Stratigraphy and tectonic implications of Paleogene stratain the Laramide Galisteo Basin, north-central New Mexico

by Spencer G. Lucas, New Mexico Museum of Natural History and Science, 1801 Mountain Road NW, Albuquerque, New Mexico 87104;Steven M. Cather, New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico 87801;John C. Abbott, Department of Geosciences, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801;

and Thomas E. Williamson, New Mexico Museum of Natural History and Science, 1801 Mountain Road NW, Albuquerque, New Mexico 87104

Abstract

We exclude the lower 0-442 m fromthe Galisteo Formation and identify it as anew, unconformity-boundedstratigraphic unit, the Diamond TailFormation. The Diamond Tail Formationis dominantly coarse-grained subarkosicto arkosic sandstone and con-glomeratic sandstone with lesseramounts of drab, green, gray, andmaroon mudstone. It crops out innorth-central New Mexico in the HaganBasin and the Madrid-Cerrillos-Galisteo area where it unconformablyoverlies Upper Cretaceous strata ofthe Mesaverde Group and isunconformably overlain by lowerEocene red-bed strata of theGalisteo Formation. The index fossil Hyra-cotherium sp. establishes theWasatchian (late Paleocene-earlyEocene) age of the upper part of theDiamond Tail Formation. The DiamondTail Formation is tentatively correlatedwith the Cuba Mesa and Regina Mem-bers of the San Jose Formation andappears to record an early phase ofLaramide trans-tensional subsidence inthe Galisteo Basin.

Introduction

The boundary between Cretaceous andPaleogene rocks in north-central New

Mexico has long been identified as thebase of the Galisteo Formation. Thisboundary is a profound regional uncon-formity between rocks of Late Cretaceousand putative early Eocene age that hasbeen interpreted to reflect a major pulse inLaramide tectonism. This pulse estab-lished the Laramide Galisteo Basin as adepocenter during Paleogene time. Here, wereinterpret the stratigraphic relationshipsof the lower part of the Galisteo Formationand modify previous concepts of the lateLaramide evolution of the Galisteo Basin.

Previous studies

Stearns (1943) established the criteriaused by subsequent workers to recognizeand map the base o f the Gal is te oFormation in north-central New Mexico.As noted by Stearns (1943), there is a dis-conformable contact between the EoceneGalisteo Formation and the Upper Creta-ceous Mesaverde Group. As summarizedby Lucas (1982, p. 9):

Mesaverde sandstones below the contactare fine to medium grained and thinlylaminated. In contrast, Galisteosandstones above the contact are coarsegrained to conglomeratic and are morethickly laminated. Most Mesaverdemudstones are gray and black (organicrich). Most mudstones of the Galisteo

Formation, however, are green andred.Although the Mesaverde-Galisteo con-tact locally appears to beconformable, regionally the contact isa major unconform i t y re p re se n t i ngmu ch o f the La te Cretaceous andprobably some of the early Tertiary(Stearns, 1943; Gorham, 1979;Beaumont, 1979).

These criteria of Stearns (1943) for se-lecting the Mesaverde-Galisteo contactare well accepted and have provided thebasis for most subsequent mapping (e.g.,Disbrow and Stoll, 1957; Bachman, 1975;Picha, 1982) and interpretation of the La-ramide depositional and tectonic historyof the Galisteo Basin (Baltz, 1978; Chapinand Cather, 1981; Gorham, 1979; Gorhamand Ingersoll, 1979; Lucas and Ingersoll,1981; Lucas, 1984; Smith et al., 1985;Cather, 1992; Lucas and Williamson, 1993;Abbott et al., 1995). In this paper, wedemonstrate that the Galisteo Formationas currently construed consists of two tec-tonosequences, i.e., two lithologically dis-tinctive, unconformity-bounded unitsdeposited as responses to separate tectonicevents. We eliminate the lower tectono-sequence from the Galisteo Formation andidentify it as a lithologically distinct, map-pable unit—the Diamond Tail Formation.We also briefly discuss the tectonic impli-cations of this revised stratigraphy.

