The Pennsylvanian System in New Mexico— overview with ...€¦ · stratigraphy of the territory, but subdivi-sion and naming of Pennsylvanian strata began in the early 1900s (e.g.,

November 2001 NEW MEXICO GEOLOGY 103

AbstractUnderstanding of Pennsylvanian lithostra-tigraphy in New Mexico has developed at anuneven pace, and the nomenclature current-ly applied to Pennsylvanian strata is in somecases antiquated, inconsistent, redundant, orinappropriate. The main Pennsylvanian rocksequences in New Mexico are reviewed, andseveral recommendations for revision of thelithostratigraphic nomenclature are pro-posed. In some cases these recommendationsbuild upon or broaden changes that havebeen implemented by other workers inrestricted areas. These recommendationsinclude: 1) abandon use of MagdalenaGroup in New Mexico; 2) raise MaderaFormation to Group rank; 3) treat previouslyused members within the Madera as forma-

Mountains); 6) reevaluate the lithostrati-graphic (formation and group) names ofThompson (1942), which remain valid and ofpotential use as members of more broadlydefined Pennsylvanian formations; and 7)use New Mexico lithostratigraphic names forsubsurface Pennsylvanian strata within thestate, rather than names applied toPennsylvanian rocks in central Texas.

IntroductionPennsylvanian strata were among the firstto be observed in detail by geologistsentering New Mexico during and immedi-ately after the American occupation, andPennsylvanian fossils were among the firstto be described from New Mexico Territory

tions, including the widely exposed and rec-ognizable units still called “lower gray lime-stone” and “upper arkosic limestone” mem-bers; 4) apply the earliest valid formal namesfor the latter two units (Gray Mesa andAtrasado, originally defined in the Lucerouplift) widely throughout the Madera Groupoutcrop area; in some cases replace other for-mation names that have been proposed, andrecognize that locally a third upper Maderaunit (Bursum Formation and equivalents)may also be present; 5) recognize MaderaGroup strata and terminology more widely(e.g., into the Caballo and RobledoMountains), yet retain current non-Maderaterminology where appropriate, especiallyfor sequences associated with rapidly sub-siding basins (e.g., San Andres Mountains,Sacramento Mountains, Sangre de Cristo

The Pennsylvanian System in New Mexico—overview with suggestions for revision of

stratigraphic nomenclatureBarry S. Kues, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131

FIGURE 1—Currently used Pennsylvanian stratigraphic nomencla-ture in New Mexico (reproduced from Armstrong et al., 1979). Theonly significant addition since 1979 has been establishment of thePorvenir Formation for the lower part of the Madera in the southeast-

ern Sangre de Cristo Mountains, with the upper part recognized as theAlamitos Formation and Madera raised to Group rank (Baltz andMyers, 1984, 1999).

104 NEW MEXICO GEOLOGY November 2001

FIGURE 2—Main exposures of Pennsylvanian strata discussed in text(reproduced from Armstrong et al., 1979). Numbers refer to mountainranges and other locations as follows: 1) northern Sangre de Cristo Moun-tains; 2) southern Sangre de Cristo Mountains; 3) southeastern Sangre deCristo Mountains; 4) Nacimiento and Jemez Mountains; 5) Sandia Moun-tains; 6) Manzanita–Manzano Mountains; 7) Los Pinos Mountains; 8)Socorro area; 9) Lucero uplift; 10) Sierra Ladrones; 11) Magdalena Moun-

tains; 12) Sierra Oscura; 13) Fra Cristobal Range; 14) Sierra Cuchillo area;15) Mud Springs Mountains; 16) Caballo Mountains; 17) Derry Hills area;18) Kingston area; 19) Santa Rita–Silver City area; 20) Big HatchetMountains; 21) Peloncillo Mountains; 22) Robledo Mountains; 23) SanAndres Mountains; 24) northern Organ Mountains; 25) northern FranklinMountains and Bishop Cap Hills; 26) northern Hueco Mountains; 27)Sacramento Mountains.

(e.g., Hall, 1856; Marcou, 1858). Later 19thcentury geologists (e.g., Stevenson, 1881)added information on Pennsylvanianstratigraphy of the territory, but subdivi-sion and naming of Pennsylvanian stratabegan in the early 1900s (e.g., Herrick,1900; Gordon, 1907) and has continuedever since. Thompson (1942) made the first

Although Pennsylvanian strata are notas widely exposed in New Mexico asPermian, Triassic, Cretaceous, and Paleo-gene strata, they include a greater diversi-ty of depositional environments and agreater variety of reported fossil species(more than 1,250) than any system exceptthe Cretaceous (Kues, 1982, table 2). Thick

real attempt to recognize, correlate, andname lithostratigraphic units throughoutNew Mexico; Kottlowski (1960a) summa-rized in detail Pennsylvanian stratigraphicsequences and nomenclature for much ofthe state; and Armstrong et al. (1979) pro-vided a general summary of New MexicoPennsylvanian strata (Fig. 1).


(500+ m [1,640+ ft]) Pennsylvanian sec-tions are exposed in many parts of north-ern and southern New Mexico (Fig. 2)chiefly in fault-block mountain rangesalong the Rio Grande rift and adjacentregions, and Pennsylvanian strata arewidely present in the subsurface in mostparts of the state, where some units arereservoirs for oil and gas (e.g., Broadhead,1999), and others are hydrogeologicaquifers producing water.

Development of our understanding ofPennsylvanian stratigraphy in NewMexico has proceeded at an uneven pace.Pennsylvanian sequences in a few areashave been intensively studied, whereas,more commonly, sequences in other areashave been examined in only moderatedetail, typically in the context of struc-tural/stratigraphic studies of individualmountain ranges. The Pennsylvanian ofsome ranges is known only at the level ofreconnaissance mapping done severaldecades ago. Similarly, the stratigraphicnomenclature applied to Pennsylvaniansequences in New Mexico has developedin a piecemeal fashion, producing a largenumber of currently used group, forma-tion, and member names, some of whichare inappropriate, redundant, informal, orapplied only to unnecessarily restrictedareas, and some of which have been wide-ly used for decades without designation oftype sections or adequate formal defini-tion.

Lateral facies changes may frustraterecognition of stratigraphic units very farfrom their type sections. This has con-tributed, on the one hand, to the establish-ment of unique successions of formationnames in individual, closely spaced moun-tain ranges, and, on the other hand, toapproaches that deemphasize the lithostra-tigraphic subdivision of thick Pennsyl-vanian sequences in favor of relying onfusulinid biostratigraphy to recognizechronostratigraphic units, such as the fivePennsylvanian series—Morrowan, Ato-kan, Desmoinesian, Missourian, and Vir-gilian—as the only subdivisions of thePennsylvanian (Kottlowski, 1960a). Inaddition, in many parts of the state, UpperPennsylvanian strata grade upwardthrough a transitional sequence with pro-gressively less marine influence, into non-marine, clastic, red-bed units near thePennsylvanian–Permian boundary. Theposition of this boundary within some ofthese transitional sequences and the termi-nology for them deserve more study.

A century after the first Pennsylvanianformation was named in New Mexico(Sandia Formation, Herrick, 1900), thispaper 1) provides an overview of Penn-sylvanian stratigraphy around the state, 2)reviews current lithostratigraphic nomen-clature, 3) proposes some revisions of thisnomenclature in some areas, and 4) indi-cates locations where additional study ofthe Pennsylvanian would be useful. A brief

summary of the major recommendationswas presented earlier (Kues, 2000). A cor-relation chart of major exposed Pennsyl-vanian sequences in New Mexico incorpo-rates proposed revisions in lithostrati-graphic nomenclature that are discussed inthis paper (Fig. 3).

In proposing revisions of currently usedstratigraphic nomenclature, the desirabili-ty of maintaining stability of nomenclatureby preserving familiar and long-usednames is recognized, even if in retrospectthey may not have been the most logical orappropriate names to apply to a particularstratigraphic unit. Separate formationnames are appropriate for distinctive litho-logical units that are mappable at a scale of1:24,000; however, the same lithostrati-graphic names should be used for similarlithologic units that may be exposed wide-ly in a region, cropping out, for example, inseveral isolated mountain ranges. Newand existing names should reflect signifi-cant lithological differences from equiva-lent units in a region, but not be estab-lished simply as a convenient way to des-ignate local successions of strata with littlereference to equivalent successions else-where in the region. In this paper, littleattention is devoted to the nomenclature ofthe transitional Pennsylvanian–Permianunits noted above. Recent proposals toraise the Pennsylvanian–Permian bound-ary to a position within the Wolfcampianstage in North America (e.g., Baars et al.,1994; Lucas et al., 2000) are likewise notaddressed. These are the focus of ongoingstudies by the author and others, and con-clusions will be presented in a futurepaper. Although the focus here is onexposed parts of the Pennsylvaniansequence, some observations are includedon terminology of subsurface units as well.

Nomenclature of Thompson (1942)Thompson (1942), in a publication titledPennsylvanian System in New Mexico,sought to classify the strata of the entirePennsylvanian using eight group names,two each for the “Derryan” (Atokan),Desmoinesian, Missourian, and VirgilianSeries; 15 formation names; and one mem-ber name, all of which were new. Hisnames were not used by the U.S.Geological Survey in its large-scale recon-naissance mapping projects in the 1940s,and ultimately none of Thompson’s nameswere used by other workers in later strati-graphic studies in New Mexico, althoughKottlowski (1960a) continued to applyThompson’s Missourian and Virgilian unitnames in the Sierra Oscura.

Thompson’s nomenclature was rejectedby later workers for essentially three rea-sons. First, he initially divided thePennsylvanian using fusulinid biostratig-raphy into the four series (chronostrati-graphic units) noted above, and then fit thelithostratigraphic units he defined to the

chronostratigraphic units, particularlywith respect to their boundaries, ratherthan defining groups and formationsentirely on a lithologic basis, as is requiredby the North American Code ofStratigraphic Nomenclature. It is a miscon-ception, however, that Thompson’s litho-stratigraphic units were nothing more thanfaunal zones, as claimed by some workers(e.g., Kelley and Silver, 1952, p. 89). Hislithostratigraphic units were defined pre-cisely, described in detail, and type sec-tions for each of them were designated.

A second difficulty with Thompson’sunits, however, was that they were basedalmost entirely on the stratigraphy of tworestricted areas. His Atokan and Des-moinesian units were based on exposuresin the Derry Hills–Mud Springs Mountainsarea, and his Missourian and Virgilianunits were based on strata in the northernSierra Oscura, with the exception of onegroup having a type section in the north-ern Sacramento Mountains. AlthoughThompson stated that many of his unitscould be recognized in areas far distantfrom their type sections, he provided littleinformation to aid such correlations. Laterfield geologists found that many of hisunits could not be recognized easily veryfar from their type areas.

Thirdly, Thompson’s groups and forma-tions largely represented restricted strati-graphic intervals that in some cases werenot easily distinguishable lithologicallyfrom underlying or overlying units, and inmany cases were not mappable at thescales required by the present Code ofStratigraphic Nomenclature.

Thompson’s work is noted here in partbecause the rejection of his stratigraphicterminology was premature and done withlittle discussion of its merits. While it istrue that Thompson’s unit names general-ly cannot be applied very widely, someunits can be recognized more than 160 km(100 mi) away from their type localities(e.g., Kottlowski, 1960a, p. 21). Other units,such as Thompson’s Bruton Formation,were well defined lithologically and recog-nizable regionally, to the extent that U.S.Geological Survey geologists adopted andmapped it, although renaming it theBursum Formation (see Wilpolt andWanek, 1951). Thompson’s Armendarisand Bolander Groups (Desmoinesian) aretogether lithologically similar to the wide-spread “lower gray limestone” member ofthe Madera of many previous workers, aunit now considered a formation with for-mal names in some areas of the state.Thompson’s group and formation namesin the Mud Springs Mountains, DerryHills, and Sierra Oscura areas remainavailable (most appropriately as membernames, because the current Code ofStratigraphic Nomenclature [NACSN,1983, p. 858] does not require that mem-bers be mappable) for units in those areas,should future workers studying the


Pennsylvanian stratigraphy in detailrequire member names.

In his 1942 study, Thompson (p. 26) pro-posed the term Derry Series for “all rocksin the central to the extreme south centralareas of New Mexico between the base ofthe Pennsylvanian system and the basalpart of the…Des Moines series,” and thisname has persisted in the more recent lit-erature as a New Mexico equivalent to theAtoka Series of the midcontinent. It is nowknown that Thompson’s type Derry sec-tion (Derry Hills, south of Truth orConsequences) includes strata of lateMorrowan and Atokan age (e.g., Clopineet al., 1991), and that thick sequences ofMorrowan strata occur at the base of thePennsylvanian in both northern and south-ern New Mexico. Recognition of theMorrowan and Atokan Series in NewMexico is straightforward, and it is there-fore preferable to use these series names(which are used widely across much of theU.S.) rather than the superfluous and localterm Derry Series.

essentially all Pennsylvanian sequences inNew Mexico, and in later decades the U.S.Geological Survey and others continued touse the term Magdalena throughout thestate, from the Sangre de Cristo to FranklinMountains. Rock units of Late Missis-sippian age (e.g., the “lower limestonemember” of the Sandia Formation of Woodand Northrop, 1946, now the ArroyoPeñasco Group) and of Early Permian age(e.g., Bursum Formation, Wilpolt et al.,1946) were also included in the MagdalenaGroup. P. B. King (1942, pp. 675–677)believed that the upper part of theMagdalena Group in New Mexico includ-ed much Wolfcampian strata and could betraced southward into the Early PermianHueco Limestone.

