hills, interpretation of welldata indicates a thickness of 40–70 m(131–230 ft) for the Ancha Formation andapproximately 30 m (98 ft) of relief on itslower contact with the Espinaso Formation(Koning and Hallett, 2000; AmericanGroundwater Consultants, 1985). Similarsubsurface relief is also interpreted for thewestern embayment east of La Cienegabased on drill-hole data (Fig. 2). In thesoutheastern Santa Fe embayment near theSangre de Cristo Mountains, well data sug-gest as much as 90 m (295 ft) of Ancha For-mation (Grant Enterprises, Inc., 1998). TheTesuque Formation generally underlies theAncha Formation in the northern Santa Feembayment, and here it is difficult to confi-dently determine the thickness of theAncha Formation and the underlying pale-otopography.

Stratigraphic position. South of Galis-teo Creek, the Tuerto formation overliesapproximately 13 m (43 ft) of granite-bear-ing gravel and sand of the Ancha Forma-tion (Fig. 3, G1). Measurements of cross-stratification and channel margins indicatea westerly paleoflow, suggesting an ances-tral Galisteo Creek deposited the AnchaFormation at the Galisteo #1 section.Southern tributaries of Galisteo Creek nearCerrillos, New Mexico, drain the OrtizMountains, and this was probably trueduring the time of Ancha Formation aggra-dation. The composition of the sedimentdelivered by these tributary drainages

Quartzite clasts compose 1–15% of thegravel in this alluvium but are significant-ly sparser to the south.

Cementation zones. In the southernSanta Fe embayment, the Ancha Formationis divided into two zones based on cemen-tation. Here, the lower Ancha Formation iscalled the well-cemented zone because it iswell-cemented with sparry calcite and typ-ically forms ledges or cliffs (Fig. 3, G1 andG2). The upper unit is called the weaklycemented zone; it typically forms poorlyexposed slopes and is mostly covered bycolluvium. These zones are generally notmappable at a scale of 1:24,000 but are dis-tinctive in outcrop. The well-cementedzone is commonly recognized in the south-ern Santa Fe embayment near GalisteoCreek, where the Ancha Formation restsupon the Espinaso Formation, GalisteoFormation, and older strata. The weaklycemented zone is more laterally extensivethan the well-cemented zone, particularlyin the north where it lies upon deposits ofthe Tesuque Formation.

Plains surface. The top of the AnchaFormation is typically modified by erosionand is best preserved locally on broadinterfluves between entrenched drainagesin the Santa Fe embayment. The relativelyflat surface preserved on these broad inter-fluves has been called the Plains surface bySpiegel and Baldwin (1963). Soils devel-oped on the Plains surface locally exhibit< 25 cm (< 9.75 inches) thick, clayey Bt orBtk horizons underlain by 50 cm (20 inch-es) to over 100 cm (39 inches) thick calcicand siliceous Bk or Bkq horizons withstage II to III+ pedogenic carbonate mor-phology. Soil development is weakerwhere the Plains surface has been erodedor affected by younger deposition.

Near major drainages, such as ArroyoHondo, erosion and subsequent depositionof younger inset stream deposits havelocally modified the Plains surface. Forexample, exposures in railroad cuts nearupper Arroyo Hondo show loose graniticsand and gravel disconformably overlyingthe Ancha Formation. The upper 1.7 m (7ft) of sandy gravel at the Arroyo Hondoreference section (unit 6), overlying a well-developed soil with a stage III calcic hori-zon, may also represent a younger insetstream deposit (Fig. 3, AH). These thin(< 4-m-thick [< 13-ft-thick]) deposits arelithologically identical to the underlyingAncha Formation, locally strip the Plainssurface and associated soils, and can bedifferentiated from the Ancha Formationonly locally because of the lack of expo-sure. Radioisotopic age control is not avail-able for these younger inset deposits.