New Mexico Geology November 1997 89

Diamond Tail Formation

Name and type section

We recognize strata previously includedin the lower part of the Galisteo as aseparate stratigraphic unit, the DiamondTail Formation (Fig. 1). The formationtakes its name from the Diamond TailRanch in the Hagan Basin ofSandoval County. The type section ofthe Diamond Tail Formation and mostof its Hagan Basin outcrops are on landsowned by the ranch.The type section of the Diamond Tail

Formation is 1.0 km north of theghost town of Hagan in theNE¼NW¼SW¼ sec. 28 T13N R6E. Thisis essentially the same section asGorham's (1979, appendix I, plate 5;Gorham and Ingersoll, 1979, fig. 2) Hagansection. Strata in this section thatGorham (1979) assigned to the lowerpart of the Galisteo Formation—his"lower buff-pebbly sandstone" through"lower buff-sandstone and mudstone"informal members—compose the typesection of the Diamond Tail Formation.These strata also correspond to theinformally designated "lower unit of thestrata of the Galisteo basin" of Abbott etal. (1995).At its type section (Fig. 2), the Diamond

Tail Formation is 442 m thick. It discon-formably overlies coal-bearing strataof the Menefee Formation (Picha, 1982)and is disconformably overlain by theGalisteo Formation. Here, three informalmembers of the Diamond Tail Formationcan be recognized.The lower member is 165 m thick and is

mostly grayish-orange andyellowish-gray, medium- to coarse-grained, troughcrossbedded sandstoneand conglomeratic sandstone. Thesandstone is subarkosic to arkosic, andconglomerate clasts are dominantlyquartzite and chert up to 0.5 cm indiameter, although a few larger (up to4 cm diameter) clasts are present. Thinbeds and lenses of olive-gray mudstoneare a minor constituent. Ironstonecannonball concretions are common (Fig.3B,C); logs and fragments of silicifiedwood are locally present. This lowermember of the Diamond TailFormation at its type section forms aprominent hogback between strikevalleys formed by Menefee coals, lignites,carbonaceous shales and lenticularsandstones below and variegatedmudstones of the middle member of theDiamond Tail Formation above. Thebasal contact is marked by a pronounceddisconformity, with conglomeraticsandstone of the basal Diamond Tailoverlying fine-grained sandstone andcarbonaceous shale of the Mesaverde(Fig. 3A).The middle member is 176 m thick and

forms a northwest-striking valley atthe type section. It consists of variegatedlight-olive to maroon mudstone andminor sandstone (Fig. 3D). Mudstones

contain abundant sideritic concretions,many of which are cannonball shaped.The upper member of the Diamond Tail

Formation at the type section is 101 m ofsandstone and mudstone similar inmany features to the lower member.Sandstones are mostly subarkoses thatare yellowish gray to grayish orange and

trough cross-bedded. Some sandstonebeds are pebbly, with numerous clasts ofgray-to-white quartzite up to 0.5 cm indiameter. Mud-stone is a subordinatepart of the unit and is greenish gray. Theupper member forms a hogback togetherwith overlying basal

New Mexico Geology November 1997 91

conglomerate and sandstone of theGalisteo Formation (Fig. 3D). The base ofthe Galisteo Formation is readily selectedabove the upper member at the base of alaterally continuous conglomeratethat has a coarse-grained, subarkosic,very pale orange to pale yellowish-brown matrix (Fig. 3E). Clasts are up to4 cm in diameter and consist mostly ofchert, quartzite, and Paleozoic limestoneand sandstone. Paleozoic clasts becomemuch more abundant higher in thesection.