Objections to use of the term MagdalenaGroup, registered in the literature for morethan 50 yrs, fall generally into three cate-gories. First, as commonly used in mostregions, the name is synonymous withPennsylvanian System and thus is super-fluous. Second, over the years, the nameMagdalena Group has been applied in somany different ways and to such different

Magdalena Group

Gordon (1907) proposed the name Mag-dalena Group to include the Sandia For-mation (of Herrick, 1900; Herrick andBendrat, 1900) and overlying MaderaLimestone (of Keyes, 1903). The name wastaken from the Magdalena Mountains westof Socorro, and the group was recognizedby Gordon in Socorro, Bernalillo, andSierra Counties, although he noted that itcould not be subdivided in Sierra County.When proposed, the Magdalena Groupwas believed to represent the lower part ofthe Pennsylvanian, the upper part beingrepresented by the Manzano Group (ofHerrick, 1900) and consisting of the Abo,Yeso, and San Andres Formations (Lee andGirty, 1909). The Permian age of the forma-tions of the Manzano Group was soon rec-ognized (e.g., Lee, 1917, 1921), and theterm Manzano Group passed out of usagein the 1930s, leaving the Magdalena Groupessentially synonymous with thePennsylvanian System. Indeed, Darton(1928) and Needham (1940) applied theterm Magdalena Group or Formation to

FIGURE 3—Stratigraphic nomenclature for major Pennsylvanian exposures in New Mexico, as pro-posed in this paper. Note that in some areas, formation-rank units within the Madera Group have yetto be established.


successions of formations that it is essen-tially meaningless in any precise strati-graphic sense. Third, the highly faulted,intruded, metamorphosed, and incom-plete Pennsylvanian section in the Mag-dalena Mountains makes this area unsuit-able for a type section, as Kottlowski (1959,p. 59) aptly noted: “Even with deliberatecare a more unsuitable type section proba-bly could not be found.” In reality, no typesection for the Magdalena Group has everbeen established, nor could an adequatetype section be established in the futurebecause of the anomalous nature of thePennsylvanian section in the MagdalenaMountains, including the absence ofUpper Pennsylvanian beds owing to fault-ing or erosion, and the lack of a strati-graphic contact with the overlying AboFormation (Kottlowski, 1960a).

As early as 1942, Thompson (p. 22) aban-doned use of the term Magdalena Groupas a stratigraphic unit in New Mexico, stat-ing that the term “Magdalena…seems tobe essentially synonymous with the sys-temic term Pennsylvanian…” and “…itseems inadvisable to attempt to preserve

this…term by merely restricting the namein any sense to a small portion of thePennsylvanian in New Mexico.” R. E. King(1945, p. 21) echoed Thompson’s recom-mendation and added his own: “…theunfortunate name Magdalena has becomemore deeply entrenched in recent geologicliterature published by the U.S. GeologicalSurvey…and it is recommended that theterm Magdalena, a relic of an antiquatedtype of stratigraphic nomenclature, be per-manently abandoned.” Pray (1961, p. 72),in rejecting use of Magdalena Group in theSacramento Mountains, stated that the“term is so broad…and the sections towhich it has been applied are so varied inNew Mexico, that it serves little practicalpurpose.” Kottlowski (1960a, p. 21) specu-lated that the main reason for continuingto use Magdalena Group rather than sim-ply Pennsylvanian might be to “indicateinclusion of more than only Pennsylvanianstrata within the group,” and noted(Kottlowski, 1962, p. 343) that “if pre andpost-Pennsylvanian units were eliminatedfrom the Magdalena Group…the remain-der is the equivalent of the Pennsylvanian

System, of which the term Magdalena is asomewhat unnecessary duplication.”Inclusion of Mississippian strata is nolonger an issue, as these have been sepa-rated out and named (e.g., Kelly Lime-stone, Arroyo Peñasco Group). And, asKottlowski undoubtedly realized, if use ofthe name Magdalena Group is an unneces-sary duplication of “Pennsylvanian,” it iseven more nebulous a term when appliedto Pennsylvanian plus Lower Permianstrata.

In recent decades, use of the term Mag-dalena Group has been discontinued inmany areas of New Mexico where it for-merly had been applied. These areasinclude the San Andres Mountains(Kottlowski et al., 1956), SacramentoMountains (Pray, 1961), Sangre de CristoMountains (Sutherland, 1963; Baltz andMyers, 1984, 1999), northern San Andresand Oscura Mountains (Bachman, 1968;Bachman and Harbour, 1970), Joyita Hills(Kottlowski and Stewart, 1970), Manzanoand Manzanita Mountains (Myers, 1973),Los Pinos Mountains (Myers et al., 1986),Nacimiento Mountains (Woodward, 1987),


and generally in central and south-centralNew Mexico (Bachman and Myers, 1975).The term Magdalena Group does notappear anywhere in the statewidePennsylvanian stratigraphic correlationchart of Armstrong et al. (1979, fig. 8).

There is clearly an ongoing, progressiverecognition that application of the nameMagdalena Group or Formation serves nouseful purpose. Further use of the nameshould cease in all parts of New Mexicowhere it is currently still used. These areasinclude the Hueco Mountains (e.g.,Williams, 1963; Stoklosa et al., 1998);Franklin Mountains (e.g., Harbour, 1972;LeMone, 1982, 1992; Kelley and Matheny,1983); Bishop Cap Hills (Seager, 1973);Silver City–Santa Rita area (e.g., Jones etal., 1967, 1970; Pratt, 1967); CaballoMountains and Derry Hills (e.g., Kelleyand Silver, 1952; Seager and Mack, 1991,1998; Mack et al., 1998); Mud SpringsMountains (Maxwell and Oakman, 1990);Fra Cristobal Mountains (Nelson, 1986);Socorro County (e.g., Siemers, 1983;McLemore and Bowie, 1987); Lucero uplift(Kelley and Wood, 1946); and SandiaMountains (Kelley and Northrop, 1975,although Lucas et al., 1999a, b, extendedinto the Sandias Myers’ [1973] terminologyfor the Manzano–Manzanita Mountains,which specifically rejects usage of the termMagdalena Group). The most recent NorthAmerican Code of Stratigraphic Nomen-clature (NACSN, 1983, p. 855) states that“…the lack of need or useful purpose for aunit, may be a basis for abandonment; so,too, may widespread misuse in diverseways which compound confusion.” Bothare true of the Magdalena Group. Perhapsthe most obvious misuse of the nameoccurs in the Franklin Mountains, where asequence of three Morrowan to Des-moinesian formations, composed of mas-sive limestone at the base, and becomingincreasingly shaly upsection, are includedin the Magdalena Group, in complete con-trast to the Magdalena as used in centralNew Mexico, which is primarily clastics atthe base and massive limestones in theDesmoinesian portion. Discontinuing useof the name Magdalena Group will notadversely affect stratigraphic understand-ing of the Pennsylvanian sequences any-where in New Mexico.

Madera Formation/GroupThe name Madera Formation was firstapplied to Upper Carboniferous blue togray beds, “the superior part of the greatlimestone formation” in the SandiaMountains by Keyes (1903). Herrick (1900)had recognized this interval between his“Sandia series” and “Manzano series” butapplied no name to it. Gordon (1907)adopted the name Madera Limestone for“the great limestone plate along the backslope of the Sandia Mountains,” named forthe former village of La Madera, and he

mations, in place of the lower “gray lime-stone” and upper “arkosic limestone”members, respectively. These revisions ofMadera terminology reflect more detailedstratigraphic study and illustrateapproaches that are applicable to otherranges as well. These approaches, whichare used below, include: 1) raising theMadera Formation to Group, a moreappropriate rank, given its great thickness(typically 300–700+ m [1,000–2,300 ft]),widespread distribution, and long dura-tion (Desmoinesian to Virgilian or earlyWolfcampian); 2) formally naming forma-tions to replace informal subdivisions suchas lower "gray limestone" and upper"arkosic limestone" members; and 3) whereappropriate, formally naming members todesignate localized but distinctive litho-logic units within formations. The MaderaGroup encompasses two or three forma-tion-rank units that are readily mappableat a scale of 1:24,000.

The Madera Group generally reflectstwo major depositional sequences. DuringDesmoinesian time marine depositionacross widespread carbonate shelf envi-ronments (e.g., Ye et al., 1996) producedmassive, gray, cliff-forming limestoneunits, typically cherty, with little interbed-ded clastic sediments. This was followedby marine environments that were increas-ingly, although not everywhere evenly,affected by influx of siliciclastic sedimentsderived from increased tectonic activity in(principally) the Uncompahgre, Pedernal,and Zuni land masses associated withAncestral Rocky Mountains tectonism dur-ing Missourian, Virgilian, and locally intoWolfcampian time. Expansion of continen-tal, transitional (e.g., delta, lagoon), coast-line, and marine, clastic shelf environ-ments at the expense of carbonate shelfenvironments occurred during this time(Ye et al., 1996). In the upper part of theMadera Group, marine limestones typical-ly still predominate, but generally in thin-ner beds and with little to no chert.Interbedded shales and siltstonesapproach (in the Missourian and Virgilian)or may locally surpass (in the Virgilian andearly Wolfcampian) the thickness of thelimestones. Siliciclastic beds in the upperMadera Group are generally gray orbrown low in the upper unit, but becomegradually more colorful upsection, withthe addition of red, maroon, purple, andgreen lithologies, which ultimately domi-nate the uppermost part of the Maderasequence. Finally, marine deposition andlimestone lithologies cease, and continen-tal nonmarine red-bed clastics of the AboFormation prevail.

The term Madera Group appropriatelyencompasses this broad depositional pat-tern, which can be recognized across muchof New Mexico. Units of formation rank, ascharacterized in the present Code ofStratigraphic Nomenclature (NACSN,1983, p. 858), are suitable for the main

recognized the formation as the upper partof his Magdalena Group in Bernalillo andSocorro Counties. No formal type sectionfor the Madera has ever been established,but a reasonable reference section was pre-sented by Kelley and Northrop (1975, p.125) at Montezuma Ridge, east of Placitasand a few kilometers north of the site of LaMadera village.

Although Thompson (1942) arguedagainst using the name Madera becauseKeyes’ (1903) definition was so poor andhe was inconsistent in his usage, the termhas proven useful for the thick, predomi-nantly limestone unit between the SandiaFormation and the Wolfcampian Abo redbeds. By 1950 the Madera Formation wasrecognized in the Sandia, Manzano,Nacimiento, Sangre de Cristo, Los Pinos,Oscura, and Magdalena ranges, the Lucerouplift, the Joyita Hills, and elsewhere inSocorro County. Detailed stratigraphic sec-tions were published for the Madera inmany of these areas, leaving no doubt as toits lithologic composition and stratigraph-ic position. In the 1940s (see Read andWood, 1947) the U.S. Geological Survey(USGS) adopted the convention in recon-naissance mapping of recognizing infor-mally a lower “gray limestone member” ofDesmoinesian age and an upper “arkosiclimestone member,” typically of Missour-ian and Virgilian age. An exception was inthe Lucero uplift area (see below), whereKelley and Wood (1946) applied formalnames to these subdivisions of the Madera.In central New Mexico, from the south endof the Manzano Mountains southward, aseparate Bursum Formation was also rec-ognized, incorporating Early Permian bedstransitional between the mainly marinesediments of the Madera Formation andthe nonmarine red-bed clastics of the AboFormation. In southern New Mexico,Madera terminology generally has notbeen used. Strata correlative with theMadera have received local formationnames, resulting in separate stratigraphicnomenclatures applied to the Pennsylvan-ian sequences in the Caballo, San Andres,Sacramento, and Franklin Mountains, andin the Silver City–Santa Rita area.

As studies progressed, the MaderaFormation in two areas was raised togroup status, its members raised to forma-tions, formal formation names given to thelower “gray limestone” and upper“arkosic limestone” members, and formalmember names given to subdivisions ofthese units. For example, in the Manzanoand Manzanita Mountains (see below) thelower member was named the Los MoyosLimestone and the upper member the WildCow Formation by Myers (1973), within aMadera Group that also included theBursum Formation. Similarly, in the south-eastern Sangre de Cristo Mountains (seebelow), Baltz and Myers (1984, 1999)raised the Madera to group rank and rec-ognized the Porvenir and Alamitos For-


lithologic units, including the lower chertylimestone sequence and the upperinterbedded limestone/siliciclastics se-quence. As previous workers have done insome areas, this upper sequence can bedivided into two formation-rank units, thelower of which is dominated by marinelimestone with no or few red, maroon, orgreen beds, and the upper of which is typ-ically thinner and dominated by coloredclastic beds, typically of nonmarine origin,with subsidiary marine limestones.

