Stratigraphy,age, and rates of deposition ol the Datil Group (Upper -Lower0ligocene), west-central New Mexico bySteren M. Cather,Willian C. Mclntosh, and Charles E. Chapin,New Mexico Bureau of Minesand Mineral Resources, Socorro, NM 87801

Introduction Stratigraphyand contacts The Datil Group, formerly called the SpearsFormation, comprises The term Datil Formation was coined by Winchester(1920), but a seriesof volcaniclasticrocks, lava flows, and ash-flow tuffs that its usagehas since evolved considerably(see, for example,Elston, crops out in a broad, west-trending swath of discontinuous expo- 1976;Cather, 1986).In this report, we employ the nomenclatureof suresin west-centralNew Mexico.The Datil Group is the oldestunit Osburn and Chapin (1983),who restrict usage of the Datil Group in the_northern Mogollon-Datil volcanic field, and ranges in thick- to the volcanic and volcaniclastic rocks that occur belowHells Mesa ness from more than 1 km to about 300 m where it onlaps late . In addition, we divide the Datil Group into two informal units, Laramideuplifts. This report summarizespart of a doctoral disser- the lower Datil and the upper Datil (Fig. 2). tation on the Datil Group (Cathea 1986)and incorporates15 new The lower Datil is dominatedby andesiticvolcaniclastic rocks that {rdiometric datesby Mclntosh et al. (1986),Mclntosh (in progress), are characterizedby phenocrystic plagioclase,amphibole, and ti- C. E. Chapin (unpublished),and Cather (1986).To facilitatediscus- tanomagnetite(+ biotite). Silicacontent in Datil andesitestypically sion of various Datil Group exposureswithin the study area, we rangesfrom 58 Io 64 wt. percent.Throughout most of the northern have divided the outcrop belt into geographicsegments'(Fig 1). Mogollon-Datil volcanic field, the lower Datil Group overlies late Laramide,synorogenic red beds of the EoceneBaca Fbrmation. The contactbetween theseunits is defined as the first up-sectionoccur- renceof megascopicvolcanic detritus (Cather,1980), and is typically marked by a pronounced change from the red colors of the Baca Formationto the grays,buffs, purples, and reddish-graysof the Datil Group. The Baca-Datilcontact is everywhere conformableand gra- dational, exceptwhere early Datil fluvial scouringhas createdlocal, apparently sharp contacts.Where deposited upon late Laramide uplifts, the Datil Group typically overliesstrata of Pennsylvanianor Permianage with angular disconformity;such contactrelations are exposedin the Lemitar Mountains, ChupaderaMountains, southern San Mateo Mountains, MagdalenaMountains, BlackRange, and at Horse Mountain (Fig. i7. In nearly all exposuresthat were examined,non-volcanic detritus identical tb that of the BacaFormation is present throughout most or all of the lower Datil Group (Fig.2). This detritus,-consisting dominantly of fragments of upper Paleozoiclimestone, sandstone, and siltstonewith subordinateamounts of Precambrianlithologies, representsthe continuationof Baca-stylesedimentation during early Datil volcanismand is important becauseit indicatesthe persistence of late Laramidepositive areasfollowinq the onset of volcanismin west-centralNe#Mexico. Abundanceof"non-volcanic detritus in the lower Datil rangesfrom individual bedsof nearlypure Paleozoicand Precambriandetritus to traceamounts of such lithologiespresent as scatteredclasts in volcaniclasticconglomerates. The upper Datil Group consistsof a fundamentallybimodal suite of basalticandesite lava flows, rhyodaciticto rhyolitic ash-flowtuffs, and volcaniclasticrocks derived from theselithologies. Basaltic an- desitesrange in silica content from about 52 to 56 wt. percent, and are characterizedbyphenocrysts of piagioclase,clinopyroxene, and titanomagnetite(+ olintr'te,amphibole). Basalticandesite lavas are widespread in the upper Datil; composite thicknessfor such lava flows locally exceeds150 m (LaRoche,1981). Four ash-flow tuffs of FIGURE l-Location map showing study area, outcrops of Datil Group and regional extent occur in the upper Datil Group. Using the termi- equivalent units, basins of Rio rift, Grande and selected structural and seo- nology of Osburn and Chapin (1983),these are the Datil Well Tuff, morphic features. Intrabasin zonesactive during Datil fault time are shown by Rock House Tuff, Blue Tuff, the heary lines: H.f .2., Hickman fault zone; R.l.f .2., Red Lake fault zone; p.f .2., Canyon Canyon and tuff of Granite Puertecito fault zone. Segmentsof outcrop belt are divided by fine dashed lines Mountain. Unlike the lower Datil Group, the upper Datil does not and are indicated by encircled numbers: 1., Quemado; 2, Alegros Mountain contain non-volcanicdetritus. arca;3, Datil Mountains-Crosby Mountains; 4, Gallinas Mountains area; 5, The contactbetween the lower and upper Datil Group is grada- northern Bear Mountains; 5, Tres Montosas area;7, Magdalena Mountains; tional and typically occurs within the volcaniclasticrocks that un- 8, Nipple Mountain-Granite Mountain; 9, Lemitar Mountiins; 10, Joyita Hills; derlie the basaltic-andesitelava flows and silicic ash-flow tuffs of the 11, northern Jornada del Muerto. Geomorphicfeatures: H, Horse Mountain; upper Datil (Fig. 2). This contact is defined as the first up-section SAB, San Agustin Basin; D, Datil Mountains;'C, Crosby Mountains; Saw, occurrenceof greaterthan 50% basaltic-andesitedetritus. The tran- Sawtooth Mountains; G, Gallinas Mountains; B, Bear Mountains; M, Mag- sition from andesitic to bimodal volcanism, as representedby the dalena Mountains; L, Lemitar Mountains; Ch, Chupadera Mountains; JH, lower Datil-upper Datil contact,marked a time of important loyita Hills; SM, San Mateo Mountains; SC, Sierra Cuihllio; BR, Black Range; changes in tectonism,sedimentation, and petrogenesisin west-central SB, Socorro Basin; AB, Albuquerque Basin; fMB, Jornada del Muerto Basin; New PB, Palomas Basin. Measured and sampled sections are denoted bv solid tri- Mexico(Cather, 1986). The upper contactof the Datil Group is placed angles, | (Cather, 1986). Base map modified from New Mexico'Geological at the base of the Hells Mesa Tuff (-32 Ma), a thick, regionally Society (1982). extensiveash-flow tuff in the northern Mogollon-Datil field. The