Thickness. The Ancha Formation rangesfrom 10 to 90 m (33–295 ft) in thicknessbased on geologic map, drill-hole, andseismic data. Surface exposures of theAncha Formation are commonly 10–40 m(33–131 ft) thick. Using drill cuttings andwell logs to differentiate the Ancha and


FIGURE 3—Type and reference stratigraphic sections of the Ancha Forma-tion in the Santa Fe embayment. Sedimentologic descriptions of the Caña-da Ancha type section (CA) are in Appendix 1. Descriptions of the La

Cienega (LC), Arroyo Hondo (AH), and Galisteo #1 and #2 (G1 and G2)sections are in Connell et al. (in prep.). Members labeled only for sectionsLC, AH, and G2; ca = coarse alluvial member; fa = fine alluvial member.


would be correlative to the Tuerto forma-tion. Thus, the arkosic sand and graniticgravel deposited by the ancestral GalisteoCreek, as observed at the Galisteo #1 sec-tion, would approximate the southernlimit of the Ancha Formation.

Near the Santa Fe River, approximately4–5.5 km (2.5–3.5 mi) north of La Cienega(Fig. 1), Ancha Formation strata withrounded quartzite pebbles are interbeddedwith basalt flows of the Cerros del Riofield. Furthermore, colluvial deposits over-lying these basalt flows contain locallyderived basaltic deposits mixed with fine-to coarse-grained sand composed of potas-sium feldspar and quartz interpreted to bederived from the Sangre de Cristo Moun-tains (Koning and Hallett, 2000). There isalso local granitic lag gravel on basaltflows approximately 3 km (2 mi) west ofLa Cienega (D. Sawyer, pers. comm. 2001).These last two observations indicate that

the Ancha Formation once extended overpart of the eastern Cerros del Rio volcanicfield, which is consistent with westwardprojection of the Plains surface (cross sec-tion in Fig. 2).

North of the Santa Fe River, the AnchaFormation pinches out against the TesuqueFormation but extends under the Cerrosdel Rio volcanic field (Figs. 1, 2). TheTesuque Formation in the Santa Feuplands, particularly at Tano Point, istopographically higher (by as much as80–100 m [262–328 ft]) than the northwardprojection of the Plains surface. The AnchaFormation extends north of the Santa FeRiver along the southwestern flank of theSanta Fe uplands to the head of CañadaAncha (Fig. 2). From here, it continuesnorthwestward toward the Rio Grande asa 10–20(?)-m-thick (33–66-ft-thick) depositunder the Cerros del Rio volcanic field, asdiscussed previously.

Pleistocene alluvial deposits with weak-ly developed soils are recognized near themouths of major arroyos draining the frontof the Sangre de Cristo Mountains. Thesedeposits overlie or partly grade into theAncha Formation but generally are not dis-tinguishable from the Ancha Formation.

Age of the Ancha FormationTephras collected at five localities fromearly Pleistocene deposits in the Santa Feembayment and one from a prominentpumice bed in the Cañada Ancha section(unit 5) were dated by the 40Ar/39Armethod and correlated to known tephrasin the region (Table 1; Winick, 1999; Peters,2000). The stratigraphic relationship forthree tephra localities in the Santa Feembayment are schematically illustrated inFigure 4. The five samples from the SantaFe embayment yield weighted mean agesbetween 1.25 ± 0.06 Ma and 1.67 ± 0.03 Ma.The results of the 40Ar/39Ar analyses indi-cate that the samples are age equivalent totephras derived from the neighboringJemez volcanic field and are interpreted toinclude the Guaje Pumice Bed (1.61 Ma;Izett and Obradovich, 1994), Cerro ToledoRhyolite (1.62–1.22 Ma; Spell et al., 1996),and Tsankawi Pumice Bed (1.22 Ma; Izettand Obradovich, 1994).

These tephra ages and stratigraphic rela-tionships indicate that the Ancha Forma-tion was aggrading in the north part of theembayment during early Pleistocene time.A bed of lapilli near the interchange of I–25and NM–14 (sample T-264; Table 1, Figs. 1,4) temporally correlates to one of the CerroToledo Rhyolite events. This tephra isapproximately 7–11 m (23–36 ft) below the

FIGURE 4—Schematic and generalized sketch showing stratigraphic relationships of tephra sam-ple localities in the northwestern Santa Fe embayment. Vertically exaggerated and not to scale.