The Madera Group reflects depositionon shelf and basin margin, and in slowlysubsiding basin conditions that were dom-inated by marine environments until near-ly the end of Madera deposition.Sequences of similar age deposited inrapidly subsiding basins (e.g., OrograndeBasin, Taos trough) are lithologically con-siderably different and generally muchthicker than Madera Group sequences.Thus, although Baltz and Myers (1984,1999) included the Porvenir and AlamitosFormations within the Madera Group innorth-central New Mexico, these Taostrough formations differ so greatly fromtypical Madera lithologies that applicationof the term Madera Group to them is notrecommended. The Desmoinesian Porve-nir Formation (see below) consists of dom-inantly clastic facies (even in the most car-bonate-rich sections, limestones—general-ly noncherty—are less than 50% of the totalthickness), in contrast to the uniform, mas-sive, cherty limestone beds of the lowerpart of the Madera Group. The lateDesmoinesian to Virgilian Alamitos For-mation consists predominantly of nonma-rine arkosic sandstones and conglomer-ates, shale, and minor marine limestones,quite different from the alternating marinelimestones and shales, with minimalcoarse clastics, that characterize the upperpart of typical Madera sequences.Similarly, rapid subsidence and filling ofthe central Orogrande Basin in theMissourian and Virgilian produced anabnormally thick (1,000 m; 3,280 ft)Panther Seep sequence that consists main-ly (74%) of coarse to fine siliciclastics(Schoderbek, 1994) with minor marinelimestone and some evaporite beds thatreflect mainly nonmarine deposition—again, so different in lithology and thick-ness from typical upper Madera units thatMadera Group terminology is clearly inap-propriate.

Within the general succession of typicalMadera strata defined above, lateral varia-tions in deposition reflect different envi-ronments, distance from land masses, andshort-term transgressive and regressivesequences owing to fluctuations in tecton-ic and eustatic activity. Where warrantedby considerable differences in lithology,these units may also be recognized as localformations, but normally recognition asmembers of more broadly defined forma-tions is recommended. Member-level

lithostratigraphic units in the MaderaGroup typically reflect local, gradationallateral or vertical facies changes, and there-fore are restricted in extent, perhaps limit-ed to individual mountain ranges. Fine-scale sequence-stratigraphic studies, suchas have been undertaken in restricted partsof the Madera Group in the SandiaMountains (e.g., Wiberg and Smith, 1993;Smith, 1999), will help to refine under-standing of the cyclic sedimentary process-es responsible for Madera lithologic unitsand ultimately, the nomenclature that isapplied to them.

Recommendations for revision of strati-graphic nomenclature for units within theMadera Group follow the philosophy dis-cussed above. The earliest formal namesapplied to the lower "gray limestone" andupper "arkosic limestone" members of theMadera Formation were established byKelley and Wood (1946) in the Lucerouplift–Sierra Ladrones region of westernValencia and northwestern SocorroCounties (see below). These members,Gray Mesa and Atrasado, respectively, inthis paper are considered formations with-in the Madera Group, and are recognizedwidely in central New Mexico, from theNacimiento and Jemez Mountains on thenorth, to the Caballo and RobledoMountains on the south, along the westernplatform margin of the Orogrande Basin.The transitional Pennsylvanian–Permianstrata at the top of the Madera Group,including the Bursum Formation andBursum-equivalent strata, whether namedor not, are not covered in detail here; afuture paper will describe and correlatethese strata and consider their nomencla-ture.

Lucero uplift–Sierra Ladrones Kelley and Wood (1946) recognized theSandia and Madera Formations within theMagdalena Group in the Lucero uplift–Sierra Ladrones region and named (inascending order) the Gray Mesa, Atrasado,and Red Tanks Members within theMadera. Thickness of the Madera rangesfrom approximately 525 m to 700 m (1,725–2,300 ft) from north to south along theLucero uplift to the Sierra Ladrones, andeach member is lithologically distinctive,more than 100 m (325 ft) thick, and can bemapped at a scale of 1:63,000 (Kelley andWood, 1946). These members meet all therequirements of formation-rank units. Theterm Magdalena Group is herein aban-doned in the Lucero uplift area, for reasonsnoted above, and the Madera is raised togroup status, with its constituent membersas formations, as has been done in theneighboring Manzano–Manzanita and LosPinos Mountains (Myers, 1973; Myers etal., 1986).

The formations of the Madera Group inthe Lucero uplift area are important, asthey are each well exposed and their

names were the first formal names to beapplied to the three subdivisions of theMadera that can be recognized widely incentral and south-central New Mexico.Kelley and Wood's descriptions of thethree formations, though brief, adequatelycharacterize them, and subsequent work-ers (Martin, 1971, for the Gray Mesa andAtrasado, and Kues and Kietzke, 1976, forthe Red Tanks) have studied the stratigra-phy of these units in detail, providing thebasis for comparing them with equivalentunits elsewhere in New Mexico.

As defined by Kelley and Wood (1946),the Gray Mesa Limestone consists of pre-dominantly massive, gray, markedly cher-ty, cliff-forming limestones, as well asminor amounts of gray shale and sand-stone, and a conspicuous tan-weatheringlimestone at the top. Thin-bedded andlocally noncherty limestones are also pre-sent, though not abundant. Martin (1971),on the basis of stratigraphic sections mea-sured on Gray Mesa (= Mesa Aparejo onUSGS topographic maps) and CarrizoMesa, Monte de Belen (= Mesa Sarca), andin the northern and southern SierraLadrones, determined the thickness of theGray Mesa Limestone to range fromapproximately 215 m to 250 m (700–820 ft),and its composition to be between 70% and88% marine limestones, 4–23% shale/silt-stone, and 4–7% sandstone/conglomerate.The highest proportion of fine to coarsesiliciclastics (30%) is in the southernmostsection (southern Sierra Ladrones). Theage of the Gray Mesa Limestone, based ondetailed fusulinid biostratigraphy (Martin,1971), ranges from at or just above theAtokan–Desmoinesian boundary to theend of the Desmoinesian. The Desmoi-nesian–Missourian boundary correspondsin all sections to an abrupt change in lithol-ogy from massive limestone beds to a thickshale sequence at the base of the overlyingAtrasado Formation.

Kelley and Wood (1946) characterizedthe Atrasado Formation as consisting ofthin- to thick-bedded, gray limestone, grayand reddish shales, and red to light red-dish-brown and gray conglomeratic sand-stone. The limestones in the Atrasado havefar less chert than those of the Gray MesaLimestone. Martin's (1971) study indicatedthicknesses ranging from approximately225 m (740 ft) on the north to possibly asmuch as 450 m (1,475 ft) in the southernSierra Ladrones. Martin (1971), usingseries divisions within the Madera, did notindicate the division between the Atrasadoand Red Tanks Formations in his strati-graphic analysis, and in the southernSierra Ladrones the boundary is not clear.Kottlowski (1960a) reported a similarthickness of approximately 420 m (1,375 ft)for the Atrasado-equivalent (Missourian–Virgilian) sequence, and Siemers (1983)reported 455 m (approximately 1,500 ft) forthis sequence in the Sierra Ladrones. TheAtrasado consists of a cyclic sequence of


from 15 to more than 20 thin-to-thick lime-stone units separated by shale intervals ofequal or greater thickness. Age rangesfrom earliest Missourian to well into, andpossibly near the end of, the Virgilian(Martin, 1971). The northernmost sectionof the Atrasado (Mesa Aparejo–MesaCarrizo) includes approximately 63% offine- to coarse-grained siliciclastic bedsand approximately 37% limestones; theproportion of limestone increases to nearly50% to the south, in the Sierra Ladrones.Sandstone and conglomerate beds are veryminor (typically approximately 2–3% in allsections), and shale units, mostly gray togreenish gray in the Missourian part of theformation, display more varied coloration(yellow, purple, red) in the Virgilian por-tion.

The Red Tanks Formation, as thick asapproximately 135 m (440 ft; Kues andKietzke, 1976) consists mainly of green,red, reddish-brown, and gray shales (thelatter locally carbonaceous with a thin coalseam), and subordinate gray marine lime-stone beds and sandstones, both essential-ly restricted to the lower 60% of the forma-tion. Kelley and Wood (1946) andArmstrong et al. (1979) believed the RedTanks to be of Late Pennsylvanian age, butlater work on its prolific and varied faunaand flora (e.g., Tidwell et al., 1999, and ref-erences therein) suggest that most of theformation is Wolfcampian. The Red Tanksis therefore coeval with, although litholog-ically distinct from, the upper unit of theMadera Group (Bursum Formation) to theeast.

Kelley and Wood (1946) did not desig-nate type sections for the three members ofthe Madera they defined, and the namesGray Mesa and Atrasado (from AtrasadoArroyo) were based on geographic fea-tures indicated on their map, which werelater replaced by the names Mesa Aparejoand Arroyo Alamitos on U.S. GeologicalSurvey topographic maps. A name change,or even the disappearance of a geographicfeature, is not a basis for abandoning alithostratigraphic name that was based onthe feature (see NACSN, 1983, pp.852–853), so Gray Mesa and Atrasado,which have been used in the Lucero upliftarea for more than a half century, are con-sidered valid names. Later workers (e.g.,Bates, 1947; Jicha and Lochman-Balk, 1958)indicated Gray Mesa (Mesa Aparejo) as thetype section for the Gray Mesa Member,indicated tributaries of Red Tanks Arroyoas the type locality for the Red TanksMember, and stated that no type sectionfor the Atrasado had been designated.However, this information was apparentlybased on the assumption that type sectionsmust be at the localities that were thesources of the names for particular units,which, of course, is not necessarily true.Here, the type section for the Gray MesaLimestone and Atrasado Formations isconsidered to be the sequence exposed

most places the top of the SandiaFormation is easily placed at the base ofthe first massive limestone of the lowerMadera. In the Sandia Mountains, thethickness of the Sandia Formation rangesfrom approximately 15 m (50 ft) to possiblyas much as 90 m (300 ft), and its age,though poorly constrained, is Atokan topossibly early Desmoinesian (Kelley andNorthrop, 1975). The Sandia Formationhas been recognized widely in central NewMexico, from the southeastern Sangre deCristo Mountains (Baltz and Myers, 1984,1999) on the north to the Mud SpringsMountains (Maxwell and Oakman, 1986)on the south.

In the Sandia Mountains, Kelley andNorthrop (1975) treated the Madera, whichis 400+ m (1,300+ ft) thick, as a formationin the Magdalena Group, using the infor-mal lower “gray limestone” and upper“arkosic limestone” member names. Lucaset al. (1999a, b) provisionally extended thenames Los Moyos Limestone and WildCow Formation from the ManzanoMountains into the Sandias, implicitlyabandoning the Magdalena Group andraising Madera to group rank. As suggest-ed elsewhere in this paper, the Lucerouplift names Gray Mesa, instead of LosMoyos, and Atrasado, instead of WildCow, may be more appropriate names forthese units, based on similarity of lithologyand priority, in the Manzano as well as theSandia Mountains. A retreat to the oldinformal members of a Madera Formationin recent mapping in the Sandia andManzano Mountains (e.g., Ferguson et al.,1996; Read et al., 1998, 1999) is basedapparently on the perception that formallithostratigraphic units cannot be recog-nized if the boundaries between them aregradational, and that the named lithostra-tigraphic units in the Manzano Mountainsare based on fusulinid biostratigraphy.Both are erroneous misconceptions, andthe informal Pennsylvanian stratigraphicdivisions of these workers are rejected.

Further work is needed in order to estab-lish the boundaries between the LosMoyos (or Gray Mesa) and Wild Cow (orAtrasado) Formations in the SandiaMountains. Read et al. (1944) for example,assigned only the lower 39% and 34% oftwo measured Madera sections (MonteLargo and Tejano Canyon) to the lower“gray limestone” member, whereas Kelleyand Northrop (1975, p. 125), in theirMontezuma Ridge reference section,included 239 m (783 ft) of the total 386 m(1,267 ft; = 62%) thickness of the Madera inthe lower member, despite their assertion(p. 34) that the lower member constitutesonly 40–45% of the total Madera thickness.In addition, their probable boundarybetween the lower and upper memberswas placed at the base of a 38-m-thick (126-ft-thick) sequence of blue-gray, chertylimestones, which would appear lithologi-cally to be better placed in the lower mem-

along the east slope of Gray (= Aparejo)and Carrizo Mesas, as measured by Martin(1971) in sec. 14 T5N R3W and sec. 7 T6NR2W southwestward to sec. 35 T6N R3W.The type section for the Red TanksFormation is the locality where Kelley andWood (1946) indicated the most completesection of this unit is exposed, and whereKues and Kietzke (1976) measured a com-plete stratigraphic section, along CarrizoArroyo near the northeast end of MesaCarrizo (= South Mesa of Kelley andWood), around the center of SW1⁄4 sec. 6T6N R2W. All of these localities are inValencia County.