August 1987 New MexicoGeology Datil-Hells Mesa contactis commonly concealedby talus due to the Location of eruptive centets cliff-formingnature of the Hells MesaTuff. Whereeiposed, however, Although dikes of probable late Datil age have been documented the contact is sharp and typically conformable, except where the in the Hills (Spradlin, 1976)and the northern Bear Mountains, paleovallevs. Joyita Hells Mesa Tuff fills local exposuresof Datil Group eruptive centers have not been docu- During deposition of the Eocenebucu Formation, two intermon- mented anywhere in the northern Mogollon-Datil volcanic field. tane basins (the Bacaand Carthage-LaJoya basins)and their sur- However, the distribution of sedimentary facies and paleoflow in rounding uplifts dominated the late Laramide paleogeographyof Datil volcaniclasticrocks provides important constraintson the lo- west-centralNew Metco (Cathea 1983;Cather and fohnson, 1984, cationof associatedvolcanic centers. For example,paleocurrent and 1985).Late Laramide basin geometriespersisted during Datil vol- faciesdata strongly suggestthat Datil andesiteand basalticandesite paleo- canism, as shown bv similarities in facies distribution and eruptive centers were present along what is now the Socorro Basin current azimuths within the BacaFormation and Datil Group (iather, of the Rio Granderift. This areacoincides with the northern portion 1986).Therefore, the terms Bacabasin and Carthage-LaJoya basin of the Laramide Sierrauplift (Cather, 1.983;Cather and Johnson, 1984; areretained in describingthe sedimentarypaleoenvironments of the 1986), which remained topographically positive throughout Datil Datil Group. Depositionalsystems of the Datil Group are identical time. to and transitionalwith the fluvial-lacustrinefacies tract of the Baca The majority of volcaniclasticdeposits in the Baca basin were Formation,except in the GallinasMountains, Datil Mountains, and derived from the south and southwest(Cather, 1986). Eruptive cen- SawtoothMountains, where debris-flow depositsare a major com- ters of probable Datil age are present in the western part of the of the lower (Cathea ponent Datil Group 1986). Mogollon-Datil volcanicfield (Ratt6et al.,1969;). C. Ratt€,written comm. 1987),and may have supplied detritus to the western portion of the study area. The location of source vents for depositsin the Averogegroin size of Poleoenvi ronmenlo I is not well constrainedbecause of the interoretolion centralpart of the study area sedimentoryrocks lack of Datil-equivalentexposures in the central portion of the Mo- gollon-Datil volcanic field. However, the similarity of facies distri- :::1"'bution and paleoflow between the BacaFormation and Datil Group suggeststhat Datil eruptive centers were developed on, or near, the southernboundary of the Bacabasin (the Laramide Morenci uplif! Cather and Johnson,L984, 1986). Although its geometry and struc- ::l-tural style are poorly understood, the Morenci uplift appears to have undergone mid- and late-Tertiary extension and collapse, to form the SanAgustin Basin and its associatedgrabens to the southwest. Proximol Availabledata suggestthat Datil sourcevents were developedon, fon or near, Laramide positive areasin west-centralNew Mexico. This spatial associationis intuitively reasonablebecause zones of prior Lovo flows Laramide deformation would tend to provide favorable pathways for magmaascent. The locationof sourcevents for Datil Group ash- Ash-f low luff flow tuffs is unknown; outcroPs are too sParse to determine vent locationsusing thicknessand distribution data, and detailedpetro- J Lovo ,,/' Ash- tloyz/ f low tutf fabric studiesof flow-direction indicatorsin thesetuffs (e.9., Elston and Smith, 1970)have not yet been attempted.