Notes:Tephras analyzed by the New Mexico Geochronological Research Labora-tory (NMGRL; Peters, 2000, and Winick, 1999) by the laser total fusionmethod, except for samples DL-HR* and 73A2** (see below). Locations oftephra sample localities given in Figure 1. Geographic coordinates (Uni-versal Transverse Mercator, 1983 North American Datum) are rounded tothe nearest 5 m (16 ft).

Abbreviations include: not applicable (n.a.) and weighted mean (WM).Single asterisk (*) indicates sample analyzed by incremental-heating-age-spectrum method on hornblende grains. Double asterisk (**) indicates sam-ple analyzed by zircon fission-track dating method (Manley and Naeser,1977). (ψ) indicates K/Ca ratio measured from graphs because tabulateddata were not available. (ϒ) Qa = Quaternary alluvium inset into QTa. (∝)= data from Read et al. (1999 and 2000).

TABLE 1—Geochronologic data for late Tertiary and early Pleistocene tephras.

Sample Map Location, Description NMGRL WM Number of WM ± 2Nno. unit UTM, NAD83, No. K/Ca sanidine age (Ma)

(Fig. 1) (Fig. 2) zone 13S (m) ratio ± N crystals (MSWD)

T-318 Qaϒ N: 3,933,270 < 10 cm lapilli bed interbedded with pebbly 51220 24.8 ± 1.3 2 (n.a.) 1.25 ± 0.06E: 400,520 sand along south margin of Bonanza Creek.

T-264 QTa N: 3,940,320 White, 80 cm thick, pebbly pumice bed 51222 29.4 ± 6.7 7 (0.88) 1.48 ± 0.02E: 406,300 overlain by 50 cm of pumiceous sand in west

roadcut just south of I–25. BS-2∝ QTa N: 3,953,470 Intersection of I–25 and Richards Avenue. 9778, 9779, 34.5 ±12.4 38 (2.66) 1.61 ± 0.02

E: 404,420 About 1–6 m below eroded Ancha Fm. surface. 9780 BS-1∝ QTa N: 3,942,350 About 1 m below eroded Ancha Fm. surface north 9777 31.2 ± 16.5 13 (5.07) 1.67 ± 0.03

E: 415,900 of Arroyo Hondo.T-40 QTa N: 3,941,940 120 cm thick, white ash and pumice mixed with 51221 60.8 ± 12.7 14 (0.82) 1.63 ± 0.02

E: 404,305 sand and sandy gravel in roadcut by aquaduct. Overlies soil with Bt horizon.

DL-HR* Tt N: 3,956,680 White lapilli bed ~20 m below basalt flow 6049 0.06 ψ n.a. 8.48 ± 0.14E: 399,870 and ~9 m below QTa(?) at Cañada Ancha section.

73A2** QTa N: 3,956,893 2 m thick, white pumice interbedded in arkosic n.a. n.a. n.a. 2.7 ± 0.4E: 399,807 sand and granitic gravel near base of Ancha Fm.

200–250 m downstream of Cañada Anchastratigraphic section.


projection of the Plains surface. Located inthe same general stratigraphic interval,tephra beds that are age equivalent to theGuaje Pumice Bed are recognized in atleast two localities in the Santa Fe embay-ment (samples T-40, BS-2, and possibly BS-1; Table 1, Figs. 1, 4). These tephra areapproximately 4–18 m (13–59 ft) belowprojections of the Plains surface. Othertephra in similar stratigraphic positions tothese sampled beds, and thus probablyCerro Toledo Rhyolite or the Guaje PumiceBed, are scattered throughout the northernpart of the Santa Fe embayment. North ofthe middle reach of Arroyo Hondo,between samples T-264 and BS-2, thesetephra are overlain by as much as 12–15 m(39–49 ft) of Ancha Formation, indicatingsignificant deposition occurred at leastlocally after their emplacement. Thesetephra have not been recognized in thesouthern part of the embayment, suggest-ing that deposition ceased before 1.6 Ma inthe south, deposits younger than ~1.6 Mahave been eroded, or that tephra correla-tive to the Guaje Pumice Bed or Cerro Tole-do Rhyolite were not deposited in thesouth. In the lower reach of Bonanza Creeknear the west side of the Santa Fe embay-ment, pumice lapilli from a deposit of peb-bly sand, interpreted to be inset against theAncha Formation, is approximately 20–25m (66–82 ft) below the projected Plainssurface (sample T-318; Table 1, Figs. 1, 4). Asample from a single pumice lapilli bedhere yields a range of ages between 1.25and 2.6 Ma. Seventy-five percent of theanalyzed sanidine crystals of this youngesttephra yield ages between 1.48 and 2.6 Ma,indicating that significant recycling ofolder Ancha Formation deposits wasunderway by 1.25 Ma. Thus, the AnchaFormation generally ceased aggradingbetween 1.48 Ma and 1.25 Ma. The cessa-tion was probably a response to incision ofthis area by drainages associated with theSanta Fe River. The Santa Fe River mayhave experienced an increase in incisionrate at this time because it had succeededin cutting through relatively resistant rocksof the Cerros del Rio volcanic field, orbecause of incision of the Rio Grande atWhite Rock Canyon (Dethier, 1997; Dethierand Reneau, 1995).