Modification of Galisteo Formationtype section

Recognit ion o f the Diamond Tai lFormation restricts the Galisteo Formationand therefore alters the content of theGalisteo Formation type section. Lucas(1982, p. 9-10) described this type sect ioneast of Cerrillos in secs. 15, 16, 21, 22T14N R8E. Units 10-21 of this section(120 m thick) are assigned by us to theDiamond Tail Formation. This limits theGalisteo Formation at its type section tounits 22-83 of Lucas's (1982, fig. 4)section, for a total thickness of 979 m.

East of Cerrillos, the Diamond TailFormation is exposed north of GalisteoCreek in secs. 16, 20, 21 T14N R8E. Thebasal bed of the formation is a locally con-glomeratic brown sandstone; clasts aregray and white quartzite and chert asmuch as 0.5 cm in diameter. This conglo-meratic sandstone rests with prominentscour on fine- to medium-grainedquartzose sandstone of the MesaverdeGroup. Overlying Diamond Tail Formationmud-stones are brown and gray, as aresandstones, which make up most of theDiamond Tail section. The sandstones aremostly medium- to coarse-grained, con-glomeratic, trough-crossbedded subarkos-es. Ironstone concretions are abundant.The base of the overlying Galisteo Forma-tion is a polymodal conglomerate of Pre-cambrian quartzite and granite with sub-ordinate Paleozoic limestone, sandstone,and chert clasts similar to beds in thelower part of the Galisteo Formation at thetype section of the Diamond TailFormation (Fig. 3F). Red and greenmudstone immediately overlie the basalGalisteo conglomerate near Cerrillos.

Contacts, distribution, mappability

The upper and lower contacts of theDiamond Tail Formation are regionalunconformities. Coarse-grainedsandstones and conglomerates at thebase of the Diamond Tail Formation arescoured into finer-grained sandstones andcoal-bearing shales of the UpperCretaceous Mesaverde Group (generallyMenefee Formation). Conglomerate atthe base of the Galisteo Formationdisconformably overlies locally pebblysandstone of the Diamond Tail Formation.

The Diamond Tail Formation has a dis-tribution similar to that of the overlyingGalisteo Formation. Major outcrop belts ofthe Diamond Tail Formation (Fig. 1) are:(1) the Hagan Basin, where it crops out ina north–south belt encompassing the typesection in T13-14N R6E [Stearns (1953),Kelley and Northrop (1975), Gorham(1979), and Picha (1982) are among thenumerous workers who mapped asGalisteo Formation strata in theHagan Basin that we assign to theDiamond Tail Formation]; (2) southwest ofCerrillos in T14N R7-8E and at theGalisteo Formation type section east ofCerrillos in secs. 16, 20, 21 T14N R8Ewhere strata mapped as the lower part ofthe Galisteo Formation by Disbrow andStoll (1957), Bachman (1975), and Lucas(1982), among others, belong to theDiamond Tail Formation; (3) farthereast, outcrops south of GalisteoCreek along and around Arroyo Choro(T13N R8-9E) and to the northeast in thevicinity of (mostly west of) Galisteo in T13-14N R9-10E, include strata mappedas the lower part of the GalisteoFormation by Bachman (1975) andJohnson (1975) that we assign to theDiamond Tail Formation.It is important to note that the Diamond

Tail Formation is not present at the Wind-mill Hill area south of San Ysidro (T14NR1E; Fig. 1) where the basal Galisteo For-mation rests directly on the Menefee For-mation (Galusha, 1966; Slack, 1973; Lucas,1982). The Diamond Tail Formation is notexposed in the St. Peter's Dome area nearLos Alamos (Cather, 1992) or in the LaBajada or La Cienega areas southwest ofSanta Fe (Sun and Baldwin, 1958),although it is possible the Diamond Tailexists in the subsurface in these areas.Outcrops of Galisteo Formation have