To complete this summary of Pennsyl-vanian strata in the Lucero uplift area, theSandia Formation is conformably presentbelow the Madera Group, resting uncon-formably on Mississippian limestones. It iscomposed of mainly shale and sandstone,and thickens southward from approxi-mately 46 m (150 ft) at Mesa Aparejo toapproximately 150 m (500 ft) in the south-ern Sierra Ladrones. Sandstones and con-glomerate, primarily quartz arenites, rep-resent approximately 72% of the totalSandia Formation to the north but dwindleto 12% in the south. In contrast, dark, oftencarbonaceous shales represent 11% of thetotal thickness to the north but increase to83% in the south. Marine limestones arelimited to between one and a few thin bedsand total no more than 17% of the forma-tion thickness at any locality. Fusulinidbiostratigraphy (Martin, 1971) indicates anAtokan to earliest Desmoinesian age forthe Sandia Formation at most localities inthe Lucero uplift area.

Sandia MountainsThe first two formation names (Sandia,Madera) in current usage that wereapplied to Pennsylvanian strata in NewMexico were based on rocks in the SandiaMountains. Herrick (1900) and Herrickand Bendrat (1900) established the “Sandiaseries” for a 46-m-thick (250-ft-thick)sequence of shales, sandstones, and con-glomerate above Proterozoic granite in theSandia Mountains, and Gordon (1907)adopted Herrick’s name for the lower partof his Magdalena Group. Kelley andNorthrop (1975) summarized the subse-quent development of the concept of theSandia Formation in the Sandia and neigh-boring ranges, including eventual recogni-tion of a basal, intermittent, thin lower car-bonate sequence as Mississippian in age.In the Sandia Mountains, the SandiaFormation consists of (in order of abun-dance) sandstone, shale, limestone, con-glomerate, and siltstone. Although lateralfacies changes are pronounced, clasticsalways dominate, and in some sectionslimestone beds are very sparse. The shaleis black, gray, or olive and has local coalseams and plant fossils. Contact with theoverlying Madera is gradational, but in


ber. Such great variation in the thickness ofthe two Madera units is less likely a resultof lateral depositional variation over shortdistances than due to inconsistent defini-tion of the two units. The Red Tanks andBursum Formations have not been rec-ognized in the Sandia Mountains, but atransitional sequence of predominantlynonmarine siliciclastic sediments (notedby Kelley and Northrop, 1975, p. 30), ofvarying thickness, is present at the top ofthe Madera Group, immediately below theAbo Formation. This transitional sequenceis thin in the Placitas area (Lucas et al.,1999b) but is much thicker to the southeast,where it is currently under study.

The member names of the Wild CowFormation established by Myers (1973) inthe Manzano and Manzanita Mountainsare explicitly not extended to the Sandiasat the present time, as more detailed strati-graphic study is required to evaluate theirpresence or absence there.

Manzano–Manzanita MountainsRead and Wood (1947) continued Gordon’s(1907) division of the Pennsylvanian in theManzano Mountains into the Sandia andMadera Formations of the MagdalenaGroup, and they divided the Madera intolower “gray limestone” and upper“arkosic limestone” members. Myers(1973), based on extensive mapping in therange, raised the Madera to group rank,named the lower member the Los MoyosLimestone, and named the upper memberthe Wild Cow Formation, and he includedthe Bursum Formation as the upper unit ofthe Madera Group. Three named membersof the Wild Cow Formation, in ascendingorder, Sol se Mete, Pine Shadow, and LaCasa Members, were also established andmapped. The ages of all of these units arewell constrained by fusulinid biostratigra-phy (Myers, 1988a, b).

In the Manzanos, the Sandia Formationranges from 15 m to 92 m (50–300 ft) inthickness; is Atokan in age; and consists ofa basal conglomeratic sandstone overlainby gray to brown sandstones, dark-gray tobrownish-gray, locally carbonaceous andplant-bearing shales, and thin, gray marinelimestones, which are more common nearthe top (Myers, 1982). Within the MaderaGroup, the Los Moyos Limestone is 180 m(approximately 600 ft) thick; is earlyDesmoinesian to earliest Missourian inage; gradationally overlies the SandiaFormation; and is predominantly gray,highly cherty limestone, calcarenitic inparts of the sequence, with much thinnerinterbeds of sandstone, siltstone, shale,and conglomerate. The overlying WildCow Formation is 225–275 m (approxi-mately 750–900 ft) thick; is earlyMissourian to earliest Wolfcampian in age;and “consists of alternating sequences ofarkosic sandstone, sandstone, conglomer-ate, gray to yellow siltstone and shale, gray

to black calcareous shale, and thin- tothick-bedded calcarenite that is locallycherty” (Myers, 1982, p. 236). The threemembers of the Wild Cow defined byMyers (1973) are of similar thickness, andeach includes sandstone, conglomerate,shale, and limestone beds that vary lateral-ly lithologically from north to south to aconsiderable degree. There do not appearto be any consistent lithologic characteris-tics of any of these members that distin-guish them from each other. In theManzanita Mountains, the Pine ShadowMember includes a deltaic sequence,exposed in Kinney quarry, which containsa remarkable mainly nonmarine fauna andflora that is documented in a book-lengthvolume (Zidek, 1992).

The Madera section in the Man-zano–Manzanita Mountains is quite simi-lar to that in the Lucero uplift, only 60–70km (37–43 mi) to the west, and it is unfor-tunate that Myers coined new names forthe formation-rank units in the Manzanoswithout detailed comparison with the pre-viously named equivalent units in theLucero uplift. With little doubt, the LosMoyos Limestone in the ManzanoMountains is the same formation as theGray Mesa in the Lucero uplift. Thedescriptions of the overlying Missourian–Virgilian Atrasado Formation provided byKelley and Wood (1946) and Martin (1971)suggest that it is the same unit describedby Myers (1973) as the Wild Cow Forma-tion in the Manzano–Manzanita Moun-tains. Detailed comparison between theAtrasado and Wild Cow Formations, interms of fine-scale vertical and lateral dis-tribution of lithologies and facies, wouldbe worthwhile. It is clear that the WildCow Formation is characterized by signifi-cant lithologic heterogeneity as a sequenceof interbedded limestone, sandstone, silt-stone, shale, and conglomerate, the relativeproportions of these lithologies varyinglaterally and vertically. The AtrasadoFormation has a similar heterogeneouslithology, although local facies differencesare present. Both formations occupy thesame position in the Pennsylvanian strati-graphic succession, both are of Mis-sourian–Virgilian age, are geographicallyclosely spaced, and possess extreme litho-logic heterogeneity of the same generalkind, “which in itself may constitute aform of unity when compared to the adja-cent rock units” (North American Code ofStratigraphic Nomenclature, 1983, p. 858).The latter point is emphasized by the codeas one of the bases on which formationsmay be recognized. It is difficult to escapethe conclusion that strata very similar tothe unit named the Atrasado Formation inthe Lucero uplift are present in theManzano Mountains (where they havebeen called the Wild Cow Formation), aswell as in the Sandia Mountains, the upliftsof Socorro County, and elsewhere wherethey have been recognized informally as

the “upper arkosic limestone member” ofthe Madera Formation.

The relationship between the lateVirgilian–early Wolfcampian La CasaMember of the Wild Cow Formation plusBursum Formation in the ManzanoMountains and the Red Tanks Formationin the Lucero uplift is less certain and cur-rently under study.

Socorro CountySiemers (1983) studied the Pennsylvanianstrata of uplifts in Socorro County, includ-ing the Magdalena, Lemitar, southeast SanMateo, and northern Oscura Mountains,Sierra Ladrones, Joyita Hills, Little SanPasqual Mountain, and the Cerros deAmado. He used the lithostratigraphicunits of earlier workers—Sandia Forma-tion and Madera Limestone within theMagdalena Group, with lower “gray lime-stone” and upper “arkosic limestone”members in the Madera—but analyzedcharacteristics of the strata for the Atokanthrough Virgilian Series within thePennsylvanian sequence.

In this region, the Sandia Formation(Atokan) is predominantly gray, green,and brown, fine-grained, terrigenous sedi-ments. Sandstones increase in amount tothe east, and carbonates increase to thesouth. The Sandia attains a greater thick-ness in this area than elsewhere, typicallymore than 100 m (325 ft) and as much as211 m (692 ft) in the Cerros de Amado, eastof Socorro.

The lower “gray limestone” member ofthe Madera is Desmoinesian in age,although beginning in the late Atokan andextending into the earliest Missourian insome places; thickness ranges fromapproximately 115 m to 250 m (approxi-mately 375–825 ft). The upper “arkosiclimestone” member, essentially the Mis-sourian–Virgilian part of the Madera, istypically approximately 180 m (600 ft) orless in thickness but reaches 455 m (1,500ft) in the Sierra Ladrones. As is the case inother regions, Desmoinesian strata are pre-dominantly gray, thick-bedded, chertylimestone units. Missourian and Virgilianstrata are also chiefly limestones (averag-ing 59% and 67% of total thickness, respec-tively; Siemers, 1983, table 2). They becomeprogressively more thinly bedded upsec-tion, and the proportion of shale and sand-stone beds in the upper Madera is abouttwice that in the Desmoinesian sequence.Siemers (1983) noted that in the Mis-sourian–Virgilian, limestones increase inabundance and terrigenous grains becomefiner from east to west across the Socorroarea, indicating derivation of siliciclasticmaterial mainly from the Pedernal uplift,the local Joyita uplift being a minor sourceof sediments.

The Pennsylvanian strata of SocorroCounty fit into the revised lithostrati-graphic nomenclature advocated in this


paper—Sandia Formation overlain by theMadera, raised to group rank, and theMagdalena Group abandoned. Kelley andWood (1946) mapped their Gray Mesa andAtrasado Members into the SierraLadrones of north-central Socorro County,and their names, as formations within theMadera Group, are appropriate for thelower “gray limestone” and upper“arkosic limestone” members used bySiemers (1983) and earlier workers.

Tidwell et al. (2000) and Lucas and Estep(2000) have extended Gray Mesa–Atrasadoterminology eastward across the RioGrande into the Cerros de Amado area. Itis also of interest that Rejas (1965) usedThompson’s (1942) stratigraphic terminol-ogy in the Cerros de Amado area, andKottlowski (written comm., 2000) notedthat Thompson’s Missourian and Virgilianformations are traceable from the SierraOscura to the Cerros de Amado, SierraLadrones, and Mesa Sarca. Thompson’sformation names might well be used inthese areas as members of the AtrasadoFormation. Although not discussed bySiemers (1983), the type section of the earlyWolfcampian Bursum Formation is also inSocorro County (Lucas et al., 2000), andhere it is considered the uppermost forma-tion of the Madera Group in areas where itcan be differentiated from the Red TanksFormation of the Lucero uplift region.

Nacimiento and Jemez MountainsWood and Northrop (1946), who mappedthis area, recognized the Sandia Forma-tion, consisting of a “lower limestone” and“upper clastic” members, and the MaderaFormation, composed of lower “gray lime-stone” and upper “arkosic limestone”members within the Magdalena Group.Subsequent work revealed the “lowerlimestone” member of the Sandia For-mation to be Mississippian (it is now theArroyo Peñasco Group, e.g., Armstrongand Mamet, 1974). The lower part of the“upper clastic” member was separated asthe 20-m-thick (66-ft-thick) Osha CanyonFormation (DuChene et al., 1977), whichconsists of bioclastic limestones and grayto tan shales of Morrowan age, and may bean erosional western remnant of the LaPasada Formation of the southwesternSangre de Cristo Mountains. The remain-ing upper strata of the Sandia Formationare Atokan in age, as much as 70 m (230 ft)thick, and consist mainly of a basal, brownquartz sandstone unit, overlain by slope-forming, green, gray, and yellow shalesand sandstones, and ledges of subordinategray marine limestones (Woodward, 1987).

Woodward (1987) used the terminologyof Wood and Northrop (1946) in describingthe Madera in the Nacimiento Mountains.Although widely exposed in the Naci-miento and Jemez Mountains, the thick-ness of the Madera varies considerablybecause of faulting and locally differing

Mesa and Atrasado Formations, and thosenames are here extended to the Maderasequence in the Nacimiento/JemezMountains. Equivalence of the uppermostpart of the Madera Group in this area tothe transitional Madera/Abo formationsrecognized farther south (Red Tanks andBursum Formations) remains to be demon-strated; if present, this interval is both thin-ner and older (middle Virgilian) than is thecase with these units farther south, closerto their type areas.

Sangre de Cristo MountainsThe Pennsylvanian of the Sangre de CristoMountains was observed and described byearly workers on New Mexico geology(e.g., Marcou, 1858; Stevenson, 1881).Reconnaissance mapping (e.g., Read et al.,1944; Northrop et al., 1946) led to extensionof the Sandia and Madera Formationnames (and Magdalena Group), and of the“lower gray limestone” and “upperarkosic limestone” members of the Madera(Read and Wood, 1947; Brill, 1952) into theSangre de Cristos. Sutherland (1963), how-ever, did not recognize a distinct lithologicbreak between rocks equivalent to theSandia and Madera Formations along thewest side of the range; instead, the onlymajor lithologic break he observed wasbetween the “lower” and “upper” mem-bers of the Madera. Accordingly, the termsMagdalena, Sandia, and Madera were notused by Sutherland in this area. He nameda Morrowan–Desmoinesian unit (La Pasa-da Formation, type section at Dalton Bluff,north of Pecos) that is equivalent to thestrata of the Sandia and “lower gray lime-stone” member of the Madera of previousworkers, and he named the “upper arkosiclimestone” member the Alamitos For-mation (type section north of Pecos).

