Faciesand age of the Oso Ridge Member (new), Abo Formation, ZuniMountains, by AugustusK. Armstrong,U.S.Geological Survey, at NMBMMR,Socono, New Mexico 87801; RobertG. Stann, U.S.Geological Survey, Reston, Virginia 22092; FrankE. Kottlowski,NewMexico Bureau of Minesand Mineral Resources, Socono, New Mexico 87801; BernardL. Mamet,Universityof Montreal, Montreal, Ouebec, Canada H3C 3J7; J. ThomasDutro, U.S.Geological Survey, Washington, D.C. 20560; andDavid J. Wury, U.S.Geological Survey, Reston, Virginia 22092

Abstract tiloid indet. The ageof the Oso Ridge sedimentaryformations are absent,at least Member is Virgilian (Late Pennsylva- The Oso Ridge Member (new), the in the central part of the uplift, owing at nian) to Wolfcampian(Early ). base of the Abo Formation in the Zuni doubtless to the presenceof a land surface area which continued until Per- Mountains, nonconformably overlies Introduction in this Proterozoicrocks. The member consists mian time." He describedand illustrated of some 30 ft (9 m) of conglomerateand Field studies in the of the Abo Formation as resting on Proter- arkose composed principally ol fuag- west-centralNew Mexico were conducted ozoic rocks. The Abo is 600-700ft (about ments of the underlying Proterozoic by Armstrong during the spring and sum- 200m) thick and consistsof massivesand- schist, gneiss, and other metamorphic mer of 1990to determine both facies and stone, brown slabby , sandy rocks; thin, fossiliferous limestone lenses age relationships of the carbonaterock at shale, and, locally, thin limestones in the are interbedded with the arkose. Biota the base of the Abo Formation (Fig. 1). from the lenses include lower part. Darton's collecting near Saw- a phylloid alga The petrographic studies of the carbonate Eugonophyllumsp.; the foraminifers Glo- yer (90P-4 area) at the headwaters of rocks were done by Mamet and Arms- biualoulinasp . , Biseriellasp. , and,Syzrania Bluewater Creek yielded brachiopodsand sp.; the conodonts Adetognnthuslautus trong; Mamet identified the calcareousalga mollusks determined by G. H. Girty (in (Gunnell), Streptognathoduselongntus and foraminifers; Stamm identified the Darton, L928,p.140) as a "Manzano (Per- Gunnell, and "Ellisonia" conflexa (Elli- conodonts, which were processedby mian) fauna." Lee and Girty (1909)de- son); the brachiopodsluresania cf. l. ne- Wearv; Dutro reexamined the brachio- scribed and illustrated the Manzano fauna braskensis(Owen), Spirit'erellasp., pods and mollusks collected by Darton. from the valley of New Mex- Compositacf. C. mexicanaHall; and the ico. In modern terms, Girty's age deter- mollusks Myalina apachesiMarcou, Previous sfudies "Pleurophorus"cf . "P." meficanasGirtv. a mination would indicate a Virgilian- bellerophontidindet., gastropods,ind Darton (1928,p. 132)found that in the Wolfcampianage range. orthocone nautiloids indet., Plagioglypta Zuni Mountains of New Mexico "the Smith (1958,1959) published a seriesof cf . "P." cannaWhite, and a coiled nau- Magdalena () and older 1.:48,000-scalegeologic maps of the Zuni Mountains in which he mapped and de- scribed Pennsylvanian arkosesand lime- stones beneath the Abo Formation. His map unit is composed"principally of frag- ments of the underlying Precambrian schist, gneiss, and other metamorphic rocks, and thin, sparsely fossiliferous limestonelenses that are interbeddedwith the arkose." He assignedthese sedimen- tary rocks a Pennsylvanian age but did not give any paleontological evidence for his age assignment. Colpitts (1989)found megafossilsin the basal limestones of the Abo Formation at La Jara Springs (90P-1 area), which in- clude planispiral gastropods,brachiopod- shell fragments, and bryozoan fragments. The age of this basal limestone and con- glomerate sequencehas been the subject of some discussion and controversy for several decades.Cheetham (1950,ltt Kot- tlowski, 1960) suggested that bryozoans collected from these carbonates are Vir- gilian in age.