We assume that the southern Santa Feembayment also ceased aggrading inresponse to incision during the early Pleis-tocene. However, this area is drained bytributaries to Galisteo Creek, such as Galli-na Arroyo, which may have a differentincision history than the Santa Fe Riverdrainage system.

Regional stratigraphic relationships andradioisotopic ages constrain the lower agelimit of the Ancha Formation to approxi-mately 2.7–3.5(?) Ma. The base of theAncha Formation is locally older than2.2–2.8 Ma flows of the Cerros del Rio vol-canic field (Stearns, 1979; Bachman andMehnert, 1978; Koning and Hallett, 2000;

WoldeGabriel et al., 1996; Manley, 1976a;Sawyer et al., 2001) and younger than the8.48 Ma tephra in the Tesuque Formationat the Cañada Ancha stratigraphic section.Near the mouth of Cañada Ancha, theAncha Formation (unit Ta of Dethier, 1997)lies below a basalt flow that gave an40Ar/39Ar age of 2.49 ± 0.03 Ma (Dethier,1997; WoldeGabriel et al., 1996). Here,these deposits are also interbedded withphreatomagmatic deposits of the Cerrosdel Rio volcanic field, which was active fol-lowing 2.8 Ma (Bachman and Mehnert,1978; WoldeGabriel et al., 1996; Sawyer etal., 2001). A 2.7 ± 0.4 Ma zircon fission-track age (Manley and Naeser, 1977) froma pumice bed probably correlative to thebasal Ancha Formation (K. Manley, pers.comm. 2002) is consistent with the abovedata and indicates that the Ancha Forma-tion near Cañada Ancha began aggradingat about 2.7 Ma.

In the northern Santa Fe embayment,there is evidence that the lower Ancha For-mation is diachronous, with the base beingolder in the west and younger to the east.North of La Cienega, at least 25 m (82 ft) ofAncha Formation underlies a 2.3–2.8 Mabasalt flow of the Cerros del Rio volcanicfield (Koning and Hallett, 2000), comparedto 12 m (39 ft) at the Ancha Formation typesection, so it is likely that the Ancha For-mation near La Cienega is older than atCañada Ancha, possibly extending back to3.5(?) Ma. Near the mountain front at theArroyo Hondo reference section, only 2 m(7 ft) of Ancha Formation underlies pumi-ceous sediment that correlates to the 1.6Ma Guaje Pumice Bed (Fig. 3; Read et al.,2000). Thus, the lower age of the AnchaFormation probably ranges from 2.7 to3.5(?) Ma in the northwestern Santa Feembayment but may be as young as ~1.6Ma to the east near the Sangre de CristoMountains.

DiscussionThe stratigraphic relationships at theCañada Ancha stratigraphic section, wherePliocene deposits correlative to the AnchaFormation unconformably overlie TesuqueFormation strata containing 8–9 Matephra, are similar to the previously men-tioned exposure in Bayo Canyon (see topof p. 78; Fig. 1). This similarity suggeststhat the associated unconformity is region-al in extent and that aggradation of theTesuque Formation continued to about 8Ma in the southern Española Basin.

