Type section of the Upper Carboniferous Bursum Formation, south-central New Mexico, and the Bursumian stage
Spencer G. Lucas, Karl Krainer, and Barry S. Kues
The term Bursum has been used to refer to a lithostratigraphic unit, a biostratigraphic zone or to a stage (Bursumian) of the earliest- Permian or latest Carboniferous in North America. The type section of the lithostratigraphic unit Bursum Formation in Socorro County, New Mexico, is 85 m thick, conformably overlies the Maya Member of the Atrasado Formation and is disconformably overlain by red-bed siliciclastics of the Abo Formation. The Bursum section is mostly shale (55% of the measured section), red-bed mudstone (23%) and marine limestone (19%), with minor conglomerate and sandstone (< 3%). We pick the base of a mappable Bursum Formation at a point of maximum lithologic contrast between a 10-m-thick interval of red-bed mudstone of the lower Bursum above light brownish gray bioclastic limestones of the Madera Group. Limestone microfacies of the type Bursum section indicate deposition in shallow, low energy shelf environments in the photic zone in waters of normal salinity. Most type Bursum shale units also were deposited in a shallow marine environment, but some pedogenically modified mudstones and arksoic sandstones indicate subaerial deposition on a coastal plain.
Invertebrate macrofossils from four limestone intervals in the type Bursum section are mostly brachiopods, but include bryozoans, bivalves, scaphopods, gastropods, nautiloids, trilobites and crinoids. These taxa generally have both Virgilian and Wolfcampian records elsewhere, and they indicate deposition in low energy, shallow marine environments.
The fusulinacean fauna of the type Bursum section, and of immediately underlying strata of the upper Atrasado Formation, is characteristic of the early Wolfcampian. Fusulinacean distribution at the type section also indicates that the base of the Bursum Formation (lithostratigraphic unit) does not correspond to the base of a fusulinacean zone. The Bursum type section is not an adequate stratotype for a Bursumian Stage. Indeed, the proposed Bursumian Stage is nothing more than a fusulinacean zone and should be abandoned.
- Lucas, Spencer G.; Krainer, Karl; Kues, Barry S., 2002, Type section of the Upper Carboniferous Bursum Formation, south-central New Mexico, and the Bursumian stage, in: Geology of White Sands, Lueth, Virgil W.; Giles, Katherine A.; Lucas, Spencer G.; Kues, Barry S.; Myers, Robert; Ulmer, Scholle, Dana S., New Mexico Geological Society, Guidebook, 53rd Field Conference, pp. 179-192.