The La Pasada Formation ( 297 m [973 ft]thick at its type section) is primarily clasticin its lower (Morrowan) part (limestone =27%, shale/siltstone = 50%, sandstone/conglomerate = 23%), but carbonatesincrease upsection, so that in the Des-moinesian part, the (upper 178 m [584 ft] ofthe La Pasada), the ratio of major rocktypes is: limestone = 67%, shale/siltstone =24%, sandstone/conglomerate = 9%. Verylittle chert is present in the limestones,which thus differ from Desmoinesian lime-stone units farther south. The late Des-moinesian–Virgilian Alamitos Formation,approximately 390 m (1,275 ft) thick at itstype locality, is a heterogeneous assem-blage of complexly interbedded fine tocoarse clastics with subsidiary limestones.

Northward from the type section, the LaPasada thickens and changes laterally intoan almost entirely marine and nonmarineclastics-dominated unit, which Sutherlandnamed the Flechado Formation (type local-ity northwest of Tres Ritos; thickness 762 m[2,500 ft]). The Alamitos also thickens tothe north of its type locality, to 1,220 m

onlap relationships upon earlier sedimen-tary and Precambrian rocks. The mostcomplete and well-exposed Madera sec-tion in the Nacimiento Mountains is inGuadalupe Box, where it totals 232 m (760ft) in thickness (DuChene, 1974; Wood-ward, 1987). Here, as in other sections, thelower “gray limestone” member is muchthinner (38 m [125 ft]) than the upper“arkosic limestone” member. Ages of thetwo Madera units are not well constrained,but the lower member appears to be lateAtokan to middle Desmoinesian (Readand Wood, 1947), and the upper membermiddle Desmoinesian to middle Virgilian(Woodward, 1987).

The lithology of the lower member ofthe Madera is predominantly dense, gray,cherty limestones intercalated with thinnerintervals of arkosic sandstone and grayshale, although in some localities (Woodand Northrop, 1946) chert is nearly absent.The upper “arkosic limestone” member isa cyclic sequence (Yancey et al., 1991;Swenson, 1996) of arkosic limestone,arkose, and shale, with siliciclastics becom-ing more abundant and variably colored,and limestones less abundant near the top(Woodward, 1987).

Sutherland and Harlow (1967) definedin San Diego Canyon a thin (9–12 m [30–40ft]), red shale unit near the top of theMadera as the Jemez Springs ShaleMember, although not including in it anoverlying marine limestone (1–2 m [3–7 ft]thick) at the top of the Madera sequence inthis area. The interval called the JemezSprings Shale is part of a Missourian–Virgilian sequence at least 80 m (260 ft)thick (Kues, 1996), in which marine lime-stones predominate over marine shales inthe Missourian part but nonmarine tomarine, gray, red, brown, and purpleshales are collectively thicker than marinelimestone ledges in the Virgilian. Goodfusulinid and macroinvertebrate dataplace the age of the top of the Madera asmiddle Virgilian, with Abo red beds con-formably overlying the Madera. Althoughthe Madera is relatively thin in theNacimiento and Jemez Mountains com-pared with other areas, Woodward (1987,fig. 5) noted thicknesses as great as 540 m(1,775 ft) along the north side of San PedroMountain, just north of the Nacimientos.

In the Nacimiento–Jemez Mountainsarea, the term Magdalena Group is aban-doned here, and the Madera is considereda group consisting of two formations.Madera sedimentation and lithologieswere influenced by local uplift of thePeñasco axis, an elongate island that exist-ed during the Pennsylvanian at theapproximate location of the presentNacimiento Mountains (Woodward, 1987).Lithologies, thickness, and durations of thetwo Madera formations therefore differ toa modest degree from that of the MaderaGroup to the south, but overall their gen-eral features broadly resemble the Gray


(4,000 ft), but its age is constricted to themiddle–late Desmoinesian, owing to earli-er onset of influx of red continental clasticsof the Sangre de Cristo Formation from thenearby Uncompahgre uplift as the Taostrough became filled with sediments.

In the southeastern part of the Sangre deCristos, detailed mapping and stratigraph-ic work by Baltz and Myers (1984, 1999)led to a different classification of thePennsylvanian sequence. They recognizeda clastic-dominated Sandia Formation thatspanned Morrowan and Atokan time andthickened northward from approximately10 m (33 ft) near Bernal to more than 1,525m (5,000 ft) east of Mora, paralleling thenorthward thickening of the Morrowan–Atokan lower part of the La Pasada/Fle-chado Formations to the west, but contain-ing fewer carbonates than the equivalentpart of the La Pasada Formation. TheMadera, above the Sandia Formation, wasraised to group status, containing thePorvenir Formation, a new name for strataformerly mapped as “lower gray lime-stone” member, overlain by the AlamitosFormation. The Porvenir Formation(Desmoinesian) is 325 m (1,065 ft) thick atits type locality west of Gallinas andincreases in thickness northward to a max-imum of almost 500 m (1,640 ft). It is repre-sented by a carbonate facies (with signifi-cant shale) through most of its distributionin the southeastern Sangre de Cristos, by asandstone-shale-limestone facies to thenorth, and by a shaly facies to the north-west (Baltz and Myers, 1999). All threefacies contain considerable shale, sand-stone, and conglomerate beds; limestonerepresents less than 50% of the totalPorvenir stratigraphic thickness even inthe carbonate facies of the type section.The carbonate facies of the Porvenir is lat-erally equivalent to the carbonate-richDesmoinesian upper part of the La PasadaFormation to the west, and the shaly faciesof the Porvenir in the northwest part of itsoutcrop belt grades westward into theupper part of Sutherland’s FlechadoFormation.

The Alamitos Formation in the south-eastern Sangre de Cristos is similar in itslithologic heterogeneity and dominance ofclastic strata to the formation in the west-ern Sangre de Cristos; in both areas are fos-siliferous limestones near the top and bot-tom that allow accurate dating of its upperand lower boundaries. To the west, theAlamitos ranges from late Desmoinesianto Virgilian in the southern part of its out-crop, but is restricted to middle and lateDesmoinesian to the north (Sutherland1963; Sutherland and Harlow, 1973). In thesoutheastern Sangre de Cristos, the forma-tion ranges from late Desmoinesian to ear-liest Wolfcampian, based on fusulinids(Baltz and Myers, 1999). In both areas, red,green, and purple clastic units and thinmarine limestones are present within atransition zone into the basal nonmarine

Sangre de Cristo Formation, and Baltz andMyers (1999, p. 96) noted some marinelimestones a little above the top of theAlamitos as defined by Sutherland (1963)in its type area.

Although there might be a tendency toexpect a unified nomenclature for Pennsyl-vanian strata in the Sangre de Cristos, thenomenclatures of both Sutherland (1963)and Baltz and Myers (1984, 1999) are basedon detailed stratigraphic studies, and eachset of formational stratigraphic names isappropriate to studies of the lithologicsequences in the areas in which they havebeen applied. As noted earlier, it is recom-mended that the term Madera Group notbe used in the Sangre de Cristo Mountains.The nature of the Pennsylvanian sequencein the Sangre de Cristos, strongly influ-enced as it is by proximity to theUncompahgre land mass and rapidly sub-siding Taos trough and other local basins,is unlike that of any other part of NewMexico, and the formational nomencla-ture, with the exception of use of theSandia Formation, is likewise restricted tothis part of north-central New Mexico.Even the Sandia Formation, recognized byBaltz and Myers (1984, 1999) in the south-eastern Sangre de Cristos on lithologicgrounds, differs from Sandia deposits tothe south in attaining a much greater thick-ness and spanning Morrowan and Atokantime. The study by Baltz and Myers (1999)represents the most detailed and compre-hensive examination and synthesis ofPennsylvanian stratigraphy yet completedin any part of New Mexico, and stands as amodel for future studies in other parts ofthe state.

Caballo MountainsCurrent nomenclature for the Pennsyl-vanian of the Caballo Mountains andneighboring small uplifts such as theDerry Hills is that of Kelley and Silver(1952). They assigned the Pennsylvaniansection to the Magdalena Group, dividedinto, in ascending order: the Red House(110 m [362 ft] thick at the type section),Nakaye (128 m [419 ft] thick), and Bar B(103 m [339 ft] thick) Formations. Althoughno age data were provided for these for-mations, Kelley and Silver (1952, fig. 11)correlated the Red House with the Sandiaand lower part of the “lower gray lime-stone” member of the Madera in centralNew Mexico; the Nakaye with most of the“lower gray limestone” member and basal“arkosic limestone” member; and the Bar Bwith most of the “arkosic limestone” mem-ber and the Bursum Formation. Later stud-ies of these units in the Derry Hills, includ-ing data based on fusulinids, conodonts,and brachiopods (Clopine, 1991, 1992;Clopine et al., 1991; Sutherland, 1991;Kaiser and Manger, 1991), indicate that thebase of the Red House Formation (and thebase of Thompson’s, 1942, “Derryan

Series”) is of latest Morrowan age; that theRed House extends through the Atokan;that the Nakaye is of early to late Des-moinesian age; and that the Bar B is of lateDesmoinesian, Missourian, and possiblyearliest Wolfcampian age, with the Vir-gilian missing because of an unconformity(Seager and Mack, 1991; Mack et al., 1998).

Lithologically, the Red House consistspredominantly of thin-bedded, gray lime-stone beds (64% of total thickness in theDerry Hills; Seager and Mack, 1991),interbedded with thin, gray shale unitsand brown to green siltstone. The Nakayeis mainly thick, massive, gray, cherty lime-stone, becoming somewhat thinner bed-ded toward the top, and having minoramounts of gray shale. The Bar B consistschiefly of thin-bedded limestone separatedby thicker intervals of gray shale (shale isapproximately 80% of the total thickness).The uppermost 15 m (50 ft) of the Bar B iscomposed of reddish-brown siltstone,limestone conglomerate, and sandstonebeds beneath the overlying Abo red beds,and appears to be Bursum-equivalent stra-ta. Curiously, although Kelley and Silver(1952, p. 89) strongly criticized Thomp-son’s (1942) units in this area as being,among other things, “scarcely mappable,”they did not map the three formations theyrecognized either; nor did Seager andMack (1991, 1998) in the Derry Hills andNakaye and Red House Mountain areas.Doubt has been expressed as to the validi-ty of Kelley and Silver’s formations as car-tographic units (Gehrig, 1958, p. 6;Bachman and Myers, 1975, p. 108).

The Red House Formation, althoughtemporally closely correlative with theSandia Formation, is lithologically distinctin its high abundance of limestone beds.The Nakaye Formation is not significantlydifferent lithologically from the wide-spread, massive, cherty, cliff-forming lime-stone unit of Desmoinesian age that cropsout throughout central New Mexico, andwhich should be designated with a singlename, which here is recommended to beGray Mesa Limestone. Although proposedby the same worker (V. C. Kelley) whocoined the name Gray Mesa in the Lucerouplift, Nakaye Formation is a good exam-ple of a redundant, unnecessary strati-graphic name.

The Bar B Formation generally resem-bles that unit earlier called “arkosic lime-stone” member of the Madera in centralNew Mexico, albeit with more shale and asmaller proportion of limestone than typi-cally is present in that unit in some otherareas. Tentatively, the name Bar B isretained, but further work may demon-strate that Atrasado and Bursum are moreappropriate terms for this unit. Use of theterm Magdalena Group is unnecessary forthis sequence of Pennsylvanian strata inthe Caballo Mountains. The upper two ofKelley and Silver’s (1952) formationsaccord well with the concept of the Madera


Group farther north, and it is recommend-ed that this term be applied in theCaballos. As noted below, Madera termi-nology has been employed in the Cuchilloand Mud Springs Mountains a short dis-tance to the northwest.

Mud Springs MountainsKelley and Silver (1952) simply mappedthe Pennsylvanian of the southern MudSprings Mountains as Magdalena Group,and Kottlowski (1960a) gave brief litholog-ic and thickness information for theAtokan through Virgilian divisions of thesequence. Earlier (see above), Thompson(1942) used the excellent section inWhiskey Canyon as type and referencesections for his Atokan and Desmoinesiangroups and formations, and Gehrig (1958)provided a detailed stratigraphic section ofThompson’s Desmoinesian units (Armen-daris and overlying Bolander Groups, withincluded formations), totaling 216 m (709ft) in Whiskey Canyon. Strata of Mis-sourian and Virgilian age in the MudSprings Mountains attain thicknesses of 98m (320 ft) and 140 m (460 ft), respectively(Kottlowski 1960a), and these are overlainby approximately 19 m (62 ft) of possiblyWolfcampian Bursum-equivalent beds.