Proterozoicbasement Goddard's (1966)geologic map and ffi Quaternarybasalt flows m Jurassicrocks studies by Mawer and Bauer (1989)indi- Tertiaryandesites and basalts Triassicrocks cate that the Proterozoicrocks of the Zuni ffi ffi Mountains are metamorphic felsic to ul- Fll fertlaryalluvial and lacustrinedeposits Permianrocks tramafic schist, gneiss, and foliated plu- ffi is at cretaceousrocks tonic rocks. Regional metamorphism lTl ffi eroterozoicrocks the lower amphibolite grade. Isotopic U- FIGURE l--Ceneralized geologic map (modified from Dane and Bachman, 1965) of the Zuni Pb ages based on zircon from felsic ig- Mountains showing the studied outcrops of the Oso Ridge Member of the Abo Formation. 90P- neous rocks indicate crystallization at about 1, La Jara Springs; 90P-2, McQue Flat; 90PJ, Log House; 90P-4, Sawyer; and 90P-5, Oso Ridge. 1.655Ga (Bowring and Condie, 1982).

May 7994 New Mexico Geology Outcrops The Oso Ridge Member is here named c as the basal unit of the Abo Formation. .9 o The name is derived from a northwest- to (5 -o southeast-trending ridge that forms the E E L o crest of the southem Zuni Mountains. The o Sawyer section (90P-4) provides the best IL exposures and is the proposed type sec- fl tion for the Oso Ridge Member of the Abo 35 Formation. Goddard's (1965)geologic map 34 !t (u provides coverage of the proposed type o o E 33 section. To the northwest of the Sawyer (! c o section,Smith's (1958)geologic map shows E o 32 the arkose and bedded limestones occur- = =o ring above the Proterozoic and beneath 31 the typical Abo over an area of several 30 square miles. The basal conglomerates of the Oso 29 Ridge Member overlie deeply weathered 28 * Echinoderm Proterozoic gneiss. The lower 8 ft (2.4 m) are water-deposited coarse clastics com- 27 (J Brachiopod posed of the weathered Proterozoicrocks (6 26 Molluscan and include maroon to brownish-red peb- o $ ble conglomerates, , and silt- 25 f enyuoiaapae (Figs.2,3). c (U s, stones Limestonesoccur at 8- o- 24 Conodont sample 1,4ft (2.4-4.6m),2l-24 ft (6.4-7.3 m), and .g $r a thin bed at 30 ft (9.1 m) and are sepa- E 23 rated by intervals of brownish-red silt- c c) 22 stonesand sandstones(Fig. 2). The upper .9 (L CL contact of the Oso Ridge Member is placed (6 o 21 at the last occurrenceof carbonate,which E o- -o o (s 20 is overlain by nearly continuous arkosic o E FE=Ef, Limestone = h o 19 sandstones of the Abo Formation. The o c o = Shale limestones are fossiliferous arenaceous o 18 packstonesto grainstones.The calcareous o o (Il o) 17 Arkosicsandstone phylloid alga Eugonophyllumis abundant E ffi and is associatedwith a rich molluskan c o = 16 FffiI Conglomerate fauna of gastropods,pelecypods, and CL o CE P-ooq .g o. lJ. 15 nautiloids. Field observations, petrogra- c f (5 o phy, and paleontology all indicate that o at 14 these carbonate rocks were deposited in o cl o estuarine 13 or lagoonal environments. g, At La fara Springs exposure,90P-1, the c c 12 limestone is about 3.5 (1.1 m) thick and c is exposedin the crestof a low ridge. God- o) : 1l IL g) dard's (1966)geologic map shows the out- 10 crop location and the regional geology. ; The arenaceouslimestones are underlain o I o. by some 50 ft (15.2m) of maroon siltstone CL 8 and quartz-conglomeratethat rest on J Proterozoic metamorphic rocks. The cal- 7 careous alga Eugonophyllumis abundant 6 and is associatedwith brachiopods, echi- noderms, bryzoans, ostracodes, and a molluskan fauna of gastropods and pe- 4 lecypods. The depositional environment of the limestone was subtidal marine. 3 The McQue Flat exposure, 90P-2, is 2 shown on Smith's (lgSgigeologicmapwith the location of the limestone that was I sampled at two outcrops near the north- 0 west edge of Sec. 30. Poor exposures of the nodular limestone bed are found on Proterozoic gneiss low hills. Construction of a stratigraphic rocks section is not possible becauseof the dis- continuous outcrops and soil cover. Pet- rographic studies show that the stratigraphically youngest limestone nod- FIGURE 2-Graphic illustration of the measured type section Sawyer 90P-4, of the Oso Ridge ules consist of brachiopod-ostracode- Member, Abo Formation, at Sawyer-Bluewater Canyon. The limestones contain the alga Eugon- echinoderm peloid packstone to wacke- ophyllum,a large molluskan fauna of bivalves, gastropods, and nautiloid cephalopods, and con- odonts.