We concur with most previous mappingof the Ancha Formation except for theinclusion of Oligocene monzonite or dior-ite gravels on the flanks of the CerrillosHills (e.g. Bachman, 1975). Rather, we pro-pose that the Ancha Formation be restrict-ed to deposits whose gravel contain morethan 5% granite clasts. This serves to clear-ly differentiate porphyry- and monzonite-bearing Tuerto formation, which was

deposited by streams draining the Cerril-los Hills and Ortiz Mountains, from thegranite-bearing Ancha Formation, whichwas deposited by streams draining thesouthern Sangre de Cristo Mountains.Within and north of the Santa Fe uplands,topographically high-level stream gravelsare preserved as terraces. The highest ter-race deposits probably predate the AnchaFormation (Koning and Maldonado, 2001),but lower terrace deposits may be partlyconcomitant with Ancha Formation aggra-dation (Koning and Maldonado, 2001).However, it is difficult to correlate theseterrace deposits among different drain-ages, and aside from a few dated terraces itis difficult to correlate these deposits withthe Ancha Formation. Thus, we concurwith Manley (1979a) that these uplandgravel deposits should not be assigned tothe Ancha Formation. This flight of rela-tively thin terrace deposits indicates long-term, post-late Miocene fluvial dissectionfor that landscape. In contrast, south of theSanta Fe River there appears to have beengeneral aggradation, as manifested by thethicker Pliocene–early Pleistocene depositspreserved there. The Pliocene–early Pleis-tocene deposits in these two areas reflectmarkedly different environments, one ofgeneral aggradation and the other of land-scape degradation, and this justifies theconcept of keeping the Ancha Formationconfined to the Santa Fe embayment andextending it northwest beneath the Cerrosdel Rio volcanic field.

Aggradation of the Ancha Formationprobably began around 2.7–3.5(?) Ma andcontinued into the early Pleistocene, whenregional incision occurred over much ofthe Santa Fe embayment. We interpret thatlarge-scale aggradation likely began as aresponse to a relative rise in base levelbecause the western, distal part of thebasin appears to have aggraded signifi-cantly before the eastern margin. Lateraggradation near the Pliocene–Pleistoceneboundary appears to have been concen-trated in the eastern Santa Fe embaymentand may have been more influenced bysediment supply and discharge factors inthe Sangre de Cristo Mountains than localbase level control. The relative rise in baselevel at the beginning of Ancha Formationaggradation was probably driven by acombination of Cerros del Rio volcanismand to a lesser degree by Pliocene tecton-ism. Basalt flow thickness, geologic maprelations, and 40Ar/39Ar ages in the TetillaPeak quadrangle (Sawyer et al., 2001) sug-gest that particular Cerros del Rio basaltflows, dated at ~2.6–2.7 Ma, may haveraised local base level by 20–50 m (66–164ft). The projected base of the Ancha Forma-tion is lower than the base of the basalts(Fig. 2, geologic cross section), so theseflows may not wholly account for Anchaaggradation, but the top of the basalts ishigher than the projected Plains surface.Moreover, Cerros del Rio volcanism proba-


FIGURE 5—Correlation and comparison of stratigraphic units used in thisreport to stratigraphic sections in the Española and Albuquerque Basins.QTg = Quaternary and Tertiary high-level gravel deposits, Qbt = TsankawiPumice Bed, Qbg = Guaje Pumice Bed. Ages of Tsankawi and GuajePumice Beds from Izett and Obradovich (1994). Age followed by (*) from

WoldeGabriel et al. (2001); ages followed by (**) from McIntosh and Quade(1995); age followed by (γ) from Smith (2000). Pattern composed of diago-nal lines represents missing stratigraphic section and an associated uncon-formity.


bly did not affect deposition in the south-ernmost Santa Fe embayment becausepaleo-drainages there likely flowed intothe ancestral Galisteo Creek and thenaround the basalt flows and into the Albu-querque Basin (Fig. 2).

