Bursum Formation


The Bursum Formation is a geologic formation in New Mexico. It preserves fossils dating back to the early Permian period.

Description

The Bursum Formation is primarily mudstone but with substantial limestone, particularly in its lower beds, with a thickness in excess of. It rests disconformably on the Atrasado Formation and grades into the overlying Abo Formation. It represents the transitional zone between the marine Madera Group and the continental Abo Formation.
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. 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.

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.
The Red Tanks Member also contains biogenic carbonate nodules at Cibola Spring 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.

Footnotes