Mid ton Age Radiometric age determinations for the Datil Group range from about 28 to 42Ma (Fig. 3; Table1), although a few of the oldest and youngestages are clearlygeologically unreasonable (see remarks in Table 1). Reasonableradiometric ages for the lower Datil (all from Debrisf Iow clasts in conglomerate) indicate an age range of about 37 to 40 Ma; averageages for ash-flow tuffs in the upper Datil range from about M to 36 Ma. Due to inherently poor precisionof I(Ar and fission- track dating, ages obtained by these methods can serve only as generalguidelines in stratigraphicanalysis of the Datil Group. Dat- ing of these rocks by the 'Ar/"Ar method (Mclntosh et aI., 1986; o Mclntosh, in progress) provides much more useful constraints be- Mid ' * fon causeof the smallrelative error of this method (approximately 0.15 m.y. for lower rocks). The first geologically reasonable date for the Rock House Canyon Tuff has been obtained recently by the *Ar/"Ar method (Table 1; Mclntosh, in progress). Previously, this tuff had yielded only three ages, all geologically unreasonable, by'Radiometric the ICAr method (Fig. 3). datesfor the Datil Group have severalimportant im- plications: grodofronolconlocl 1) Age determinations for the Eocene-Oligoceneboundary range from 35.7 -r 0.4 Ma (Montanari et al., 1985)to 35-37 Ma (Prothero et al., 1982;Prothero, 1985;Berggren et al., 1985).Therefore, it ap- pearsthat most or all of the lower Datil volcaniclasticrocks, in places E as much as 680 m thick, are of Eocene age. This interpretation is compatiblewith Eoceneage determinationsfor correlative(?)early andesitic rocks in south-central New Mexico (Palm Park Formation, Macho Andesite, and the lower, andesitic part of the Rubio Peak FIGURE 2-Representative stratigraphic section of Datil Group showing in- Formation),which have yielded l?Ar agesranging from 35.7 to 51 formal units and distribution of lithologies. Section measured in northern Ma (Kottlowski et al., t969; Seager et al., 1975; Marvin and Cole, ]ornadadel Muerto Basinby S. M. Catlier.

