The Pennsylvanian System in New Mexico— overview with suggestions for revision of stratigraphic nomenclature Barry S. Kues, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131 Abstract tions, including the widely exposed and rec- Mountains); 6) reevaluate the lithostrati- ognizable units still called “lower gray lime- graphic (formation and group) names of Understanding of Pennsylvanian lithostra- stone” and “upper arkosic limestone” mem- Thompson (1942), which remain valid and of tigraphy in New Mexico has developed at an bers; 4) apply the earliest valid formal names potential use as members of more broadly uneven pace, and the nomenclature current- for the latter two units (Gray Mesa and defined Pennsylvanian formations; and 7) ly applied to Pennsylvanian strata is in some Atrasado, originally defined in the Lucero use New Mexico lithostratigraphic names for cases antiquated, inconsistent, redundant, or uplift) widely throughout the Madera Group subsurface Pennsylvanian strata within the inappropriate. The main Pennsylvanian rock outcrop area; in some cases replace other for- state, rather than names applied to sequences in New Mexico are reviewed, and mation names that have been proposed, and Pennsylvanian rocks in central Texas. several recommendations for revision of the recognize that locally a third upper Madera lithostratigraphic nomenclature are pro- unit (Bursum Formation and equivalents) posed. In some cases these recommendations may also be present; 5) recognize Madera Introduction build upon or broaden changes that have Group strata and terminology more widely Pennsylvanian strata were among the first been implemented by other workers in (e.g., into the Caballo and Robledo to be observed in detail by geologists restricted areas. These recommendations Mountains), yet retain current non-Madera entering New Mexico during and immedi- include: 1) abandon use of Magdalena terminology where appropriate, especially Group in New Mexico; 2) raise Madera for sequences associated with rapidly sub- ately after the American occupation, and Formation to Group rank; 3) treat previously siding basins (e.g., San Andres Mountains, Pennsylvanian fossils were among the first used members within the Madera as forma- Sacramento Mountains, Sangre de Cristo to be described from New Mexico Territory FIGURE 1—Currently used Pennsylvanian stratigraphic nomencla- ern Sangre de Cristo Mountains, with the upper part recognized as the ture in New Mexico (reproduced from Armstrong et al., 1979). The Alamitos Formation and Madera raised to Group rank (Baltz and only significant addition since 1979 has been establishment of the Myers, 1984, 1999). Porvenir Formation for the lower part of the Madera in the southeast- November 2001 NEW MEXICO GEOLOGY 103 FIGURE 2—Main exposures of Pennsylvanian strata discussed in text tains; 12) Sierra Oscura; 13) Fra Cristobal Range; 14) Sierra Cuchillo area; (reproduced from Armstrong et al., 1979). Numbers refer to mountain 15) Mud Springs Mountains; 16) Caballo Mountains; 17) Derry Hills area; ranges and other locations as follows: 1) northern Sangre de Cristo Moun- 18) Kingston area; 19) Santa Rita–Silver City area; 20) Big Hatchet tains; 2) southern Sangre de Cristo Mountains; 3) southeastern Sangre de Mountains; 21) Peloncillo Mountains; 22) Robledo Mountains; 23) San Cristo Mountains; 4) Nacimiento and Jemez Mountains; 5) Sandia Moun- Andres Mountains; 24) northern Organ Mountains; 25) northern Franklin tains; 6) Manzanita–Manzano Mountains; 7) Los Pinos Mountains; 8) Mountains and Bishop Cap Hills; 26) northern Hueco Mountains; 27) Socorro area; 9) Lucero uplift; 10) Sierra Ladrones; 11) Magdalena Moun- Sacramento Mountains. (e.g., Hall, 1856; Marcou, 1858). Later 19th real attempt to recognize, correlate, and Although Pennsylvanian strata are not century geologists (e.g., Stevenson, 1881) name lithostratigraphic units throughout as widely exposed in New Mexico as added information on Pennsylvanian New Mexico; Kottlowski (1960a) summa- Permian, Triassic, Cretaceous, and Paleo- stratigraphy of the territory, but subdivi- rized in detail Pennsylvanian stratigraphic gene strata, they include a greater diversi- sion and naming of Pennsylvanian strata sequences and nomenclature for much of ty of depositional environments and a began in the early 1900s (e.g., Herrick, the state; and Armstrong et al. (1979) pro- greater variety of reported fossil species 1900; Gordon, 1907) and has continued vided a general summary of New Mexico (more than 1,250) than any system except ever since. Thompson (1942) made the first Pennsylvanian strata (Fig. 1). the Cretaceous (Kues, 1982, table 2). Thick 104 NEW MEXICO GEOLOGY November 2001 (500+ m [1,640+ ft]) Pennsylvanian sec- summary of