Neogene tectonism appears to haverestricted the aggradation and preserva-tion of thick late Pliocene–early Pleistocenealluvial deposits to the Santa Fe embay-ment. The Santa Fe uplands to the northwere likely a topographic high duringAncha Formation deposition, whichexplains why the Ancha Formation pinch-es out northward against them. Theuplands may have formed by differentialuplift of the rift hanging wall during thelate Miocene–Pliocene (Koning and Mal-donado, 2001; Smith and Roy, 2001).Between 2.7 and 0.5 Ma, as much as 500 m(1,640 ft) of offset occurred along the mid-dle part of the La Bajada fault, and thefootwall experienced relative uplift(Sawyer et al., 1999). Consequently, theAncha Formation thins against the Espina-so and Galisteo Formations in the footwallwest of La Cienega (Fig. 2, geologic crosssection).

The Pliocene–Pleistocene Ancha, Puye,and Tuerto formations are generally grav-elly and unconformably overlie older, gen-erally tilted, and finer strata of the Santa FeGroup (Fig. 5). Pliocene sand and gravelsuccessions are recognized in the upperArroyo Ojito and Sierra Ladrones Forma-tions in the adjacent Albuquerque Basin(Fig. 5; Connell et al., 1999; Smith andKuhle, 1998; Maldonado et al., 1999). Thecoarse-grained character of these upperSanta Fe Group units, which are exposedover a wide area and in different structur-al basins, suggests that significant regionalerosion and accompanying depositionmay have been triggered by paleoclimaticfactors that increased stream discharge andcompetence. However, we interpret thatlate Tertiary tectonic factors and emplace-ment of the Cerros del Rio volcanic fieldwere largely responsible for the significantdifferences of stratal thickness between theSanta Fe embayment versus the piedmontregions near the Ortiz Mountains south ofthe embayment and the generally degrad-ed upland regions to the north of theembayment.

ConclusionsRecent mapping and sedimentologic studyin and near the Santa Fe embayment, alongwith 40Ar/39Ar dating of tephra layers,demonstrate that the lower three-quartersof the Ancha Formation partial type sec-tion of Spiegel and Baldwin (1963) encom-passes the Tesuque Formation. Conse-quently, we restrict the type section toinclude 12 m (39 ft) of arkosic sedimentlocated above the Tesuque Formation andbelow deposits and flows of the Cerros delRio volcanic field. We also propose a gran-

ite clast content of 5% for differentiation ofthe granite-rich Ancha Formation from thegranite-lacking Tuerto formation. Paleocli-matic influences may be responsible fordeposition of regionally extensive, coarse-grained, Pliocene–early Pleistocene strata.However, preservation of thick deposits inthe Santa Fe embayment may be attributedto a relative rise of local base level becauseof Cerros del Rio volcanic flows and Neo-gene tectonism. Based on 40Ar/39Ar datingof tephras, the Ancha Formation aggradeduntil regional incision occurred between1.25 and 1.48 Ma.

Outcrop and subsurface data suggestthat the Ancha Formation is generally10–90 m (33–295 ft) thick and pinches outagainst older and topographically higherTesuque Formation deposits north of theSanta Fe River. The suite of thin terracedeposits in the uplands north of the SantaFe River should not be included in theAncha Formation because of correlationambiguities. Instead, the Ancha Formationshould be restricted to Pliocene–earlyPleistocene, arkosic, basin-fill sedimentwithin the Santa Fe embayment andextending beneath the Cerros del Rio vol-canic field.

Acknowledgments

The authors thank Steve Maynard, JohnHawley, John Rogers, Adam Read, DaveLove, Charles Stearns, David Dethier, JackFrost, David Sawyer, Florian Maldonado,Ralph Shroba, and Charles Ferguson fordiscussions on the geology of the Santa Feand Hagan embayments. Dave Love col-lected the tephra sample at the CañadaAncha section. We especially thank KimManley and Shari Kelley for revisiting out-crops in Cañada Ancha. Discussions withGary Smith greatly improved an early ver-sion of this paper. This study was funded,in part, by the New Mexico Statemap Pro-gram of the National Cooperative Geolog-ic Mapping Act of 1992 (P. W. Bauer, Pro-gram Manager), and New Mexico Bureauof Geology and Mineral Resources (P. A.Scholle, Director). We thank Mr. GaylonDuke, Ms. Barbara Harnack, and theBonanza Ranch for allowing access ontotheir lands for this study. We also thankJack Frost for sharing drill-hole andaquifer data for the Santa Fe area, andCharles Hibner for soil-profile data. GlenJones prepared the shaded relief base forFigure 1. Gary Smith and Spencer Lucasreviewed the manuscript.