New Merico Geology Augost 7987 1978;Clemons, 1982;Loring and Loring, 1980;Seager et al., 1982) Sedimentationrates and a vertebratefossil of Duchesneanage (Lucas,1983). Averagesedimentation rates can be estimatedfrom the Datil Group 2) The transition from fundamentally andesitic to bimodal vol- sectionin the western GallinasMountains, where four tuffs (Datil canismin the northern Mogollon-Datil field (asrepresented by the Well Tuff, Rock House Canyon Tuff, Blue Canyon Tuff, and Hells contactbetween the lower and upper Datil) approximately coiniides Mesa Tuff) are present and have been dated by the precise ooAr/"Ar with the Eocene-Oligoceneboundary (Fig. 3). This is at odds with method. Assuming that the oldest dated clast(39.6 -r 1.5 Ma the previousplacement of the Eocene-Oligocene [I(Ar boundary at or near datel; see Fig. 3 and Table 1) from the basal part of the Gallinas the Baca-Datilcontact (Lucas, 1983). Mountains section is representativeof the approximatebeginning 3) Throughout the study area, the lower Datil Group is every- of volcaniclastic sedimentation, average sedimentation rates for the where thicker than the underlying BacaFormation. Becauselocal Datil Group can be estimated (Fig. a). Datil Group sedimentation basin geometrieswere similar during Baca and Datil deposition, rates ranged from about 32 to 177 nr.lnr..y.,and may have diminished these thicknessrelationships indicate that the lower Datil Group is through time. Episodicdeposition of major debrisflows may account the volumetricallydominant Eoceneunit in west-central New Mex- for high sedimentation rates during early Datil time in the western rco. GallinasMountains. Becauseaverage sedimentation rates were no 4) Stratigraphicand structuraldata (Cather, 1985)indicate a maior doubt strongly influenced by the effects of intrabasin structures on transition in lithospheric stressregime in west-central New Mexico local subsidence(Cather, 1986),sedimentation rates in other areas accompanied (and probably caused) the change from early Datil of the Datil outcrop belt were probably significantly different from andesiteto laterbimodal volcanism.Radiometric dates from the Datil those depictedin Fig. 4. Ratesof deposition comparableto those of Group suggestthe tectonictransition from waning Laramide short- the Datil Group have recentlybeen determinedfor Plio-Pleistocene ening to incipient mid-Tertiary extensionoccurred about 35 Ma- sedimentsin the Mesilla Basinof southern New Mexico (25to 60 mi approximatelycoincident with the Eocene-Oligoceneboundary. m.y.; Vanderhill, 1986;J. W. Hawley, oral comm. 1987). No have been reported from any of-the Datil Croup ex- posuresin the study area.However, the Red Rock Ranchpaleoflora from the PlacitasCanyon lakebeds(upper Datil equivalent?;in the Summary southernSan Mateo Mountains (Farkas,1969; Axelrod ,1975; Axekod Beginning about 40 Ma, andesitic eruptive centersbegan to de- and Bailey, 1976)shows similarities to the Florissantflora of early velop on or near late Laramide positive areasin west-centralNew Oligoceneage (about 34-35 Ma; Epis et al., 1980)in centralColorado. Mexico.These volcanoes shed detritus and pyroclasticmaterial into The PlacitasCanyon lakebedsare directly overlain by a basaltican- two basins of Laramide ancestry,the Baca and Carthage-LaJoya desite flow (Uvas Canyon andesiteof Farkas, 1969)that has been basins.Andesitic volcanism and associatedsedimentation continued dated at 35.1 + 1.8 Ma by the ICAr method (Cather. 1986). until about 37 Ma, and are representedtoday by the lower Datil mammalsfrom the upper part of the RubioPeak Formation in the Group. Bimodal volcanism suiplanted andesitic'volcanismin the BlackRange (Lucas, 1986; Harrison, 1986)are early Oligocene(Chad- northern Mogollon-Datil field by about 36 Ma. Basalticandesite ronlan) ln a8e. lavas, silicic ash-flow tuffs, and associatedvolcaniclastic deposits

|-+

o q* F<-{

IF {c H,-

l---H

o K/Ar Sporsefossils of Bridgerionlo Duchesneonoge o Fission lrock o%rzth, a 40 38 28 26 Eocene

