Stratigraphy, Age, and Rates of Deposition of the Datil Group (Upper

Stratigraphy,age, and rates of deposition ol the Datil Group (Upper Eocene-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 Tuff. 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 (1986),Mclntosh (in progress), are characterizedby phenocrystic plagioclase,amphibole, and ti- C. E. Chapin (unpublished),and Cather (1986).To facilitatediscus- tanomagnetite(+ biotite).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 Sierra uplift (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

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