Arroyo Penasco Group

The Arroyo Penasco Group is a geologic group exposed in the Nacimiento, Jemez, Sandia, and Sangre de Cristo Mountains of northern New Mexico.[1] It preserves fossils characteristic of the late Mississippian.[1]

Arroyo Penasco Group Stratigraphic range: Mississippian
Arroyo Penasco Group resting on Precambrian rock
Type	Group
Sub-units	Espiritu Santo Formation, Tererro Formation
Underlies	Log Springs Formation
Overlies	Precambrian basement
Thickness	95 m at type section
Lithology
Primary	Limestone
Other	Sandstone
Location
Coordinates	35.640635°N 106.852934°W
Region	New Mexico
Country	United States
Type section
Named for	Arroyo Penasco (35.5591955°N 106.8647114°W)
Named by	A.K. Armstrong (1955)
Arroyo Penasco outcrop map

Description

The Arroyo Penasco Group consists of marine sedimentary formations, primarily massive limestone but with some sandstone. It is divided into the lower Espiritu Santo Formation and the upper Tererro Formation.

The lowermost part of the Espiritu Santo Formation is designated the Del Padre Member and is a transgressive siltstone, sandstone, and shale unit, which interfingers with the carbonate rocks of the upper Espiritu Santo Formation. A similar sequence is seen in the Tererro Formation, whose base is a collapse breccia.[2]

The group rests everywhere on Precambrian basement that shows remarkably little relief, suggesting that the group was deposited on a peneplain. Its outcrops are spotty and its upper contact shows indications of well-developed karst topography, often filled in by the iron-rich sediments of the Log Springs Formation, indicating that it was heavily eroded before deposition of the Sandia Formation.[3]

The Espiritu Santo Formation has undergone dolomitization, dedolomitization, and calcitization of gypsum and the Macho Member has experienced dissolution and brecciation. Together with the presence of paleosilcretes in the Manuelitas Member, this suggests a history of at least four episodes of sea level fluctuation in the Mississippian in northern New Mexico. Diagenetic fabrics can be traced across the region, demonstrating that the group was deposited across an extensive surface of low relief.[4]

• 
  Massive limestone beds of the Arroyo Penasco Group on the rim of Guadelupe Box
• 
  Closer view of massive limestone of the Arroyo Penasco Group at Guadelupe Box

Fossils

The fossil assemblage suggests an age of Osagean to Meramecian.[5]

Brachiopoda
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Eumetria	verneuiliana Hall	Penasco Canyon[6]
Eumetria	vera Hall	Penasco Canyon[6] San Pedro Mountains[7]
Linoproductus	ovatus Hall	Penasco Canyon[6]
Composita	aff. lewisensis Weller	Penasco Canyon[6]
Spirifer		Penasco Canyon[6] San Pedro Mountains[7] Tecolote Creek[8]

Mollusca
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Staparollus		Penasco Canyon[6]
Leptodesma		Penasco Canyon[6]

Bryozoa
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Fenestella		Penasco Canyon[6]
Archimedes		Penasco Canyon[6]

Foraminifera
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Endothyra		Penasco Canyon[6] San Pedro Mountains[7] Lujan Canyon, Rincon Range[9] Gallinas Canyon[9] Tecolote Creek[8] El Macho, Pecos River Canyon[8]
Plectogyra		Penasco Canyon[6] San Pedro Mountains[7] Lujan Canyon, Rincon Range[9] Gallinas Canyon[9] Tecolote Creek[8] El Macho, Pecos River Canyon[8]

Cnidaria
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Triplophyllites		Lujan Canyon, Rincon Range[9]
Conularia		Lujan Canyon, Rincon Range[9] Tecolote Creek[8]

History of investigation

The massive limestone beds of the Arroyo Penasco Group were originally included as the base of the Pennsylvanian Sandia Formation. The discovery of Endothyra and Plectogyra in 1951 showed that the beds were actually Mississippian in age, and they were split off as the Arroyo Penasco Formation.[10] The Arroyo Penasco was later found to correlate to the Espiritu Santo Formation and Tererro Formation in the Sangre de Cristo Mountains, and it was promoted to group rank.[2]

See also

• List of fossiliferous stratigraphic units in New Mexico
• Paleontology in New Mexico

Footnotes

1. Armstrong 1955, p.3
2. Armstrong and Mamet 1974
3. Armstrong 1967
4. Ulmer and Laury 1984
5. Ulmer and Laury, 1984, p.91
6. Armstrong 1955, p.6
7. Armstrong 1955, p.17
8. Armstrong 1955, p.25
9. Armstrong 1955, p.20
10. Armstrong 1955, pp.1-3

References

• Armstrong, Augustus K (1955). "Preliminary Observations of the Mississippian System of Northern New Mexico" (PDF). Circulars of the New Mexico Bureau of Mines and Mineral Resources. 39. Retrieved 21 May 2019.
• Armstrong, Augustus K (1967). "Biostratigraphy and Carbonate Facies of the Mississippian Arroyo Penasco Formation, North-Central New Mexico" (PDF). Memoirs of the New Mexico Bureau of Mines and Mineral Resources. 20. Retrieved 21 May 2019.
• Armstrong, Augustus K; Mamet, Bernard L. (1974). "Biostratigraphy of the Arroyo Penasco Group, Lower Carboniferous (Mississsippian), north-central New Mexico" (PDF). Annual NMGS Fall Field Conference Guidebooks. 25. Retrieved 22 May 2019.
• Ulmer, Dana S.; Laury, Robert L. (1984). "DIAGENESIS OF THE MISSISSIPPIAN ARROYO PEN ASCO GROUP, NORTH-CENTRAL NEW MEXICO" (PDF). New Mexico Geological Society Field Conference Series. 35: 91–100. Retrieved 22 April 2020.