been mapped near Placitas by Stearns(1953), Kelley (1977), Kelley and Northrop(1975), and Menne (1989). These rocks aresandstone, drab mudstone, and rare,slightly pebbly sandstone that have beenvariably oxidized beneath the basalunconformity of the Santa Fe Group. Weregard these beds as probably belongingto the Mesaverde Group. It is possible,however, that these strata belong to theDiamond Tail Formation, although nounconformity is apparent between themand the carbonaceous Cretaceous rocksbeneath.Conglomeratic sandstone and drab

mudstone exposed near the towns ofGalisteo and Lamy (Fig. 1) appear to beDiamond Tail Formation. The geographicdistribution and the nature of the upperand lower contacts of the Diamond Tail inthese areas, however, are in need offurther study.Mappabi l i ty o f the Diamond Tai l

Formation can readily bedemonstrated at various scales. Inmost areas it can be easi ly mappedat a scale o f 1:24,000, and it can beseparated from the Galisteo Formationon broader regional-scale maps such as

our Fig. 1.

Thickness and lithology

The max imum th i ck ne s s o f theDiamond Tail Formation is 442 m at itstype section. In the Cerrillos area, a moretypical thickness is 100-150 m. Dramaticchanges in thickness of the Diamond TailFormation take place laterally over shortdistances. For example, in the HaganBasin, the Diamond Tail Formation is 442m thick at its type section, but in Arroyodel Tuerto, 6.5 km to the north, it is only210 m thick. A further 5 km to the north, itis less than 20 m thick.

The thickness distribution of theDiamond Tail is similar to that of theoverlying Galisteo Formation in theHagan Basin (see Gorham, 1979). Bothunits thin to the northwest and exhibittheir maximum thicknesses near theghost town of Hagan. The basalunconformity of the Galisteo Formation,which we select at the base of Gorham's(1979) "lower white conglomeraticsandstone" near Hagan, appears toclimb northwestward in paleoelevation.This is shown by the northwesterly onlapof progressively younger units withinthe Galisteo against the basalunconformity (see Gorham, 1979, plate 5).The northwestward thinning of theDiamond Tail Formation appears to be theresult of both greater pre-Galisteo erosionto the northwest and syndepositionalthickening of the Diamond Tail to thesoutheast near Hagan.

Most of the Diamond Tail Formation isyellow, orange and gray, medium- tocoarse-grained arkose and subarkose (seeGorham, 1979, for petrographic descrip-tions). Trough-crossbedding is the domi-nant bedform of these sandstones, andlocally they contain abundant petrifiedwood and ironstone cannonballconcretions.

Conglomeratic sandstone (and, morerarely, conglomerate) also is a commonrock-type in the Diamond Tail Formation.These conglomeratic units contain mostlysmall (<0.5 cm diameter), well-sortedclasts dominantly of white and grayquartz i te and chert . Diamond Tai lFormation conglomeratic sandstones andconglomerates are thus easilydistinguished from Galisteo Formationconglomerates, which are commonlycoarser-grained (>2 cm diameter) andcontain a greater varie ty o f c lastl i tho logies (quartzite, chert, limestone,granite, sandstone). Clast compositionssuggest that coarse detritus in theDiamond Tail was recycled primarily fromtexturally and mineralogically matureclasts derived largely from Mesozoicstrata. In contrast, clasts in the GalisteoFormation in the Hagan area undergo anabrupt upsection evolution from aDiamond Tail-like suite in the basalconglomerate to a more litho-logicallydiverse, texturally and miner-

92 November 1997 New Mexico Geology

alogically less mature suite above. In otherareas, such as near Cerrillos, thebasal Diamond Tail-like suite is missing,and the polymodal suite comprises thebasal conglomerate. Theserelationships indicate that sedimentsabove the basal Galisteo unconformitywere largely supplied by ris ingbasement upl i f ts that exposedPaleozoic and Precambrian rocks except inlocal areas, such as near Hagan,where scouring and resedimentation ofDiamond Tail deposits was initiallydominant. The detrital record ofapparent unroofing shown by theDiamond Tail /Galisteo sequence isbroadly similar to the unroofing of lateLaramide uplifts described elsewhere inNew Mexico (Seager and Mack, 1986;Gather, 1991, 1992).