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Unit Description Thickness(m) (ft)

Basalt and pyroclastic deposits of the Cerros del Rio volcanic field (25 m):

12. Basalt; basalt flows underlain by 60–70 cm of planar lam-inated to thinly bedded, welded, sand-size lapilli; somebeds are wavy; lower 0.5–3 m of basalt flows are flowbreccia; sharp lower contact; no quartzite or graniticclasts are seen on top of the flows.

11. Sandy silt, very pale brown (10YR 8/2); massive; con-tains 5% very fine to medium basaltic pebbles with 10%granitic pebbles; paleosol with a stage II calcium carbon-ate horizon is present 60–90 cm below top of unit; upper60 cm is light brown (7.5YR 6/4) and contains a possibleBt soil horizon overprinted by contact thermal alterationfrom overlying basalt; sharp lower contact. Interpretedas a bioturbated eolian deposit.

10. Basaltic lapilli, black; in very thin, even, and tabularbeds or planar laminations; some beds are wavy; sand-size lapilli may be welded; minor bombs as much as 25cm in diameter are on top of some beds and form loaddepressions on the underlying sediment; lapilli arepoorly to well sorted and may contain as much as 20%granitic pebbles and sand grains; sharp lower contact.Interpreted as ash-fall deposits related to phreatomag-matic eruptions.

Ancha Formation (12 m):9. Silty sand with 10–15% pebbles, pale-brown (10YR 6/3);

massive and matrix-supported with 2–3% very thin tothin, planar beds or lenses of basaltic lapilli; pebbles arevery fine to medium, subrounded, and have ~40% gran-ite and ~60% basalt; sand is very fine to very coarsegrained, poorly sorted, lithic-rich (~4:6, feldspar:lithicsratio), and subrounded (lithic grains) to subangular(feldspar grains); abundance of granitic clasts decreasesto < 5% in the upper half of the unit; lower contact notwell exposed; loose to moderately consolidated. Thetops of two prominent buried paleosols are 89 cm and200 cm below the top.

8. Gravelly sand (very poorly exposed), light yellowish-brown (10YR 6/4); surface gravel are cobbles and peb-bles consisting of 3–5% quartzite, 25% granite, and 70%basalt clasts (some or all of latter is slough from overly-ing units); sand is subangular to subrounded, arkosic,and loose with strong effervescence in dilute hydrochlo-ric acid. Composition, stratigraphic position, and weak-ly consolidated nature suggest correlation to Ancha For-mation of Spiegel and Baldwin (1963). Basal contact isgenerally covered, but presumably disconformable.Gravel contains slight increase in quartzite compared tounderlying units.

Tesuque Formation (55 m):Note: Sand is very fine to very coarse, poorly sorted & subangular to

subrounded, unless noted otherwise. Interpreted as fluvialpiedmont deposits, unless noted otherwise.

7. Sandy cobble and pebble conglomerate, strong-brown toyellowish-red (5–7.5YR 5/6); very thin to thin and vaguebeds; gravel is clast supported, with subequal pebbles tocobbles and 1–3% quartzite, 5% amphibolite, < 1%basalt(?) clasts; sand has trace to 10% clay, is moderatelyto poorly sorted, and arkosic; sand has 5% lithics but nobasalt or pumice identified; scoured lower contact; looseto moderately consolidated. This is correlated toTesuque Fm. because of gross similarities with unit 4;however, its cobbles and quartzite % are characteristic ofPliocene deposits in the vicinity.

14.0 45.9

1.6 5.2

3.5 11.5

5.6 18.3

6.1 20.0

5.0 16.4

Measured section of Ancha Formation partial type section of Spiegel andBaldwin (1963). Measured and described along west slope of CañadaAncha, from floor of arroyo to top of cliff-forming basalt of the Cerros delRio volcanic field by D. J. Koning on October 17, 2000. Base at N:

3,956,720 ± 20 m, E: 399,955 ± 20 m (zone 13, NAD 83), Horcado Ranch 7.5-min quadrangle, Santa Fe County, New Mexico. Colors determined dry.Numerical unit designations established upsection, but listed in descend-ing stratigraphic order.