New Mexico Ceology May 1994 FIGURE 4-Log House outcrop 90P-3. View to the east, illustrating the FIGURE 3-Pennsylvanian limestonesexposed in a roadcut near Sawyer generally poor exposures in the west Zuni Mountains. The low ridge (90P-4). The arkosicbed below the limestonesis clearly visible. The blocky beneath the truck is formed by the nodular dolostones in a siltstone nature of the lower limestone and the somewhat nodular-bedded lime- matrix. The soil in the foreground, beneath the ridge, is formed by the stone near the top of the section are apparent. These carbonatescontain basal siltstone and sandstone overlying Proterozoic rocks. The petro- abundant phylloid algae, brachiopods, conodonts, gastropods,and echi- graphic and microfossil sampleswere collected from bedrock exposed in noderms. The light meter is 3 in. (7.5 cm) long. the roadcut.

stone. The carbonaterocks near the Prot- kg from 0-3.5 ft in the sectionyielded 11 the carbonate section, immediately over- erozoic contact are arenaceous-dolomitic Pa elements of Adetognathuslautus (Gun- lain by phylloid-algal mats. lime mudstones. nell), 4 Pa elements of Streptognathodus G. H. Girty (ln Darton L928,p. l4l) The Log House exposure, 90P-3, also elongatusGunnell, 18 elements oI "ElIi- identified several brachiopods and mol- on Smith's (1958)geologic map, shows sonia"conflexa (Ellison), and 13 fragments lusks that he thought could indicate either that the outcrop of carbonaterocks is about (Fig. 5). Thesespecies indicate an agerange a Virgilian or Wolfcampian age. The for- 0.3 mi (0.48 km) long. The nodular do- of Late Pennsylvanian to Early Permian aminifers, algae,and conodonts, and Du- lostone is exposedas a low ridge some 20 (Virgilian to late Wolfcampian). tro's reexamination of these collections, ft (5.1 m) above the Proterozoic contact all support Girty's original age assign- Gig. a). Petrographic studies show that McQue Flats, 90P-2 ment for the limestone. the dolostone is formed of euhedral dol- SpongeJump lime mudstone with bird's- omite rhombs 25 to 300 pm across. eye structures, formed in a very shallow Paleoenvironments The Oso Ridge outcrop, 90P-5, pro- marine to supratidal environment. The outcrops at Sawyer (90P-4)and La vides the poorest exposuresof all those Jara Springs (90P-1) both contain echi- discussedabove. Above the noderm fragments that indicate normal nonconformity Log House, 90P-3 and weathered Protero- marine salinities, but the restricted nature Dolomitized recrystallized zoic gneiss ate 2-4 ft (0.6-7.2m) of ma- sponge{ump of the fauna (no fusulinids, paucity of grainstone. roon to reddish-brown siltstone and brachiopods) suggests some inhibiting sandstoneoverlain by a 1-ft-(0.$m-) thick, factors in the environment. The calcare- nodular dolomitic limestone. Sawyer,90P-4 ous alga Eugonophyllumis abundant in the Phylloid-algal buildup associatedwith carbonate rocks at both La Jara Springs Paleontology, lithology, and age abundant gastropods, nautiloids, some and Sawyer. The Sawyer outcrop has a crinoids, and echinoid spines. This is a rich molluskan fauna of gastropods, pe- La fara Springs, 90P-1 molluskan facies. Reworking this 90P-4 lecypods, and nautiloids. Field observa- Dissolved, mud-coated phylloid-algal lithology could result in the 90P-1 facies. tions, petrography, and paleontology all grainstone contains bioclasts of echino- Algae againbelong to Eugonophyllum.The suggest that these carbonate rocks were derms, brachiopods, numerous produc- mollusks are Myalina npachesiMarcou, deposited in shallow