Except in the vicinity of the type section,Diamond Tail mudstones are greenor gray; no red-bed mudstones arepresent. Mudstone in the upper partof the Diamond Tail Formation in thearea of its type section, however, is maroon(dark reddish brown and grayish red)and contains numerous sideriticnodules. In contrast, red mudstones ofthe Galisteo Formation are brick red(moderate reddish brown) and lacksideritic nodules.

Age and correlation

Until this study, no fossils otherthan petrified wood were known fromthe Diamond Tail Formation, so nodirect evidence ,of its age wasavailable. All previously reportedfossils from the strata of the GalisteoBasin were found in the restrictedGalisteo Formation, above the DiamondTail.

NMMNH (New Mexico Museum ofNatural History and Science) P-25046 fromNMMNH locality 3043 in the DiamondTail Formation is a left dentary fragmentwith a partial P2 P3-4, and partial M1 (Fig. 4).The dentition is highly worn, indicating arelatively mature individual. The specimenis referred to Hyracotherium sp. based onits size and premolar morphology. TheP2 is narrow (width = 2.5 mm) butpossesses a small talonid. The posteriorpremolars (P3 -4 ) are molariform andpossess prominent metaconids in alingual position and large hypoconids in alabial position. Both lack distinctparaconids but have weak paracristidsat the anterior of the tooth. The labialfaces of both teeth are marked by adiscontinuous basal cingulid.

The tooth size (P3 length = 6.5 mm,width = 3.5 mm; P4 length = 6.7 mm,width = 4.4 mm; M, anterior width =5.3 mm) falls within the range of H.angustidens and H. vasacciense (Kitts,1956). Unfortunately, the charactersused to distinguish these species(e.g., presence/absence of diastemabetween P1 and P2 , P3 morphology:Kitts, 1956) are not present in thisspecimen. The Wasatchian land-mammal"age" (lma) is typified by the occurrence ofHyracotherium. Hyracotherium is not

known before the Wasatchian lma (latePaleocene–early Eocene) but rangesinto the Bridgerian Ima (Krishtalka et al.,1987).

The above specimen wasrecovered from the upper part of themiddle member of the Diamond Tail,1.1 km northeast of Hagan (NE¼SE¼sec. 28 T13N R6E). The youngest datedCretaceous unit it overlies, theMenefee Formation, is of early Campanianage. The oldest age-diagnostic foss i l sf rom the ove r ly ing Gal is teoFormation are middle Wasatchian (earliestEocene) mammals found 248-303 mabove the Diamond Tail–Galisteocontact near Cerrillos (Lucas, 1982;Lucas and Williamson, 1993). TheHyracotherium fossil from the DiamondTail Formation must be older than theLysitean mammals from the lowerpart of the overlying Galisteo For-mation, but is not older than earliest Wa-satchian (Graybullian). The dataprovided by adjacent stratigraphicunits are consistent with a latePaleocene–early Eocene age for theupper part of the Diamond Tail Formation.

We tentatively correlate the Galisteo-Diamond Tail unconformity with thebase of the upper persistentsandstone of the Llaves Member of theSan Jose Formation in the San JuanBasin. The underlying Regina Member(and correlative Ditch Canyon andlower Llaves Members, see Smith,1988) is dominantly drab, variegatedmudstones and subarkosicsandstones(Baltz, 1967; Smith, 1988;