6. Pebbly sandstone and sandy pebble conglomerate; simi-lar to unit 4 except unit is mixed with varying amounts ofsand-size pumice (< 20%) and volcanic lithic (5–7%) frag-ments. Unit contains 10% slightly (< 5%) muddy and peb-bly sand, pink to very pale brown (7.5–10YR 7-8/3); thesebeds are lenticular (over 10s of meters distance) andmedium to thick (25–80 cm); mostly matrix supported;have 10–15% very fine to medium pebbles with ~60%granite and ~40% pumice; sand is poorly to moderatelysorted and contains 20% pumice, 10% lithics, and 70%arkosic grains; sharp but relatively planar lower contact;generally loose to moderately consolidated.

5. Pumice lapilli; white (5Y 8/1) pumice bed with scatteredgravel; probably lenticular and lacks sedimentary fabric;clast supported; a:b axis ratio is 9–11:7–9 cm; pumice ismixed with 1–3%, 0.5–6-mm-diameter clasts of graniteand rhyolite(?); lower contact is scoured; weakly to mod-erately consolidated; interpreted to be slightly fluviallyreworked. Sample of pumice taken near N: 3,956,680 m;E: 399,869 m (UTM zone 13S, NAD 83) is dated using the40Ar/39Ar method at 8.48 ± 0.14 Ma (NMGRL Lab#50512-01).

4. Pebbly sandstone and sandy pebble conglomerate, red-dish-yellow (5YR 6/6); lenticular and very thin to medi-um (2–35 cm) beds; gravel has about 5% cobbles and isclast supported, poorly to moderately sorted, and con-sists of 1% quartzite, 1–2% amphibolite, 97–98% graniticclasts, and trace dacite(?); sand is generally coarse to verycoarse, moderately sorted, and has 5–10% lithic frag-ments (mostly basalt, pyroxene(?), or olivine), 10–15%quartz, 10–15% plagioclase, and 60–75% potassiumfeldspar; 3% of sediment is well cemented by 2–15-cm-thick, discontinuous layers of calcium carbonate (whichare more common near the bottom of the unit); lowercontact is scoured and slightly wavy (~5 cm of relief onlower contact); loose to weakly consolidated. Trace ofsediment is yellowish-red (5YR 5/6) clayey sand.

3. Pebbly sandstone, very pale brown (10YR 7/4) andslightly (< 5%) muddy; bed is lenticular and internallymassive; clast and matrix supported; pebbles compose5–10% of sediment, are poorly sorted, and consist ofgranitic clasts; sand consists of 3% pumice, 30–50% lithicfragments (basalt(?), pyroxene(?), olivine), 10–15%quartz, 20% potassium feldspar, and 15–40% plagioclasegrains; moderately consolidated; lower contact slightlywavy and scoured (5–10 cm of relief).

2. Pebbly sandstone and sandy pebble conglomerate, light-brown (7.5YR 6/4); beds are thin to medium (9–14 cm);gravel is clast supported, subangular to subrounded,poorly sorted, and consists of granitic clasts with tracebasalt(?), trace amphibolite, and trace quartzite; arkosicsand; lower contact not exposed; loose.

1. Covered; probably same as unit 2 except for one exposedbed of pumiceous sandstone about 11 ± 1 m above thebase. This bed is a 30-cm-thick pebbly sand, pale-brownto light yellowish-brown (10YR 6/3–4), poorly sorted,and matrix-supported; about 10–15% granitic pebbles; alithic-rich feldspathic wacke sand with 10–15% pumicegrains. Base not exposed.

Unit Description Thickness(m) (ft)

3.7 12.0

0.3 1.0

27.0 88.6

0.4 1.3

0.5 1.6

18.0 59.1

Appendix 1

nmgv24n3: redefinition of the ancha formation and pliocene … · 2006. 3. 27. · sangre de cristo...

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