FIGURE 3-Summary of radiometric ages for Datil Group and overlying Hells Mesa Tuff. Bracketsindicate one standard deviation about the mean; vertical bars indicate averagesfor,individ gl units; dashed vertical line is approximate Eocene-Oligoceneboundary according to Montanari et al. (1985).Sources for Datil datesare listed in Table1. Hells Mesa Tuff datesfrom-C. E. Chapin (unpublished),Kedzie ei al. (1985);Weber(1971), Burke et al. (1,963),Mclntosh et al. (1986),and Marvin et al. (in press).

August 1987 New Mexko Geology TABLE l-Radiometric age determinations for the Datil Group. *Has been recently recorrelated to tuff of Taylor Well by Mclntosh et al. (1986). References:1. Cather, 1986-dates by F. W. McDowell, University of Texas(Austin); 2. C. E. Chapin, unpublished dates; 3. Bornhorst et al., 1982; 4. Burke et a1.,7963and Weber, 1971;5. Mclntosh et a1., 1986and Mclntosh, in progress.Except where noted, l?Ar ages were calculated(or recalculated)for the revised1976 IUGS decayconstants using Dairymple's (1979)tables.

Unit Age (Ma) Method Location Reference Remarks lower Datil 38.4+ 0.6 I(Ar northern Jornada Andesite conglomerate clast from about 350 m above base of Group (biotite) del Muerto section. lower Datil 41.8+ 3.0 I(Ar northern Jornada From same clast as above. This date is considered to be in Group (plagioclase) del Muerto error because of poor reproducibility of K and Ar analyses. lower Datil 38.9+ 0.9 I(Ar northern Jornada Andesite conglomerate clast from about 140 m above base of Group (plagioclase) del Muerto sectl0n. lower Datil 39.6t 1.5 ICAr western Gallinas Andesite clast in debris-flow deposit. Located on southeast Group (biotite) Mountains side of Dog Springs Canyon, about 4.8 km southwest of Martin Ranch. lower Datil 38.6+ 1.5 fission track Datil Andesite clast from SW1/Esec. 7, T1S, R10W. Group (zircon) Mountains lower Datil 5/.L 1 t.J ICAr Joyita Date calculated using old decay constants. Hornblende Group (biotite) Hills andesite clast from Tspl unit of Spradlin (1976), near Cibola Canyon.

+ Datil Well 33.8 0.7 I(Ar northern Jornada Tuff is about 770 m above base of section. Tuff* (sanidine) del Muerto

Datil Well 35.6 L 0.7 I(Ar Datil SW1/+sec. 2, T2S, R10W. Tuff (sanidine) Mountains

Datil Well 37.7+ 1.8 fission track Datil SW1A sec. 2, T2S, R10W. Tuff (zircon) Mountains

RockHouse 28.7 t 4.2 I(Ar northern Jornada Tuff is about 850 m above base of section. Date is Canyon Tuff (feldspar) del Muerto geologically unreasonable, possibly due to impure mineral separate.

Rock House 43.4! 2.7 l?Ar western Gallinas Date is geologicaily unreasonable. Canyon Tuff (feldspar) Mountains

RockHouse 27.6! 1.0 l?Ar western Gallinas Date is geologically unreasonable. Canyon Tuff (sanidine) Mountains

Blue Canyon 33.2 ! 7.7 fission track Crosby Mountains SW1/esec. 1, T2S, R12W. Tuff (zircon) vicinity

Blue Canyon l?Ar western Gallinas NWl/+, SE1/r sec. 36, T1N, R9W. Tuff (biotite) Mountains

Blue Canyon 29.7+ 2.2 I(Ar western Gallinas SE1/nsec. 1, T3S, R10W. Tuff (biotite) Mountains

Blue Canyon 35.4+ 1.3 I(Ar Joyita Location is about 300 m north of road to Rosa de Castillo Tuff (biotite) Hills Windmill, on east side of hogback.