estuarine or la- tid-brachiopod spines, and ostracodes. "Pleurophorus"cf . "P." mexicanusGirty, an goonal environments. Phylloid blades are mostly Eugonophyllum orthocone indet., and a coiled nautiloid The Log House (90P-3), McQue Flat sp. Identifiable foraminifers are apterri- indet. Conodontsfrom a9. -kgsampleat (90P-2), and Oso Ridge (90P-5) outcrops nellids, Globhnloulirasp., Bisuiellasp., and 8-14 ft in the section include 9 Pa ele- in the western part of the Zuni Mountains Syzraniasp. The latter is a cosmopolitan ments of Adetognathuslaufr.rs (Gunnell), 1 contain thin or nodular beds of carbonate genus that first occurs in the Moscovian, Pa element of Streptogmthoduselongatus rocks that are composedof limestone and and the oldest possible age thus is Des- Gunnell, 32 elementsof "Ellisonia"conflexa dolostone in gray to dark red shale and moinesian (late Middle Pennsylvanian). (Ellison), and 60 fragments. A 3.8-kg sam- siltstone. The carbonate rocks have very Megafossilsinclude Juresaniacf-. J. nebras- ple from 21-24 ft in the section yielded few fossils and sedimentary features,sug- knensis(Owen), Spiriferellasp., Composita one Streptognnthodussp.,29 elements of gesting intertidal to supratidal environ- cf. C. mexicanaHall,Plngioglypta cf.. P. unru "Ellisonia" conflem (Ellison), and 61 frag- ments of deposition. The sedimentary White, a bellerophontid indet., and a pe- ments (Fig. 5). The environment of dep- features include laminations formed by lecypod indet. A conodont sampleof 5.6 osition was a supratidal flat at the baseof microbial mats, small rip-up clasts, and

May 1994 New Mexico Geology the Oso Ridge Member in containing an apparent Virgilian-Wolfcampian fauna without definitive fusulinids. As de- scribed by Kues and Kietzke (1976), the member is about 450ft (I37 m) thick and consistsof red and green interdistributary or delta-embaymentshales and siltstones intertongued with fossiliferous gray cal- careous marine shales and limestones. Kues' (1984;Zidek and Kietzke, 1993)de- tailed study of the faunas suggests that the lower 90 ft (27 m) are Virgilian and the middle and upper parts are Wolfcam- Plan. Along Bonito Canyon in the Defiance uplift, 40 mi (64 km) northwest of Log House, Supaired beds (Permianor Penn- svlvanian-Permian) overlie Proterozoic quartzites.There are no limestonesin the lower Supai. In the north NacimientoMountains, 90 mi (145 km) northeast of Oso Ridge on the eastside of the San|uan Basin,Wood- ward (1987)reported intertonguing of up- per Madera (Pennsylvanian)marine limestonewith lower Abo (Permian)red beds. Thus the Oso RidgeMember equiv- alent is in the gradational Pennsylvanian- Permian sequence. Subsurfacecontrol around the Zuni Mountains is sparse. Petroleum tests drilled show Abo red beds, with no lime- stones, unconformableon Proterozoic rocks at least as far south as Quemado, 55 mi (89km) south of Oso Ridge.Twenty miles (32km) eastof La JaraSprings, about 480 ft (1.46m) of marine Pennsylvanian strata (in Larrazolo-Gottlieb oil test) over- lie the Proterozoic.Uppermost beds may interfinger with the Abo, a facies similar to the Red Tanks Member. Similar rela- tionships may be recordedin wells drilled 20 (32 km) to 30 mi (a8 km) to the north- east and north. Interestingly, Colpitts (1989)noted a carbonate-rockbed 65 ft (20 m) above the Proterozoic-Abo contact, using the resis- tivity log from the Southland Royalty Lu- cero 1-14 Federalwell drilled 20 mi (32 km) south of La |ara Springs.Thus lenses of the Oso Ridge Member of the Abo may extend that far south.

