La Pasada Formation
The La Pasada Formation is a geologic formation in the southern Sangre de Cristo Mountains of New Mexico. It preserves fossils dating back to the early to middle Pennsylvanian.[1]
| La Pasada Formation | |
|---|---|
| Stratigraphic range: | |
 La Pasada Formation at its type section at Dalton's Bluff | |
| Type | Formation |
| Underlies | Alamitos Formation |
| Overlies | Tererro Formation |
| Thickness | 973 ft (297 m) |
| Lithology | |
| Primary | Limestone |
| Other | Shale |
| Location | |
| Coordinates | 35.662°N 105.693°W |
| Region | New Mexico |
| Country | United States |
| Type section | |
| Named for | Upper La Pasada (abandoned Spanish settlement) |
| Named by | P.K. Sutherland |
| Year defined | 1963 |
  La Pasada Formation (the United States)  La Pasada Formation (New Mexico) | |
Description
The formation is a cyclic carbonate consisting of alternating limestone and shale with some thin sandstones. Total thickness is 973–1,002 ft (297–305 m). The formation is more clastic towards its base (50% shale and siltstone) than towards its upper portion (24% shale and siltstone). The shales are noncalcareous and greenish towards the base but become gray, calcareous, and often fossiliferous towards the upper portion. The formation shows considerable lateral variability, grading into the Flechado Formation to the north.[1]
The lower half of the formation is interpreted as a shallow marine nearshore sequence with occasional nonmarine intervals with thin coal beds. The upper half was deposited under neritic offshore marine conditions with infrequent nonmarine intervals.[1]
Massive limestone bed of La Pasada Formation at Dalton's Bluff.
La Pasada Formation in road cut north of Pecos.
Fossils
The formation contains abundant fossils of Mesolobus and other brachiopods, fenestrate bryozoans, crinoid fragments, and less common pectinid bivalves, as well as small numbers of trilobites, including Ditomopyge scitula and Ameura missouriensis.[2]
History of investigation
The formation was first defined in 1963 by Patrick K. Sutherland, who considered it correlative with the lower part of the Madera Formation.[1] However, in 2004, Barry Kues and Katherine Giles recommended restricting the Madera Group to shelf and marginal basin beds of Desmoinean (upper Moscovian) to early Virgilian age, which excluded the La Pasada Formation.[3] Spencer G. Lucas and coinvestigators also exclude the La Pasada Formation from the Madera Group.[4]
References
- Sutherland, P.K. (1963). "Paleozoic rocks". In Miller, J.P.; Montgomery, Arthur; Sutherland, P.K. (eds.). Geology of part of the southern Sangre de Cristo Mountains, New Mexico. New Mexico Bureau of Mines and Mineral Resources Memoir 11. pp. 22–44. Retrieved 15 December 2021.
- Kues, B.S. (2004). "Pennsylvanian trilobites from the Sangre de Cristo and Jemez Mountains, north-central New Mexico" (PDF). New Mexico Geological Society Field Conference Series. 55: 326–334. Retrieved 15 December 2021.
- 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 (Special Volume 11). New Mexico Geological Society. pp. 95–136.
- Lucas, Spencer G.; Krainer, Karl; Vachard, Daniel (2016). "The Pennsylvanian section at Priest Canyon, southern Manzano Mountains, New Mexico" (PDF). New Mexico Geological Society Field Conference Series. 67. Retrieved 11 June 2020.