Blue Canyon 36.1+ 1.3 ICAr Joyita Same as above. Tuff (biotite) Hills

{Ar/3eAr lower Datil 37.02+ 0.I5 foyita Andesite clast in debris-flow deposit (Tsp-afl of Spradlin, Group (hornblende) Hills 7976).

Datil Well 35.51+ 0.15 4Ar/3eAr Datil Tuff (sanidine) Mountains

Blue Canyon 33.93+ 0.15 {Arl3eAr western Gallinas Tuff (sanidine) Mountains

Rock House 34.81+ 0.15 {Ar/3tAr western Gallinas Canyon Tuff (sanidine) Mountains

New Mexico Geology Augost 1987 HellsMeso Tuf f west-central New Mexico and eastem Arizona: New Mexico Bureau of Mines and r,ooo {.32.04tO.l5 Mo) Mineral Resources, Circular 792,33 pp Cather, S. M., and Johnson , B. D., 1'986,Eocene depositional systems and tectonics in BlueConyon Tuff m/m.y. west-central New Mexico and eastem Arizona: American Association of Petroleum Geologists, Memoir 41, pp. 623-652. (33.93t O.l5 Mo) Peak quadrangle, Luna County, New Mexico: 147 m/m.y. Clemons", R. E., 1982, Geology oI Massacre RockHouse Conyon Tuff New Mexico Bureau of Mines and Mineral Rbsources, Geologic Map 51, scale 1:24,000' to new 800 - (34.81t O.l5Mo) Dalrymple, G. 8., 1979, Critical tables for conversion of lSAr ages from old constants: Geology, v. 7, PP. 558-560. volcanic 65 m/m.y. Elston, W E.,7975, Glossary of stratigraPhic terms of the Mogollon-Datil DotilWell Tuff province, New Mexico; in Elston, W E., and Northrop, S A (eds ), Cenozoic vol- (35.5tt O l5 Mo) ianism in southwestern New Mexico: New Mexico Geological Society, Special Pub- lication no. 5, pp.737-144. Elston, W. E., and Smith, E. 1., 7970, Detemination of flow direction of rhyolitic ash- 600 flow tuffs from fluidal textures: Geological Society of America Bulletin, v. 81, pp. o o 3393_3405. q) Epis, R. C., Scott, G. R., Taylor, R. 8., and Chapin, C. E., 1980, Summary of Cenozoic c^ upperDoiil Group ^geomorphic, xF volcanic, and tectonic features of central Colorado and-adjoining areas; o.= .E toier oitiGroup l-7 fent, g. C., and Porter, K. W. (eds.), Colorado geoiogy: Rocky Mountain Asso- oation oI Geologists, pP. 135-156. F Socono and Sierra 400 Farkas, S. 8., 796E, Geology of the southern San Mateo Mountains, Counties, New Mexico; Unpublished Ph.D thesis, University of New Mexico (Al- buquerque), 137 pp. 177m/my. Haniion, R. W.,1986, General geology of the Chloride mining district, Sierra and Catron Counties, New Mexico: New Mexico Geological Sociefy, Guidebook to 37th Field Conference, pp. 255-272. aAr/reAr Kedzie, L. L., Sutter, J. F,, and Chapin, C. E , 1985, High-precision ages of widespread Oligocene ash-flow tuff sheets near Socono, New Mexico: EOS, Trans- actioni of the American Geophysical Union, v. 77' p. 625 Koitlowski, F. E., Weber, R. H., and Willard, M. E.' 1969, Tertiary intrusive-volcanic mineralization episodes in the New Mexico retion (abs ): Geological Society of Amer- ica, Abstracts with Programs, pt.7, PP 27A-280. closl (39.6t 1.5Mo) LaRoche, T. M., 1981, Geology of the Gallinas Peak area, Socorro County, New Mexico: Unpublished M.S thesis, New Mexico Institute of Mining and Technology (Socono), o 145 op. 40 38 36 34 32 Loring, A. K., and Loring, R. 8., 1980, K-Ar ages of middle Tertiary rocks from southem Age (Mo) New Mexico: IsochronAVest, no. 