The Lead Camp limestone and it correlatives in south-central New Mexico
George O. Bachman and Donald A. Myers
Nomenclature of Pennsylvanian rocks in New Mexico has posed many problems. Facies changes that cross time lines (faunal zones) are characteristic of these rocks. This characteristic has contributed to two major types of nomenclature: (1) naming of mappable or rock-stratigraphic units, and (2) naming of faunal zones or biostratigraphic units, with little regard to how they relate to each other. Both nomenclatures result in a great number of stratigraphic names, but most workers have applied the first type because it follows the conventions of mappability as outlined by the Code of Stratigraphic Nomenclature (American Commission on Stratigraphic Nomenclature, 1970). Some workers, however, have applied biostratigraphic names, because of the intricate biostratigraphic relations of these rocks and because it appears more practical, in some places, to correlate faunal zones from one mountain range to another than to try to correlate rock units. In deep basins in the subsurface, it is more difficult to divide Pennsylvanian rocks on a lithologic basis alone, and faunal zones serve a useful purpose.
Our approach to this problem of stratigraphic nomenclature has been to use principles of both physical stratigraphy and biostratigraphy. We have traced rock units in the field and have attempted to fit these units and their changes in facies into mappable formations. While tracing units we collected fusulinids; these fusulinids were used as the basis for a framework of faunal zones for the formations.
Owing to the problems of correlating rock units from one mountain range to another, our approach encourages a multiplicity of stratigraphic names. Regardless of this disadvantage, we believe that this approach will lead ultimately to a better understanding of the stratigraphy and to a more precise interpretation of the paleogeography. Our correlations are shown on Table 1 and Figure 2.
Local names are preferable to the usage of "Magdalena Formation" or "Magdalena Group." The Magdalena has been used in the past to designate rocks of predominantly Pennsylvanian age in New Mexico and West Texas. Where exposed, individual formations can usually be mapped on the basis of lithology; the term Magdalena Group is superfluous and its use should be discouraged.
The fusulinid zones used in this report are those outlined by Thompson (1964, p. C389-393). The generic name Beedeina is now used for most North American species that were formerly referred to Fusulina (Ishi, 1957, 1958). Hence, the "zone of Beedeina" is used instead of "zone of Fusulina."
The zone of Millerella includes latest Mississippian and Early Pennsylvanian time; the zone of Profusulinella, early Middle Pennsylvanian time; the zone of Fusulinella, middle Middle Pennsylvanian time; the zone of Beedeina, late Middle Pennsylvanian time; and the zone of Triticites, Late Pennsylvanian time.
- Bachman, George O.; Myers, Donald A., 1975, The Lead Camp limestone and it correlatives in south-central New Mexico, in: Las Cruces Country, Seager, William R.; Clemons, Russell E.; Callender, Jonathan F., New Mexico Geological Society, Guidebook, 26th Field Conference, pp. 105-108.