Maxwell and Oakman (1986), in a pre-liminary map of the Mud SpringsMountains, recognized a thin SandiaFormation (46 m [150 ft] thick) only in thesouthern part of the range, overlain by athick Madera Formation (457 m [1,500 ft]),consisting of three units: 1) a lower partconsisting of thin-bedded, locally chertylimestones and considerable gray to greenshale beds; 2) a middle part of mainly mas-sive cherty limestone; and 3) an upper partof thin-bedded limestones alternating withgray shale and greenish to reddish shaleand siltstone. The upper part of theMadera was said to grade into the overly-ing, 50-m-thick (160-ft-thick) BursumFormation, composed mainly of red, green,and purple shales and minor sandstoneand limestone.

In the final version of their map(Maxwell and Oakman, 1990), the termMadera was not used, the Pennsylvanianunits were assigned names derived fromthe Caballo Mountains, and these units,together with the overlying Bursum For-mation, were included in the MagdalenaGroup. Specifically, the middle part of theMadera of the earlier map was termedNakaye Formation, the upper Madera wascalled Bar B (although the uppermost partof this unit was included in the BursumFormation on the later map, as the thick-ness of the Bursum increased from 50 to 90m [160 to 300 ft] from the earlier to latermap), and the Sandia Formation (plusapparently the lower part of the Madera)of the earlier map was included in the RedHouse Formation on the final map. A noteon the preliminary map (Maxwell and

Mountains to the south, recognizing theRed House, Nakaye, and Bar B Formationswithin the Magdalena Group, whichencompasses the entire Pennsylvanian sec-tion. Unfortunately, too little detailedstratigraphic information on these units isprovided to allow comparison with otherPennsylvanian sequences in the region. Anearlier study (Cserna, 1956), summarizedby Kottlowski (1960a), described threePennsylvanian units: 1) a lower shalymember (81 m [266 ft] thick), 2) a medialcherty limestone member (331 m [1,087 ft]thick), and 3) an upper shaly member (81m [266 ft] thick) having a thin (6-m [20-ft])transition zone of purplish limestone andshale grading into the overlying AboFormation. The lithology of these unitssuggests the presence of the SandiaFormation at the base, overlain by the“lower gray limestone” (= Gray MesaLimestone) and “upper arkosic limestone”(probably the Atrasado Formation) of theMadera Group.

Fusulinid biostratigraphy (Verville et al.,1986) in a section south of Cserna’s morecomplete section records only 10 m (33 ft)of Atokan strata; 260 m (853 ft) ofDesmoinesian strata, equivalent toCserna’s medial cherty limestone unit; 60m (197 ft) of Missourian strata, mainlygray, medium- to thick-bedded limestoneswith less chert than the Desmoinesianlimestones; and 50 m (164 ft) of Virgilianstrata that grade upward into the red bedsof the Abo. The stratigraphically highestfusulinids, a few meters below the base ofthe Abo, are no younger than middleVirgilian.

Robledo MountainsThe Pennsylvanian sequence exposed inthe Robledo Mountains was deposited onthe Robledo shelf along the west side of theOrogrande Basin. It is unusually thin,especially compared with the basinalsequences to the east, and unusually richin carbonates. Not much clastic materialreached the Robledos during thePennsylvanian from the Pedernal andFlorida land masses (Kottlowski, 1960b).Kottlowski (1960a, b) published a relative-ly detailed stratigraphic section of Penn-sylvanian and early Wolfcampian stratathat included Atokan (30 m [100 ft] thick),Desmoinesian (69 m [225 ft]), Missourian(52 m [170 ft]), and Virgilian (73 m [240 ft])strata totaling approximately 225 m (735 ft)thick (Kottlowski and Seager, 1998), over-lain by a 55-m-thick (180-ft-thick) Bursuminterval. Atokan strata, thinned by partialassimilation in an Oligocene sill, consist ofblackish, silty limestone and greenish-grayshale; the Desmoinesian predominantly ofmassive, cherty limestones interbeddedwith minor limy shale and calcareoussandstone; the Missourian of slope-form-ing limestones and limy shales with sever-al ledge-forming limestones; and the

Oakman, 1986) stating that it had not beenreviewed for conformity with U.S. Geo-logical Survey stratigraphic nomenclatureperhaps explains the changes in nomencla-ture introduced on the final map.

The contrasting nomenclature used byMaxwell and Oakman (1986, 1990) illus-trates the similarities of the central NewMexico Madera sequence to that of theCaballos, with its local stratigraphic termi-nology, and the application of differentnomenclatural philosophies to the MudSprings Mountains Pennsylvaniansequence. As in the Caballo Mountains, itseems clear to me that the unit calledNakaye is the Gray Mesa, and that theGray Mesa–Bar B–Bursum sequence of theMud Springs Mountains represents theMadera Group. Similarly, use of the termMagdalena Group should be discontinuedin this range, as Maxwell and Oakman(1986) implicitly suggested. Possibly someof Thompson's (1942) lithostratigraphicnames might be useful as member namesin the Mud Springs Mountains.

Cuchillo MountainsIn the Cuchillo Mountains of northwesternSierra County, the Pennsylvanian sequenceis relatively poorly exposed and has beenlittle studied. It was divided into theSandia and Madera Formations (of theMagdalena Group) and as much as 67 m(220 ft) of “Magdalena transition beds”below the overlying Abo Formation byJahns (1955) and Jahns et al. (1978). TheSandia Formation, as much as 53 m (173 ft)thick, consists principally of shale andgreenish-gray to red-brown siltstones andsandstones interbedded with gray, locallycherty, limestone ledges. Strata assigned tothe Madera Limestone attain a thickness of275–300 m (900–985 ft) and are predomi-nantly gray, thin-bedded to massive, oftencherty limestones separated by minor bedsof shale and sandstone. Stratigraphic infor-mation is currently insufficient to allowsubdivision of the Madera or to compare itin detail with Madera sections elsewhere.The “transition beds” are greenish, brown,or maroon clastics, interbedded with lime-stone beds and some sandstone, limestone,and quartzose conglomerates, similar tothe uppermost Madera regionally andprobably in part equivalent to the lowerBursum Formation to the northeast.

More detailed information on thisPennsylvanian sequence would be desir-able. For now, the term Magdalena Groupshould be abandoned, and Madera shouldbe raised to group rank, though with for-mations yet to be defined.

Fra Cristobal RangeThe most recent geological study of the FraCristobal Range (Nelson, 1986) uses thePennsylvanian terminology introduced byKelley and Silver (1952) in the Caballo


Virgilian chiefly of several thick, nonchertylimestone cliffs interbedded with thin-bed-ded limestone and shale. The interval con-taining early Wolfcampian “Bursum”fusulinids (Thompson, 1954) is predomi-nantly limestone and is not similar to thelithostratigraphic unit called BursumFormation to the north; Jordan (1975) didnot recognize the Bursum Formation in theRobledos, nor have more recent workerssuch as Krainer et al. (2000) and Wahlmanand King (in press).

To date there has been little publisheddetailed information on the Pennsylvanianin the Robledo Mountains, and no litho-stratigraphic names have been applied tothis sequence. The Atokan section litholog-ically resembles the Sandia Formation, andthe Desmoinesian interval is closely com-parable in lithology, if not in thickness, tothe lower Madera Group formation farthernorth for which the name Gray MesaLimestone is recommended. The Mis-sourian and Virgilian units are tentativelyregarded as a southern, thinner expressionof the Atrasado Formation, although witha higher proportion of carbonates in theVirgilian part.

Silver City, Santa Rita, and Kingstonareas

The incomplete, faulted, and generallypoorly exposed Pennsylvanian sequencein the Silver City–Santa Rita area was ini-tially included with Mississippian andPermian strata within the Fierro Formation(Paige, 1916). Spencer and Paige (1935) dis-carded the term Fierro and divided thePennsylvanian section into the lowerOswaldo Formation (130 m [425 ft] thick)and overlying Syrena Formation (120 m[395 ft] thick) of the Magdalena Group,because the sequence appeared lithologi-cally different from the Sandia and MaderaFormations used elsewhere. These forma-tions were summarized by Kottlowski(1960a) and discussed by Jones et al. (1967,1970), Pratt (1967), and LeMone et al.(1974), among others. The OswaldoFormation consists almost entirely of lime-stone, locally cherty, with shale interbedsnear the top and locally a shale unit at itsbase. The Syrena Formation includes abasal, thick (as much as 40 m [131 ft]), darkshale or limestone unit and an uppersequence of interbedded gray limestonesand nodular limestones and brown, yel-low, and red shale.

LeMone et al. (1974) reported aMorrowan–earliest Missourian age for theOswaldo, an early Missourian to Virgilianage for the Syrena, and suggested thatthese strata were deposited on open-marine shelf environments between thoseof extreme southwestern New Mexico andthe Robledo shelf to the east. Lithologicallythe Oswaldo Formation reflects the pre-vailingly carbonate character of Lower and

Middle Pennsylvanian strata in southernNew Mexico (e.g., Lead Camp Limestoneof the San Andres Mountains), and theSyrena reflects increased siliciclastics in theUpper Pennsylvanian, comparable to theAtrasado and equivalent units to the northand east. The unneeded term MagdalenaGroup is abandoned here, but the namesOswaldo and Syrena are retained as usefullocal divisions of an incomplete Pennsyl-vanian sequence.

To the east, near Kingston, Kuellmer(1954) described in detail three incompletesections of Pennsylvanian strata, indicat-ing a composite thickness of approximate-ly 200 m (655 ft) or more, assigned to theMagdalena Limestone but not further sub-divided. Faulting and quartz monzoniteintrusions complicate interpretation ofthese outcrops. In two sections, 15–23 m(50–75 ft) of basal, Sandia Formation-like,dark shales and sandstones rest uncon-formably on Mississippian limestone, andare overlain by as much as 20 m (65 ft) ofmassive, cherty limestones similar toDesmoinesian units deposited widelyacross the state. The thickest section (III, ofKuellmer, 1954) rests on a quartz mon-zonite intrusion and consists mainly ofmassive, gray, cherty limestones (130 m[425 ft]) overlain by approximately 60 m(200 ft) of gray and brown shaly limestone,yellow shale, thin-bedded and nodularlimestone, and limestone-pebble conglom-erate. At the base is approximately 10 m(33 ft) of interbedded dark shale and lime-stone. The isolation of these sections fromothers and the fact that fusulinid ages ofthese strata have not been reported com-plicate correlation, but the thickest section,although thinner than is typical of theMadera, lithologically resembles theMadera Group in its lower cherty lime-stone formation and upper interbeddedlimestone and siliciclastic formation. Moredetailed study and comparison with thenearest Pennsylvanian sequences to theeast (Caballo Mountains), north (CuchilloMountains), and west (Santa Rita–SilverCity area) would increase our understand-ing of all of these sequences.

Southwestern New Mexico Pennsylvanian strata in southwestern NewMexico, best exposed in the Big HatchetMountains, are assigned to the HorquillaFormation, which also includes units ofWolfcampian age (Zeller, 1965; Ross, 1979;Thompson and Jacka, 1981; Drewes, 1991).The time represented by Horquilla deposi-tion has been reliably determined byfusulinid biostratigraphy as Morrowan toWolfcampian, with the following seriesthicknesses reported by Thompson andJacka (1981) for the Big Hatchet Peak sec-tion: Morrowan, 75 m (247 ft); Atokan, 115m (378 ft); Desmoinesian, 305 m (1,001 ft);Missourian, 142 m (466 ft); Virgilian, 128 m(419 ft); and Wolfcampian, 219 m (719 ft).

The total thickness of the Horquilla isapproximately 1,000 m (3,280 ft), of whichapproximately 765 m (2,500 ft) is ofPennsylvanian age. Zeller (1965) andThompson and Jacka (1981) informallydivided the Horquilla into lower (420 m[1,380 ft] thick; Morrowan to upperDesmoinesian) and upper (570 m [1,870 ft]thick; upper Desmoinesian to Wolf-campian) members. The lower member is90+% gray limestones with moderate chertbeds, and the upper member is 70% lime-stone and 30% dolostone with little chert.

In the Big Hatchet Mountains, these car-bonates were deposited primarily in shal-low marine shelf environments. The shal-low carbonates of the upper memberchange laterally to the southwest into basi-nal deposits of dark mudstones and lime-stones, reflecting onset of subsidence of thePedregosa (or Alamo Hueco) Basin at afaster rate than deposition occurred (e.g.,Greenwood et al., 1977; Thompson andJacka, 1981; Wilson, 1989). Large, mainlyLate Pennsylvanian and Wolfcampianphylloid algal bioherms are conspicuouson outcrop and mark the shelf marginsadjacent to the subsiding basin (e.g., Zeller,1965). Deposition of the Horquilla For-mation was very cyclic, and its cyclic stra-tigraphy can be correlated with midconti-nent Pennsylvanian cyclothems (Connollyand Stanton, 1992).

Deposition and thickness of theHorquilla were strongly influenced by sub-sidence of the Pedregosa (or AlamoHueco) Basin and represent an unusuallylong span of nearly completely carbonatedeposition that is unlike that of thePennsylvanian–Early Permian of any otherpart of the state. It is a well-defined, litho-logically homogeneous, lithostratigraphicunit, appropriately considered to be of for-mation rank. The Horquilla has beentraced northward in New Mexico to thePeloncillo Mountains, where a highlyfaulted and intruded section approximate-ly 450 m (1,475 ft) thick apparently extendsfrom Atokan to Wolfcampian time(Gillerman, 1958; Kottlowski, 1960a).Deposition of carbonate shelf sedimentsextended continuously northeastwardfrom the area of the Pedregosa Basin to theSilver City and perhaps Robledo shelfareas, but in those regions the Penn-sylvanian sequence is both much thinnerand contains considerably more siliciclas-tic beds than the Horquilla.