Thermal alteration s--:'F]!i::yi\_,co1lte11@llison); 1 (USNM 4760s0),pb IISyRI element, La Jara Springs e0p_1, 0_ (CAl 3.5 ft; 2,3 (USNM 476057,476052), M elements,Sawyer g0p_4,B_74 ft; i,5,6^(US"NM 476053, The color-alteration index is L for 476054,476055), Sa, Sb?, Sc elements, respectively,Sawyer g0p_4,2124 ft. the Oso Ridge Member conodontsin the Adetognathuslaatus (Gunnell); 7 (USBM 476056),pa element, Sawyer g0p_4,g_74 fr. Zuni Mountains (Epsteinet al., 1977).T\is streptognathodus CAI value indicates that the rocks have elongatusGunnell; B, 9, t0 (usNM 4z60sz, 4760s9,4760s9), pa elements, La Jara Springs 90P-1,0-3.5 ft;11 (USNM 426060),Pa elernent.Sawver g0p_4.B_74ft not beenheated to more than 50"C;there- fore, they have not been subjectedto a regionalmetamorphic event and havenot beenburied under more than about 3,500 mudcracks. These carbonate rocks appear red beds are east of Mesa Lucero in the to 4,000ft (1,000-1,200m) of overburden. to represent a shoreward facies to the Carrizo Mesa area, 60 mi (97 km) to the The persistentpositive relief of the Zuni northwest of the Sawyer and La Jara out- east-southeast of La Jara Springs. This is Mountains region from the Paleozoicto crops. the northmost outcrop of the Virgilian- the Holocene is in marked contrast to the some 90 mi (145km) to correrations u:l':lxFi:l"Ti.lhti:^*,Tff: :i"li: the north, where there is at least 15,000 The nearest outcrops of Pennsylva- ward to the west side of the Ladron ft (5,000m) of sedimentaryrock abovethe nian-Permianlimestonesinterbeddedwith Mountains). The Red Tanks is similar to Proterozoic (Huffman, 1987). r?

New Merico Geology May 7994 Conclusions ico Bureau of Mines and Mineral Re- Mineral Resources,Bulletin 66,187 pp.,13 pls , 34 sources. We wish to thank Charles E. figs. Throughout the early Paleozoicand Kues, B S., 1984,Bivalves from the Red TanksMem- , the Zuni Mountains re- Chapin, Director of the New Mexico Bu- ber, Madera Formtion, of central New Mexico (abs.): mained a positive area in comparison to reau of Mines and Mineral Resourcesfor New Mexico Geology, v 5, no. 4, p, U. his support of this study. Kues, B. S., and Kietzke,K K.,1976, Paleontology adjacentareas such as the Sanfuan Basin and stratigraphy of the Red TanksMember, Madera to the north, the Black Mesa Basin to the Fomation near Lucero Mesa, New Mexico; in Tec- west, and the PedregosaBasin to the south References tonics and mineral resourcesof southwestem North (Armstrong et al., 1992). Armstrong, A. K., Mamet, B. L , and Repetski,J. 8., America: New Mexico Geological Society, Special 192, Strati$aphy of the Mississippian System, Publication 6, pp. 102-108. During the late Mississippian and Early south-central Colorado and north-central New Lee, W T, and Girty, G. H., 1909, The Maroano Pennsylvanian, the Zuni highlands were Mexico: U.S Geological Survey, Bulletin 1787, Group of the Rio Grande Valley, New Mexico: U.S. rejuvenated by tectonic events related to Chapter EE, pp. 1J0. Ceological Survey, Bulletin 389,141 p. the Ouachita Bowring, S. A , and Condie, K. C.,7982, U-Pb zircon Mawer, C K., and Bauer, P. W., 1989, Precambrian (Szabo agesfrom northern and central New Mexico (abs.): rocks of the Zuni uplift: a summary with new data and Wengard, 1975;Ross and Ross, Geological Societyof America, Abstractswith Pro- on ductile shearing: New Mexico Geological Soci- 1985). However, by the Late Pennsylva- grams, v 14, pp. 3tX. ety, Guidebook to 40th Field Conference, pp.143- nian they were eroded to low hills and Cheetham, A. H., 1950,Preliminary suruey of some 147 Iowlands of Proterozoic rocks; a marine New Mexicobryozoans: Unpublished Seniorthesis, Ross, C A, and Ross,J. P., 1985,Paleozoic tectonics New MexicoSchool of Mines, 107pp., 28 pls., 18 and sedimentation in west , southem Ari- transgression resulted in carbonate-sedi- figs. zona; in Dickerson, P W., and Muehlberger, W. R. ment production. Colpitts, R M, 1989, Permian referencesection