Williamson and Lucas, 1992).Lithologic similarities between theDiamond Tail and Cuba Mesa–ReginaMembers include basal sandstone-dominated sequences, subarkosicsandstones with numerouscannonball concretions and silicifiedwood, generally drab mudstones withnumerous sideritic concretions, andconglomerate dominated by well-roundedclasts of quartzite and chert. Thesesimilarities, as well as comparablestratigraphic position and age,allow a tentative correlation of theDiamond Tail Formation with theCuba Mesa and Regina Members ofthe San Jose Formation in the SanJuan Basin (Fig. 5). Correlation withthe Paleocene Nacimiento Formation(see Baltz, 1967, p. 57) can probablybe ruled out because of thegenerally fine grained nature of theNacimiento and the fact that it hasnot yielded any fossils as young asWasatchian (Williamson and Lucas,1992; Williamson, 1996). A more completeunderstanding of the temporalcorrelation between strata of the SanJuan and Galisteo Basins mustawait better constraints on the age ofthe lower part of the Diamond TailFormation and of the upperNacimiento and lower San Jose(Cuba Mesa Member) Formations.

Tectonic significance

The Galisteo Basin is one of severalwrench-related basins of late Laramide

New Mexico Geology November 1997 93

age (Echo Park-type basins of Chapin andCather, 1981) in New Mexico. The Galisteodiffers from these other basins in atleast three major ways: (1) theGalisteo Basin has a transtensional,rather than a transpressional, origin(Cather, 1992; Abbott et al., 1995); (2)the late Laramide strata of the GalisteoBasin contain two distincttectonosequences, whereas otherEcho Park-type basins in New Mexicoappear to contain only one; and (3)the Galisteo Basin contains a thicksequence of late Laramide clasticdeposits that were derived in largepart from an extensive catchment inthe San Juan Basin area of theeastern Colorado Plateau (Cather,1992, fig. 7). In contrast, sediments inwrench-related basins to the northand south (El Rito, Carthage—La Joya,and Cutter Sag—Love Ranch basins;see Chapin and Cather, 1981) appearto have been locally derived fromsmaller catchments ontranspressional uplifts along the east-ern boundary of the Colorado Plateau.The three attributes of the Galisteo

Basin described above may beinterrelated and share a commonorigin. The extensional aspect of theGalisteo Basin appears to have beenrelated to transtensional deformationalong the Tijeras—Cañoncito fault zone,which acted as a releasing bend in theright-lateral wrench system of theLaramide southern RockyMountains (Cather, 1992). Thisextensional aspect of deformation wouldtend to produce alocal gap of low

topographic relief in the otherwise high-standing transpressional welt thatformed along the eastern margin of theColorado Plateau during late Laramidetime. Low relief in the Galisteo Basinarea would have facilitated theintegration of paleodrainage across thePlateau margin and may explain theexternal paleodrainage of the SanJuan Basin (Smith, 1988, 1992a). Incontrast, other late Laramide basinson the eastern part of the ColoradoPlateau [Baca Basin (Cather andJohnson, 1986); Piceance CreekBasin (Johnson, 1988)] were boundedon the east by transpressional upliftsand show evidence for paleohydrographicclosure.

In the Hagan area, the DiamondTail and Galisteo Formations bothdisplay their maximum thicknessesto the southeast, adjacent to theTijeras—Cañoncito fault zone. Bothunits thin markedly to the northwest, andthe basal beds of the Galisteo appearto lap northwestward across theeroded top of the Diamond Tail. TheTijeras—Cañoncito fault zone was an activeintrabasin structure in the GalisteoBasin during the Eocene, as shown bypreservation of Upper Cretaceousrocks and beds of the Diamond TailFormation on the up-thrown, southeastblock (Abbott et al., 1995). Theserelationships suggest that the DiamondTail and Galisteo Formationsrepresent two distinct tectonosequencesin part of a basin that was subsidingand tilting southeastward a long theTi je ras- Cañoncito fault zone. The

Tijeras—Cañoncito fault zone wasright-oblique normal during theLaramide (Abbott , 1995) , whichlends support to the half-grabenmodel for the Galisteo Basin proposedby Cather (1992).