San Andres MountainsThe San Andres Mountains extend fornearly 140 km (85 mi) from near the SierraOscura on the north to the OrganMountains on the south. Pennsylvanianstrata crop out along the entire length ofthe range but are most accessible in a seriesof canyons, which from north to south are:Mockingbird Gap, separating the SanAndres from the Oscura range, and


Rhodes, Hembrillo, San Andres, Ash, andBear Canyons. The San Andres Mountainsalso occupy the western side of thePennsylvanian–Early Permian OrograndeBasin, resulting in a thick Pennsylvaniansequence that differs markedly, especiallyin the Missourian and Virgilian, from thatof the broad, stable Robledo shelf to thewest (Robledo, Caballo, Mud SpringsMountains and Sierra Cuchillo), and fromthe Pennsylvanian sequence depositedalong the steep, fault-bounded easternmargin of the basin in the SacramentoMountains.

The definitive work on the stratigraphyof the San Andres Mountains is byKottlowski et al. (1956), with subsequentsummaries and updates by Kottlowski(1960a, 1975) and Kottlowski and LeMone(1994), among others. As described bythese authors, Atokan strata (34–105 m[107–345 ft] thick) are mainly siliciclastic tothe north and carbonates to the south;Desmoinesian beds (56–190 m [183–622 ft]thinning to the south) are chiefly massiveto medium-bedded cherty limestones withsome clastic and calcarenitic units; andMissourian strata (44–83 m [145–271 ft]thinning to the south) consist of interbed-ded argillaceous limestone and calcareousshale, with local massive cherty lime-stones. Bachman and Myers (1969, 1975)combined this predominantly carbonatesequence into the Lead Camp Limestone,which is 262 m (861 ft) thick at the typelocality in San Andres Canyon in thesouthern part of the range. They recog-nized possible Morrowan fusulinids at thebase and early Missourian fusulinids at thetop. The lower, mainly siliciclastic beds inthe Atokan part of the section to the northwere assigned to the Sandia Formation,which thus grade southward into themostly cherty limestones of the lower LeadCamp Limestone. The upper part of theLead Camp Limestone is lithologicallysimilar to units named Gray Mesa and LosMoyos Formations to the west and north.

Above the Lead Camp Limestone is anabnormally thick sequence, named thePanther Seep Formation by Kottlowski etal. (1956), which thickens progressivelysouthward from approximately 250 m (820ft) near Mockingbird Gap (Kottlowski etal., 1956) to 865 m (2,885 ft) in the AshCanyon area and 975+ m (3,200 ft) in thesubsurface of the central Orogrande Basin(Schoderbek, 1994). The Panther Seep con-sists of cyclic (Schoderbek, 1994; Soreghan,1994), brackish-water, deltaic, stream chan-nel and marginal marine, brown to dark-gray carbonaceous shales and coarse tofine-grained sandstones, argillaceous andcalcarenitic limestones, calcareous sand-stones, phylloid algal bioherms (nearRhodes and Hembrillo Canyons, seeToomey, 1991; Soreghan and Giles, 1999a,b), and several gypsum beds near the topnear Ash Canyon and southward.Terrigenous clastics represent about 74% of

Franklin Mountains, NewMexico–west Texas

The Pennsylvanian sequence in the Frank-lin Mountains was first studied in detail byNelson (1940), who assigned it to theMagdalena Formation and divided it into(in ascending order) the La Tuna, Berino,and Bishop Cap Members. Harbour (1972),in mapping the range, added an unnamedupper member between the Bishop Capand the overlying Permian HuecoLimestone, which was later found to be thePanther Seep Formation, described initial-ly (Kottlowski et al., 1956) in the SanAndres Mountains. Current nomenclature(LeMone, 1982, 1992) recognizes theMagdalena Group as comprising the LaTuna (as much as 156 m [511 ft] thick),Berino (as much as 157 m [514 ft] thick),and Bishop Cap (180 m [590 ft] thick)Formations, overlain by the Panther SeepFormation (213 to possibly 376 m[700–1,235 ft] thick). These formationshave been accurately dated using fusulin-ids and conodonts (e.g., Wilson, 1989;Clopine et al., 1991; Clopine, 1992).

The La Tuna Formation (Morrowan–early Atokan) is predominantly cliff-form-ing, gray, cherty limestones with algalmounds and thin lenses of shale near thetop. The Berino (middle Atokan–middleDesmoinesian) consists of alternating graycherty limestone and gray shale beds inabout a 70:30 ratio. The Bishop CapFormation (middle Desmoinesian–earlyMissourian) comprises mainly gray tobrown shale (65–75%) alternating withthin ledges of gray limestone. As notedpreviously, this sequence differs greatly inits lithology from the sequences in centralNew Mexico to which the term MagdalenaGroup was first applied, and therefore useof the name Magdalena is both confusingand inappropriate; I strongly recommendthat this group name be abandoned in theFranklin Mountains.

The Panther Seep Formation in theFranklin Mountains is a poorly fossilifer-ous yellow-gray shale and siltstonesequence, with minor carbonates, severalgypsum beds near the top, and a basal, 6-m-thick (20-ft-thick), chert-pebble con-glomerate, marking an unconformablecontact with the underlying Bishop CapFormation (LeMone, 1982). Its age is mid-dle Missourian to possibly earliestWolfcampian, as it conformably underliesthe basal limestones of the Hueco Group,which are known to be of early (but notearliest) Wolfcampian age (Williams, 1966).

In the Bishop Cap Hills, approximately10 km (6 mi) north of the FranklinMountains, only the La Tuna and BerinoFormations are exposed (Harbour, 1972),with thicknesses of approximately 80 m(260 ft) and 150 m (500 ft), respectively(Seager, 1973, 1981).

total Panther Seep thickness (Schoderbek,1994, p. 92). The age of the Panther Seepranges from middle or late Missourian toearly Wolfcampian (Bachman and Myers,1969, 1975), and its lithology is quite differ-ent from that of any other Late Pennsyl-vanian sequence in the state. Rapid subsi-dence of the Orogrande Basin during thistime was matched by enormous amountsof sediment shed from the Pedernal landmass, keeping the basin filled nearly to orabove sea level, while glacioeustatic fluctu-ations of sea level imprinted cyclicity onthe depositional sequences (Schoderbek,1994).

Kottlowski et al. (1956) recognized from23 m (76 ft) of Bursum Formation inHembrillo Canyon to 81 m (287 ft) ofBursum in Rhodes Canyon unconformablyoverlying the Panther Seep and underly-ing the Hueco Formation in the north partof the range. Bachman and Harbour (1970)apparently included both Bursum andHueco strata in the upper part of thePanther Seep north of Rhodes Canyon.Similarly, Kottlowski et al. (1956) reportedthe Panther Seep in Mockingbird Gap, butBachman (1968) referred these strata to theupper member of the Madera Formation.Although Madera Group terminology isnot appropriate for the Lead Camp andPanther Seep Formations through most ofthe San Andres Mountains, these forma-tions, and the Sandia Formation below theLead Camp in the northern part of therange, grade laterally into the familiarSandia/Madera Group units to the northand west, where deposition occurred onthe more stable shelf areas along the mar-gins of the Orogrande Basin. Similarly, theunits of the predominantly basinal Penn-sylvanian sequence exposed in the SanAndres Mountains can be traced in detailthrough individual cycles within faciesand thickness changes to the narrow, tec-tonically active shelf to the east, in theSacramento Mountains (e.g., Wilson, 1967;Algeo et al., 1991).

Organ MountainsIn the northern Organ Mountains, immedi-ately south of the San Andres range, theLead Camp Limestone (200–265 m[650–870 ft] thick) intertongues with theoverlying Panther Seep Formation(approximately 600 m [1,970 ft] thick). Thelithologies of the two formations in theOrgan Mountains are not greatly differentfrom their lithologies in the San AndresMountains, except for several thick gyp-sum beds near the base and near the top ofthe Panther Seep (Seager, 1981). The upper120 m (400 ft) of the Panther Seep is transi-tional into the overlying lower unit of theEarly Permian Hueco Formation.


Hueco Mountains, New Mexico andTexas

Beede (1920) first assigned the Pennsyl-vanian sequence in the Hueco Mountainsto the Magdalena Group, and King et al.(1945) divided what they called theMagdalena Limestone into three informaldivisions—lower, middle, and upper. Thisnomenclature has persisted to the present(e.g., Williams, 1963; Seewald, 1968;Stoklosa et al., 1998). The upper part of theMagdalena sequence of these authors isbounded by an unconformity at the base ofthe Lower Permian unit (PowwowConglomerate) of the overlying HuecoGroup, which rests on progressively olderbeds from the north (Wolfcampian) to thesouth (Atokan in Powwow Canyon).Thompson (1954) recognized a thin (6 m[20 ft]) “Bursum” unit based on fusulinidsat the top of the Magdalena and below theunconformity, and Williams (1963)described an additional 60-m (200-ft) ofWolfcampian (“Pseudoschwagerina beds ofthe Magdalena Limestone”) along the westside of the northern part of the range. Cys(1975), however, interpreted these beds asbeing part of the lower Hueco Group, withthe Powwow Conglomerate being below,rather than above them.

In the central and southern HuecoMountains, according to Seewald (1968),the “lower division” of the Magdalena (162m [530 ft] thick; Morrowan–early Atokanin age) is composed mainly of massive,gray, very cherty limestones, with abun-dant oosparites (see also Connolly andStanton, 1983). The “middle division” (90m [300 ft] thick; middle Atokan–middleDesmoinesian) is poorly exposed shaleand thin limestone beds with lithology andfauna “strikingly similar” to that of theBerino Formation in the FranklinMountains. The “upper division” (145 m[480 ft] thick; late Desmoinesian–earlyWolfcampian) is lithologically heteroge-neous and characterized by cyclic sedi-mentation, large algal mounds (Toomey,1991), and a chiefly carbonate environ-ment, interrupted by thin shales and con-glomerates. Hardie (1958) pointed out thatthe lithology of the upper 365 m (1,200 ft)of the Magdalena in the northern (NewMexico) part of the Hueco Mountains isconsiderably different from that in thesouthern part of the range, consistingmainly of gray to grayish-red shale (50+%),massive gray limestone (35–40%), espe-cially in the upper part, and lesser propor-tions of limestone-pebble conglomerates,and one gypsum bed as much as 23 m (75ft) thick near the top. Although preciseages for the northern sequence were notavailable, Hardie believed most of thesequence to be of Virgilian and earlyWolfcampian age, and Kottlowski (1960a)interpreted the sequence as “Panther Seepformation” types of sediments.

Developing a formal, reasonable, litho-

stratigraphic nomenclature for the Penn-sylvanian of the Hueco Mountains couldbe done with detailed stratigraphic study,dating, and correlation of units in thenorthern and southern parts of the range,and then correlation with Pennsylvaniansequences elsewhere in the region, espe-cially in the Franklin Mountains, approxi-mately 40 km (25 mi) to the west. Probablythe Hueco Mountains section could beaccommodated by the formations definedin the Franklin Mountains, as Wilson(1989, p. 9, fig. 2B) suggested for the Mor-rowan–Desmoinesian part of the sequence,and Kottlowski (1960a) for the upper(“Panther Seep”) portion of the Penn-sylvanian–early Wolfcampian sequence inthe northern part of the range. Applyingthe name Magdalena Limestone or Groupto this sequence in the Hueco Mountains isinappropriate for the same reasons as inthe Franklin Mountains, and the nameshould be abandoned in future work.

Sacramento Mountains The Pennsylvanian sequence of the Sacra-mento Mountains was divided into (inascending order) the Gobbler, Beeman, andHolder Formations by Pray (1959, 1961), allbased on a continuous type section nearLong Ridge and Mule Canyon southeast ofAlamogordo. There, the Gobbler Forma-tion (?Morrowan or early Atokan–late Des-moinesian in age; Bachman and Myers,1975; Wilson, 1989) is approximately 400 m(1,300 ft) thick, and the Beeman (Missou-rian–possibly early Virgilian in age; Bach-man and Myers, 1975; Raatz and Simo,1998) is 120 m (400 ft) thick. The HolderFormation (early–late Virgilian) attains amaximum thickness of approximately 275m (900 ft) north of the type section.