for (eds.), Structureand tectonicsofTrans-Pecos Texas: Smith's (1958,1959) and Goddard's southeasternZuni Mountains, Cibola County, New West TexasGeological Society, Publication 85-81, (1956)geologic maps show many miles of Mexico: New Meico Geological Society, Guide- pp 221-230. map Inscription Rock exposed contact between the basal Abo book to 40th Field Conference,pp 777-1.80 Smith, C. T., 1958,Geologic of Dane, C H , and Bachman, G. O., 1965, Geologic fifteen-minute quadrangle: New Mexico Bureau of Formation and the Proterozoic metamor- map of New Mexico: U.S GeologicalSuruey, scale Mines and Mineral Resources,Geologic Map 4, rale phic rocks. The contact was walked out l:500.000. 1:,18,000. for this study, and it was verified that the Darton, N H,7928, "Red beds" and associatedfor- Smith, C T., 1959,Geologic map of Foster Canyon mations in New Mexico: U.S. Geological Suwey, quadrangle, Valencia [now Cibolal and McKinley basal carbonate rocks of the Oso Ridge Bulletin 794, 355 pp Counties. New Mexico: New Mexico Bureau of Mines Member are found at only a few localities. Epstein,A G, Epstein,J B, andHanis, L D., 79n, and Mineral Resources, Geologic Map 9, scale This disjunct distribution of the carbonate Conodont color alteration-an index to organic 1:,18,000 rocks indicates that they were deposited metamorphism: U S. Geological Suroey, Profes- Szabo,8., and Wengard,S A.,1975, Stratigraphy sional Paper 995,27 pp. and tectogenesisof the Paradoxbasin: Four Corners in low areasover an irregular surface.The Goddard, E. N, 1956,Geologic map and sectionsof GeologicalSociety, Guidebook 8, Canyonlands,pp. carbonatebeds in the Oso Ridge Member the Zuni Mountains fluorspar district, Valencia 193-210. are 6-8 ft (1.8-2.4 m) thick, which sug- County, New Mexico: U S. G-eologicalSurvey, Mis- Woodward, L A., 1987, Geology and mineral re- gests only a short period, perhaps a few cellaneousGeologic Investigation Map I-454 sources of and vicinity, New Huffman, A. C., 1987, Petroleum geology and hy- Mexico: New Mexico Bureau of Mines and Mineral thousand years,of sedimentdeposition. '25-27. drocarbon plays of the San Juan Basin petroleum Resources,Memoir 42, pp province: U S. GeologicalSuruey, Open-file Report Zidek, J., and Kietzke. K. K., 1993,Pre-Pemian ver- 87-4508,67pp tebratesof New Mexico with remarkson someEarly Outcrop tegister Kottlowski, F. E., 1950, Summary of Pennsylvanian Pemian specimens;ir Vertebrate paleontology in Sawver: field localitv 90P-4:NW'/+ sec. L9 sections in southwestern New Mexico and south- New Mexico: New Mexico Museum of Natural His- of Mines and tory and Science,Bulletin 2, pp.1,-lO T1lN R13W;Lat.35610.5', Long. 108'15.0'; easternArizona: New Mexico Bureau n Kettner Canyon 7.5' quadrangle; Cibola County. Section can be found west of junction of ForestRoutes 50 and 482.See Goddard, 1966. La Jara Springs: field locality 90P-1; NE '/+sec. 13 T10N R12W Lat. 35'6.0',Long. 108'2.58';Paxton Springs 7.5' quadrangle; Cibola County. Section is on side of low hill a few hundred feet north of Forest Route 49. SeeGoddard,1956. McQue Flat: field locality 90P-2; NWr/+ sec. 30 T13N R15W Lat. 35'20.0',Long. t08"29.0'; Page7.5' quadrangle;McKinley County. Exposures are 200 to 500 ft (61- 152 m) south of Forest Route 402. See Smith, 1959. Log House: field locality 90P-3;SW'/+ sec. 32 T13N R15W; Lat. 35'18.0', Long. 108"26.5'; Page 7.5' quadrangle; Cibola County. Exposuresare locatedon an east- west dirt road, next to and south of an abandoned farm house, approximately 300 ft (91 m), east of Forest Route 157. See Smith, 1958. Oso Ridge: field locality 90P-5; SW1/aSE 'L sec.6 T12N R15W;Lat.35"77.5' , Long. 108"28.0';Page 7.5' quadrangle; Cibola County. Poorly exposed outcrops on dip slope in dry stream beds of large valley. AcxNowr-nocMEr\ms-The manuscript was reviewed by fohn E. Repetski and Anita G. Harris, U.S. GeologicalSurvey, Reston, Virginia and firi Zidek, New Mex-

May 7994 Nao Mexico Geology