28, pp 17-18. Lucas, S. G., 1983, The Baca Formation and the Eocene-Oligocene boundary in New FIGURE4-Graph showing Datil Group sedirnentation rates, as determined Mexico: New Mexico Geological Society, Guidebook to 34th Field Conference, pp by stratigraphic thickness and radiometric dates in the westem Gallinas 787,192. Mountains. Lucas, 1986, Oligocene mammals from the Black Range, southwestern New Mexico: New Mexico Geolocical Society, Guidebook to 37th Field Conference, pp.261'-263. Maruin, R. F., and CoIe, J. C., 1978,Radiometric age-compilation A, U S Geological dominate the upper Datil Group (32-36 Ma) and record the late, Suruey: Isochron,Aryest, no. 22, P. 3. Marvin,'R. F., Naeser, C. W., Bikerrnan, M., Mehnert, H H , and Ratt6, J. C., in press, bimodal phase of Datil volcanism. Sedimentation rates during Datil Isotopic ages of post-Paleocene igneous rocks within and bordering the Clifton 1'x deposition in the westem Gallinas Mountains area ranged from about 2'quadrangle, Arizona-New Mexico: New Mexico Bureau of Mines and Mineral 32ro177 m/m.y., and appear to have diminished through time. The Resources, Bulletin, 118. Mclntosh, W. C., in progress, Stratigraphic framework of Mogollon-Datil volcanic end of Datil Group deposition was marked by the eruption of Hells {0Ar/loAr^the and high-precision ages: Unpublished Ph D (32 field based on paleomignetism Mesa Tuff Ma). Radiometric data indicate that the major petro- thesis, New Mexico Institute of Mining and Technology (Socono). genetic, tectonic, and sedimentologic changes represented by the Mclntosh, W C., Sutter, J. F., Chapin, C. E., Osburn, G. R , and Ratt6, J. C , 1986, A lower Datil-upper Datil contact occurred at about the Eocene-Oli- stratigraphic framework for the ea^stern Mogollon Datil volcanic field based on pa- oAr/3eAr gocene boundary. leomignetism and high-precision dating of ignimbriteFa Progress rePort: Geological Society, Guidebook to 37th Field Conference, pp 183-195 AcrNowuocMENrs-The New Mexico _ manuscript was improved by reviews Montanari, A., Drake, R., Bice, D. M., Alvarez, W., Curtis, G. H., Tunin, B. D., and from R. M. Chamberlin, f . W. Hawley, S. G. Lucas, and |. C. Ratt€. DePaolo, D. I., 1985, Radiometric time scale for the upper Eocene and Oligocene tAr/t'Ar All dating work was performed in cooperation with J. F. based on lSAr and Rb/Sr dating of volcanic biotites from the pelagic sequence of Sutter at the USGS laboratory in Reston, Virginia. F. W. McDowell Gubbio, Italy: Geology, v. 13, pp. 596-599 Society, 1982, New Mexico highway geologic map; New Mexico (Austin) New Mexico Geological of the University of Texas contributed six new ICAr ages. Geological Society, scale 1:1,000,000. G. R. Osburn provided valuable field assistance. Financial support Osburn, G. R., and Chapin, C. E.,'1983, Nomenclature for Cenozoic rocks of northeast for this study was provided by the New Mexico Bureau of Mines Mosollon-Datil volcanic field, New Mexico; New Mexico Bureau of Mines and Mineral and Mineral Resources G. E. Kottlowski, Director). Resources, StratiSraPhic Chart 7, 7 p. Proihero, D. R., 1985; Chadronian (early Oligocene) magnetostratiSraphy of eastern Wyoming-implications for