Bursum Formation
The Bursum Formation is a geologic formation in New Mexico. It preserves fossils dating back to the early Permian period.[1]
| Bursum Formation Stratigraphic range: lower Permian | |
|---|---|
| Type | Formation |
| Underlies | Abo Formation |
| Overlies | Atrasado Formation |
| Thickness | 35 m (115 ft) |
| Lithology | |
| Primary | Mudstone, limestone |
| Location | |
| Coordinates | 33.812698°N 106.478451°W |
| Region | New Mexico |
| Country |  |
| Type section | |
| Named for | Bursum triangulation point |
| Named by | Wilpolt, R.H., MacAlpin, A.J., Bates, R.L., and Vorbe, George |
Description
The Bursum Formation is primarily mudstone but with substantial limestone, particularly in its lower beds, with a thickness in excess of 100 m (330 ft). It rests disconformably on the Atrasado Formation and grades into the overlying Abo Formation.[1] It represents the transitional zone between the marine Madera Group and the continental Abo Formation.[2]
The Red Tanks Member is composed mostly of variegated shale, mudstone, and siltstone of nonmarine origin, with some beds of marine limestone and shale forming six transgressive depositional sequences. Its type section is at Carrizo Arroyo (34.7790683°N 107.1068066°W). The Bursum Formation has a more consistently marine character further south. The fact that the transgressive sequences are fewer in number than the cyclothems of the Pennsylvanian-Permian boundary in the North American mid-continent shows that tectonics had more influence on the development of this formation than glacial cycles.[1]
Fossils
Fossils found in the formation include the fusilinid Triticites from which its early Permian age is determined. The formation also preserves fossils of the forams Bradyina lucida and Eostaffella. There are two Lagerstätten in the Red Tanks Member at Carrizo Arroyo that are of early Asselian age, while the recent definition of the base of the Permian as the first appearance of the conodont Streptognathus isolatus pushes the earliest part of the formation, and the associated North American Wolfcampian Stage back into the latest Pennsylvanian.[1]
The Red Tanks Member also contains biogenic carbonate nodules (oncolites) at Cibola Spring (34.231151°N 106.6787895°W) in a distinctive, widespread limestone horizon. These are unusual in being composed mostly of formans rather than cyanobacteria and are interpreted as having formed in a shelf environment below the euphotic zone or normal wave base but above the storm wave base.[3]
Footnotes
- Lucas et al. 2016
- Kues and Giles 2004, pp.98-100
- Scholle et al. 2016
References
- Kues B.S. Giles K.A., 2004, The late Paleozoic Ancestral Rocky Mountain system in New Mexico, in Mack G.H. Giles K.A., eds., The geology of New Mexico. A geologic history: New Mexico Geological Society Special Volume 11, p. 95–136.
- Scholle, Peter A.; BouDagher-Fadel, Marcelle; Ulmer-Scholle, Dana S.; Love, David W. (2016). "On the origin of carbonate nodules in the Bursum Formation at Cibola Spring, Socorro County, New Mexico" (PDF). New Mexico Geological Society Field Conference Series. 67: 369–376. Retrieved 12 June 2020.
- Lucas, Spencer G.; Barrick, James E.; Krainer, Karl; Schneider, Jorg W. (2016). "Pennsylvanian-Permian boundary at Carrizo Arroyo, central New Mexico" (PDF). New Mexico Geological Society Field Conference Series. 67: 303–311. Retrieved 11 June 2020.