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: early Pennsylvanian to middle Pennsylvanian
La Pasada Formation at its type section at Dalton's Bluff
TypeFormation
UnderliesAlamitos Formation
OverliesTererro Formation
Thickness973 ft (297 m)
Lithology
PrimaryLimestone
OtherShale
Location
Coordinates35.662°N 105.693°W / 35.662; -105.693
RegionNew Mexico
CountryUnited States
Type section
Named forUpper La Pasada (abandoned Spanish settlement)
Named byP.K. Sutherland
Year defined1963
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 less clastic towards its base (50% shale and siltstone) than towards its upper portion (24% shale and silstone). 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]

History of investigation

The formation was first defined in 1963 by Sutherland, who considered it correlative with the lower part of the Madera Formation.[1] However, in 2004, Kues and 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.[2] Lucas et al. also exclude the La Pasada Formation from the Madera Group.[3]

See also

Footnotes

  1. Sutherland 1963
  2. Kues and Giles 2004
  3. Lucas 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 (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.
  • Sutherland, P.K. (1963). "Paleozoic rocks" (PDF). 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 29 July 2020.
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