the major recommendations chronostratigraphic units, particularly tions are exposed in many parts of north- was presented earlier (Kues, 2000). A cor- with respect to their boundaries, rather ern and southern New Mexico (Fig. 2) relation chart of major exposed Pennsyl- than defining groups and formations chiefly in fault-block mountain ranges vanian sequences in New Mexico incorpo- entirely on a lithologic basis, as is required along the Rio Grande rift and adjacent rates proposed revisions in lithostrati- by the North American Code of regions, and Pennsylvanian strata are graphic nomenclature that are discussed in Stratigraphic Nomenclature. It is a miscon- widely present in the subsurface in most this paper (Fig. 3). ception, however, that Thompson’s litho- parts of the state, where some units are In proposing revisions of currently used stratigraphic units were nothing more than reservoirs for oil and gas (e.g., Broadhead, stratigraphic nomenclature, the desirabili- faunal zones, as claimed by some workers 1999), and others are hydrogeologic ty of maintaining stability of nomenclature (e.g., Kelley and Silver, 1952, p. 89). His aquifers producing water. by preserving familiar and long-used lithostratigraphic units were defined pre- Development of our understanding of names is recognized, even if in retrospect cisely, described in detail, and type sec- Pennsylvanian stratigraphy in New they may not have been the most logical or tions for each of them were designated. Mexico has proceeded at an uneven pace. appropriate names to apply to a particular A second difficulty with Thompson’s Pennsylvanian sequences in a few areas stratigraphic unit. Separate formation units, however, was that they were based have been intensively studied, whereas, names are appropriate for distinctive litho- almost entirely on the stratigraphy of two more commonly, sequences in other areas logical units that are mappable at a scale of restricted areas. His Atokan and Des- have been examined in only moderate 1:24,000; however, the same lithostrati- moinesian units were based on exposures detail, typically in the context of struc- graphic names should be used for similar in the Derry Hills–Mud Springs Mountains tural/stratigraphic studies of individual lithologic units that may be exposed wide- area, and his Missourian and Virgilian mountain ranges. The Pennsylvanian of ly in a region, cropping out, for example, in units were based on strata in the northern some ranges is known only at the level of several isolated mountain ranges. New Sierra Oscura, with the exception of one reconnaissance mapping done several and existing names should reflect signifi- group having a type section in the north- decades ago. Similarly, the stratigraphic cant lithological differences from equiva- ern Sacramento Mountains. Although nomenclature applied to Pennsylvanian lent units in a region, but not be estab- Thompson stated that many of his units sequences in New Mexico has developed lished simply as a convenient way to des- could be recognized in areas far distant in a piecemeal fashion, producing a large ignate local successions of strata with little from their type sections, he provided little number of currently used group, forma- reference to equivalent successions else- information to aid such correlations. Later tion, and member names, some of which where in the region. In this paper, little field geologists found that many of his are inappropriate, redundant, informal, or attention is devoted to the nomenclature of units could not be recognized easily very applied only to unnecessarily restricted the transitional Pennsylvanian–Permian far from their type areas. areas, and some of which have been wide- units noted above. Recent proposals to Thirdly, Thompson’s groups and forma- ly used for decades without designation of raise the Pennsylvanian–Permian bound- tions largely represented restricted strati- type sections or adequate formal defini- ary to a position within the Wolfcampian graphic intervals that in some cases were tion. stage in North America (e.g., Baars et al., not easily distinguishable lithologically Lateral facies changes may frustrate 1994; Lucas et al., 2000) are likewise not from underlying or overlying units, and in recognition of stratigraphic units very far addressed. These are the focus of ongoing many cases were not mappable at the from their type sections. This has con- studies by the author and others, and con- scales required by the present Code of tributed, on the one hand, to the establish- clusions will be presented in a future Stratigraphic Nomenclature. ment of unique successions of formation paper. Although the focus here is on Thompson’s work is noted here in part names in individual, closely