the age of the Eocene-Oligocene boundary; Journal of References Ceology. v. c3, PP. 555-565 Axelrod, D.1.,7975, Tertiary floras from the : New Mexico Geological Prothero-,'D. R., Denham, C. R., and Farmer, H. G.,798?, Oligocene calibration of the Society, Guidebook to 26th Field Conference, pp. 85-88. magnetic polarity time scale; Geology, v. 10, pp' 550-653 Axelrod, D. I., and Bailey, H. P.,1,976, Tertiary veSetation, climate, and altitude of the Ratt6;1. C.,iandis, E. R., Gaskill, D. L., and Raabe, R C., 1969, Mineral resources of Rio Grande depression, New Mexico-Colorado: Paleobiology, v.2, pp. 235-254. the Blue Range primitive area, Greenlee County, Arizona, and Catron County, New Berggren, W A., Kent, D. V, Flynn, J. J., and van Couvering, J. A., 1985, Cenozoic Mexico: U.S. Geological Survey, Bulletin 1267 E,9"1 pp : Geological Society of America Bulletin, v.96, pp. 1407-141,8. Seager, W R., Clemons, R. E., and Hawley, J. V'l-, 7975' Ceology_of Sierra Alta,quad- Bornhorst, T. 1., Jones, D. P, Elston, W. E., Damon, P E., and Shafiqullah, M.,1,982, rangle, Dona Ana County, New Mexico: New Mexico Bureau of Mines and lv{ineral New radiometric ages on volcanic rocks from the Mogollon-Datil volcanic field, south- Resources, Bulletin 102, 56 PP. western New Mexico: Isochron,/Vvest, no. 35, pp. 13-15. Seager, W. R., Clemons, R. 8., Hawley, J W., and Kelley, R. E , 7982, Geology of the Burke, W. H., Kenny, G. S., Otto, J. 8., and Walker, D., 1963, Potassium-argon dates, n6rthwest part of Las Cruces 1'x 2' sheet, New Mexico: New Mexico Bureau of Mines Socorro and Sierra Counties, New Mexico: New Mexico Ceological Society, Guidebook and Mineril Resources, Geologic Map 53, 3 sheets, scale 1:125,000. to 14th Field Conference, p. 224. Spradlin, E. J.,7976, ShatigraPhy of Tertiary volcanic rocks, Joyita Hills area, Socorro Cather, S. M., 1980, Petrology, diagenesis, and genetic stratigraphy of the Eocene Baca County, New Mexico: Unpublished M.S thesis, University of New Mexico (Albu- Fomation, Alamo Navaio Reseruation, Socono County, New Mexico: Unpublished querque), 73 pp. M.S. thesis, Universify of Texas (Austin), 2a3 pp. Vanderhill, J.8.,1,986, Lithoshatigraphy, vertebrate paleontology, and magnetoshatiS- Cathet S. M., 1983, Laramide Siena uplift; ewidence for major pre-rift uplift in central raphv of PliePleistocene sediments in the Mesilla Basin, New Mexico: Unpublished and southern New Mexico: New Mexico Geoloeical Societv, Guidebook to 34th Field Ph.D. thesis, University of Texas (Austin), 311 pp. Conference, pp. 99-101. Weber, R. H., 1977, K-Ar ages of Tertiary igneous rocks in central and western New Cather, S. M.,7986, Volcano-sedimentary evolution and tectonic implications of the Mexico: Isochron,/West, no. 7, pp.33-45. Datil Group (latest Eocene-early Oligocene), west-central New Mexico: Unpublished Winchester, D. E.,1920, Geology of Alamosa Creek Valley, Socorro County, New Mexico; Ph.D. thesis, University of Texas (Austin), a82 pp. with special reference to the occurrence of oil and gas: U.S. Geological Suruey, Bulletin Cather, S. M., and Johnson, B. D.,1984, Eocene tectonics and depositional setting of 715-4, 15 pp.

August 1987 Nal MexicoGeology