We cannot demonstrate that theunconformity that intervenes betweenthe Diamond Tail and GalisteoFormations is tectonic in origin. It ispossible, for example, that thisunconformity was the result ofsedimentary offlap and subsequentonlap driven by climatically influencedrates of sediment supply in a uniformlysubsiding half graben. We feel,however, that the abrupt influx oflocally derived Precambrian andPaleozoic clasts at or slightly abovethis unconformity argues for a tectonicorigin.

The upper Galisteo Formation tectono-sequence of early to late Eocene age in theGalisteo Basin is lithologically similarto the fill of other Echo Park-typebasins in New Mexico, which consist ofvariegated red-bed sequences that containpolymodal conglomerates dominatedby local ly derived Paleozoic andPrecambrian lithologies. Althoughnot well constrained everywhere, theage of the fill in these other basins ismiddle to late Eocene (see summary inLucas and Williamson, 1993), coeval withthe upper part of the Galisteo Formation.In contrast, the upper Paleocene—lowerEocene tectonosequence in the GalisteoBasin (i.e., Diamond Tail Formation)has no lithologic or temporal equivalentin the Echo Park-type basins of NewMexico. The Cuba Mesa—Regina section inthe San Juan Basin, however, islithologically similar to the DiamondTail Formation, being dominated by adrab sequence of mudrock,sandstone, and minor conglomeratecharacterized by well-rounded clasts ofquartzite and chert (Smith, 1988;Williamson and Lucas, 1992).Although the Cuba Mesa Member isundated pale-ontologically, its upperpart intertongues with the ReginaMember, which has yielded Wasatchianfossils. The Regina Member (andlaterally equivalent lower Llaves andDitch Canyon Members) is overlain bythe upper persistent sandstone of theLlaves Member and the red beds ofthe Tapicitos Member of middleWasatchian (Lysite) age. We have shown apossible unconformity at the base of theupper persistent sandstone of theLlaves Member in Fig. 5, although thisis speculative. Smith (1988, 1992b)argued that an increased contribution ofcoarse, basement-derived detri tus tomudrocks of the Tapic itos Membermay have allowed greater oxidationand reddening during Eocene pedo-genesis than in the finer grained,more poorly drained paleoso ls o fthe drab Regina Member. A similarexplanation may apply to depositsof the Galisteo Basin, where anincrease in basement -deriveddetritus at or near the base of theGalisteo Formation (as shown by clast

94 November 1997 New Mexico Geology

lithology) coincides approximately withthe change from drab to red coloration.The probable tectonic origin of the uncon-formity beneath the Galisteo Formationsuggests that an episode of middle Wasat-chian (-53 Ma) tectonism caused wide-spread unroofing of basement terranes inthe Laramide Brazos-Sangre de Cristo up-lift, which contributed detritus to both theGalisteo Formation and the TapicitosMember of the San Jose Formation.The lower (Diamond Tail) tectonose -

quence of the Galisteo Basin is uniqueamong the Echo Park-type basins of NewMexico, both lithologically and chronolog-ically. Although we can only speculateabout the origin of this early tectonose-quence, it is possible that the unique trans-tensional nature of the Galisteo Basincaused sediments to be trapped soonerthan in basins along the high-standing,transpressional regions to the north andsouth. Additionally, the extensive SanJuan Basin catchment that apparently exit-ed through the Galisteo Basin (Cather,1992) would have supplied voluminoussediments to provide a stratigraphicrecord of any subsidence events within thebasin.

ACKNOWLEDGMENTS- The New MexicoBureau of Mines and Mineral Resources(C. E. Chapin, Director) and New MexicoMuseum of Natural History and Sciencesupported this research. The owners of theDiamond Tail Ranch allowed access totheir property, and Gary Morgan assistedin the field. Earlier versions of the manu-script were kindly reviewed by C. E.Stearns, B. S. Brister, G. A. Smith, and L. N.Smith.

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