The basal 60–150 m (200–500 ft) of theGobbler Formation consists of slope-form-ing quartz sandstone, gray to black shale,and ledges of dark, cherty limestone.Above this clastic interval, two facies of theGobbler are recognized: 1) a northerndetrital facies composed almost entirely ofnonmarine to nearshore marine quartzsandstone and shales with minor lime-stone and 2) a cliff-forming, massive, local-ly cherty marine limestone facies, namedby Pray (1961) the Bug Scuffle LimestoneMember, which is present in the northernSacramentos, dominates the central andsouthern parts of the range, and interfin-gers with and grades into the detritalfacies. The Bug Scuffle Limestone repre-sents carbonate deposition widely acrossthe narrow Sacramento shelf and adjacentslope along the east side of the OrograndeBasin. The detrital facies should be namedformally as a member of the Gobbler bythose actively engaged in studying the for-mation. It represents a wedge of terrige-nous clastic sediments derived from thePedernal land mass immediately to theeast and deposited within an intrashelf

graben, the Alamo trough of Algeo et al.(1991), which divided the Sacramento shelfinto two independent tectonic blocks. TheGobbler sequence is very cyclic, and the20–25 independent cycles can be tracedlaterally from shelf to slope to basinalfacies (Algeo et al., 1991; Algeo, 1996).

The Beeman Formation consists mainlyof interbedded calcareous shale and thin-bedded, locally argillaceous limestone. Thelower third of the formation contains sig-nificant sandstone bodies (Pray, 1961) thatrepresent cyclically deposited shelf to basi-nal environments (see Raatz and Simo,1998, for detailed facies and sequencestratigraphic analysis). Basinal sequenceswithin the Beeman are considerably thick-er and have many more parasequencesthan the shelf sequences.

The Holder Formation consists of a widevariety of sedimentary strata, primarilymarine but increasingly brackish to non-marine toward the top. Holder strata arevery cyclic, and the cycles can be tracedwestward into the thicker basinal sequence(Panther Seep Formation) of the Oro-grande Basin (Cline, 1959; Wilson, 1967).The basal Holder is characterized by largephylloid-algal bioherms as much as 23 m(75 ft) high (e.g., Toomey et al., 1977;Toomey, 1991), which grew along the shelfmargin. Overlying the Holder conform-ably in the northern part of the Sacra-mentos is the 150-m-thick (500-ft-thick)Laborcita Formation, initially consideredlatest Virgilian to early Wolfcampian in age(Otte, 1959) but later determined on thebasis of fusulinid biostratigraphy to beentirely Wolfcampian (Steiner and Willi-ams, 1968). To the south, red beds of theAbo Formation unconformably overlieeach of the three Pennsylvanian forma-tions at various localities (Pray, 1961, fig.26).

Clearly, local paleogeography and thetectonic processes operating on it havegreatly influenced Pennsylvanian sedi-mentary deposition in the SacramentoMountains, resulting in a distinctivePennsylvanian sedimentary sequence thathas appropriately received unique forma-tion and member names. These units havebecome a focus for sequence stratigraphicstudies in the past decade, with the resultthat some of the cyclic sequences recog-nized here can be traced not only to thewest, into the axis of the Orogrande Basin,but also eastward to the midcontinent area(e.g., Raatz and Simo, 1998).

Subsurface stratigraphyAlthough the emphasis in this paper is onexposed Pennsylvanian rocks, one com-ment on subsurface stratigraphic nomen-clature in New Mexico is needed. Mostreports have assigned intervals of thePennsylvanian petroleum-bearing strata ineastern New Mexico to divisions such asthe Strawn, Canyon, and Cisco Series (e.g.,


Broadhead and King, 1988). These unitsare actually lithostratigraphic units(groups) composed of many formationswith their type sections in north-centralTexas and should not be used as chrono-stratigraphic units (series). According toKier et al. (1979, p. S5) there was anattempt in the 1940s to redefine thesegroups as series to facilitate subsurfacecorrelation between basins, but “…severalattempts have been made to apply [this]classification in the field” and “…suchapplications have proven difficult, if notentirely inappropriate.” Present usage ofthe Strawn, Canyon, and Cisco names inTexas is as groups (e.g., Kier et al., 1979; theGeologic Atlas of Texas maps), the stan-dard U.S. series names (e.g., Des-moinesian, Missourian, Virgilian) as themajor chronostratigraphic units. If thesenames are not used as series names inTexas, there is no justification for usingthese names as series names in the subsur-face of New Mexico. Nor is there any justi-fication for using them as group terms inNew Mexico as these units do not extendlaterally from north-central Texas intoNew Mexico, and the lithologies of thesegroups in north-central Texas do notresemble the lithologies of the subsurfaceunits in New Mexico to which these nameshave been applied. For example, theStrawn Group in Texas “consists predomi-nantly of cyclic terrigenous clastic faciesdeposited by…fluvial-deltaic systems”(Kier et al., 1979, p. S13), whereasDesmoinesian strata in most of NewMexico are massive, cherty, marine lime-stones. Use of these names in the NewMexico subsurface should be abandoned,and lithostratigraphic terms based onexposed New Mexico strata (or if this is notpossible, the standard series names)should be used for subsurface Pennsyl-vanian stratigraphy. Permian lithostrati-graphic units established on the basis ofoutcrops in and near New Mexico, such asHueco, Abo, Yeso, and San Andres Forma-tions, are routinely used as Permian sub-surface stratigraphic units in the state, andthere is no reason why the same cannot bedone with Pennsylvanian units.

Summary and conclusions This review of Pennsylvanian stratigraphyin New Mexico is not intended to be com-prehensive, but rather to provide a broadoverview of the major Pennsylvaniansequences across the state and the nomen-clature that has been applied to them. Therevisions suggested herein are designed toadapt the lithostratigraphic nomenclatureto more appropriately and accuratelyreflect the major features of the variedPennsylvanian successions exposed, andto provide an integrated nomenclaturalframework that can be consistentlyapplied as studies of Pennsylvanian strati-graphy around the state proceed. The cor-

implemented by some workers in areassuch as the Manzanita, Manzano, and LosPinos Mountains.

3) Units previously regarded as mem-bers within the Madera Formation shouldbe treated as formations, as they have all ofthe attributes of formations as outlined inthe present Code of Stratigraphic Nomen-clature. This especially applies to the infor-mal “lower gray limestone” and “upperarkosic limestone” member names widelyused beginning in the 1940s and continu-ing today in many parts of central NewMexico. These two units represent twovery different, widespread, easily recog-nized sedimentary sequences; the lower,an essentially Desmoinesian sequence ofcherty, massive, cliff-forming, carbonate-shelf marine limestones and the upper, acyclic Missourian–Virgilian sequence ofalternating, generally thinner and chertlessmarine limestones and siliciclastic units.Locally, a third unit at the top of theMadera may be recognized (e.g., Bursumand Red Tanks Formations), composed pri-marily of variably colored, nonmarine sili-ciclastic beds with subordinate marinelimestones, reflecting final regression ofthe Madera sea and replacement of marinestrata with continental red beds near thebeginning of Permian time.

4) Formal formation names should beapplied to the “lower gray limestone” and“upper arkosic limestone” members. Theearliest valid, adequately defined, formalnames given to these widely distributedand easily distinguished units within theMadera are Gray Mesa Limestone andAtrasado Formation, respectively, in theLucero uplift area (Kelley and Wood,1946). These formations can be recognizedwidely in central New Mexico, and they, aswell as the Madera Group, are here extend-ed southward into the Caballo andRobledo Mountains area. The Maderasequence in the Lucero uplift area is animportant reference section for theselithostratigraphic units. Only in caseswhere the lithostratigraphy departs con-siderably from that of the type Gray Mesaand Atrasado Formations need differentformation names be considered.

5) Madera Group terminology reflectssedimentary sequences deposited on sta-ble platforms or along the margins of sub-siding basins, as is the case with the Lucerouplift sequence. Broadly similar Maderasequences, though variable locally, can berecognized along the western and northernmargins of the Orogrande Basin, andnorthward to the Peñasco uplift area.Pennsylvanian sequences deposited morecentrally in subsiding basins, such as in theSan Andres Mountains or the Sangre deCristo Mountains, or within unique tecton-ic regimes, as along the eastern side of theOrogrande Basin in close proximity to thePedernal uplift (Sacramento Mountains)display strata that differ markedly fromtypical Madera Group sequences, and dif-

relation chart (Fig. 3) indicates the litho-stratigraphic terminology of New MexicoPennsylvanian strata advocated in thispaper. In some areas, detailed studies ofthe Pennsylvanian sequences are neededto test the validity of the proposed nomen-clature.

The paleogeography and tectonic histo-ry of New Mexico during the Pennsyl-vanian Period were complex. Althoughdeposition was mainly in shallow marineenvironments across much of the state dur-ing all but the earliest (Morrowan) part ofthe period, stable shelf environments wereinterrupted, to varying degrees at differenttimes, by four major subsiding basins: Taostrough and Orogrande, Delaware, andPedregosa Basins. These basins accumulat-ed unusually great thicknesses of sedi-ment, although generally in shallow ratherthan deeper marine environments, fromfour large (Uncompahgre, Sierra Grande,Zuni, and Pedernal) and several smallerisland uplifts that provided fluctuatingvolumes of siliciclastic sediments tomarine and coastal environments (Fig. 2).The interplay between paleogeography,tectonic activity, eustatic sea level changes,and climate through the Pennsylvanianproduced depositional sequences of signif-icant lithologic variability. Most Pennsyl-vanian sequences accessible for detailedstudy are within relatively recently (Ceno-zoic) uplifted, isolated, fault-block moun-tain ranges, which have tended to empha-size apparent differences between localsequences, rather than the broader deposi-tional patterns that tie them together.However, it is these broad patterns that areused here as the basis for lithostratigraph-ic units such as formation and groups,with the view that for nomenclatural pur-poses, most local lithologic variability isproperly accommodated within localmember-rank units of rather broadlydefined and geographically extensive for-mations. This approach is not new; it wasforeshadowed, albeit somewhat irregular-ly and informally, by the extensive recon-naissance mapping and stratigraphic stud-ies undertaken by U.S. Geological Surveygeologists in the 1940s to 1960s.

The main recommendations for revisionof current Pennsylvanian lithostratigraph-ic nomenclature are as follows:

1) Use of the term Magdalena Group inthose parts of the state (mainly southernNew Mexico) where it is still beingemployed should be abandoned. Thisterm, a relic of the earliest attempts to sub-divide the Pennsylvanian in New Mexico,has been applied in so many confusing andcontradictory ways as to render it mean-ingless, and nearly a century of subse-quent, more detailed stratigraphic workhas deprived it of any utility it might oncehave possessed.

2) The Madera Formation is most appro-priately regarded as a Group wherever itoccurs in the state, an assessment already


ferent sets of lithostratigraphic names,some including Morrowan and Atokanstrata not normally present in the MaderaGroup, have appropriately been appliedby previous workers.

6) Pre-Madera (generally pre-Desmoin-esian) strata are heterogeneous and havereceived distinct formational names (e.g.,Sandia and Red House Formations), or areincluded in formations (such as the LeadCamp Limestone in the southern SanAndres Mountains) that show no signifi-cant breaks in sedimentation from Early toMiddle or even into Late Pennsylvaniantime. The Atokan, predominantly clasticSandia Formation is widely presentbeneath the Madera Group, but it becomesgradationally more carbonate-rich to thesouth, where other names have beenappropriately used for this interval.

7) The lithostratigraphic formation andgroup names proposed by Thompson(1942), which were never widely used andare all but forgotten today, should bereevaluated. They designate legitimate,well-defined lithostratigraphic units, andsome of them may be appropriate as mem-ber names locally within the more broadlyconceived formations discussed above.They also have priority over most otherlithostratigraphic names used for parts ofthe Pennsylvanian sequence in NewMexico.

8) Lithostratigraphic names used forPennsylvanian strata in the subsurface ofNew Mexico should, as far as possible, bethe same as those defined from exposedstrata in the state, rather than names basedon surficial units in central Texas, whichhave no apparent relationships with NewMexico strata.

Acknowledgments. I thank Frank Kot-tlowski, New Mexico Bureau of Mines andMineral Resources, Spencer Lucas, NewMexico Museum of Natural History andScience, and Elmer Baltz for reviewing anearlier version of this paper and offeringsuggestions that improved it. Thanks alsoto Mabel Chavez for word processing themanuscript. This paper is dedicated toFrank Kottlowski in recognition of 50 yrsof contributions to our knowledge andunderstanding of New Mexico Penn-sylvanian stratigraphy and to many otheraspects of New Mexico’s geological record.

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Wahlman, G. P., and King, W. E., in press, LatestPennsylvanian and earliest Permian fusulinidbiostratigraphy, Robledo Mountains and adjacentranges, south-central New Mexico: New MexicoBureau of Geology and Mineral Resources,Circular 208.

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Wilson, J. L., 1967, Cyclic and reciprocal sedimenta-tion in Virgilian strata of southern New Mexico:

On October 15, 2001, the University of New Mexico Printing Servicesclosed its doors after nearly 70 years of service. New Mexico Bureau ofGeology and Mineral Resources was among many state-affiliated enti-ties that regularly did business with UNM Printing Services. Theyprinted every issue of New Mexico Geology since its inception inFebruary 1979. However, this association between the two organiza-tions goes back much farther. This long partnership owed its success tothe dozens of dedicated and skilled employees who worked alongsidebureau editors to make beautiful and accurate geologic publicationsand maps. We wish all of those employees well in their new endeavors.

Documents

The Pennsylvanian System in New Mexico— overview with ...€¦ · stratigraphy of the territory, but subdivi-sion and naming of Pennsylvanian strata began in the early 1900s (e.g.,