Petrified Forest Member

Petrified Forest Member
Stratigraphic range: late Triassic, 240–215 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Petrified Forest Member at its type location, north of Tiponi Point, Petrified Forest National Park
Type	member
Unit of	Chinle Formation
Underlies	Rock Point Formation
Overlies	Poleo Formation
Lithology
Primary	Mudstone
Other	Sandstone, siltstone
Location
Coordinates	35.074°N 109.780°W / 35.074; -109.780
Region	New Mexico
Country	United States
Type section
Named for	Petrified Forest, Navajo and Apache Counties, AZ[1]
Named by	Herbert E. Gregory
Year defined	1947
Petrified Forest Member (the United States) Show map of the United States Petrified Forest Member (Arizona) Show map of Arizona

The Petrified Forest Member is a stratigraphic unit of the Chinle Formation in Arizona, Colorado, New Mexico, Nevada, and Utah.^[2] It preserves fossils dating back to the Triassic period.

Subunits

Beds (are in alphabetical order, asterisks (*) indicate usage by the U.S. Geological Survey, other usages by state geological surveys):^[2]

• Capitol Reef Bed (UT*)
• Correo Sandstone Bed (NM*)
• Sonsela Sandstone Bed (AZ*,NM*)

In the Chama Basin of New Mexico, the Chinle Formation is promoted to group status and the Petrified Forest Formation has the following members:^[3]

• Painted Desert Member
• Mesa Montoso Member

The Mesa Montosa Member is up to 22 meters (72 feet) thick and is mostly composed of sandstone (44%) and mudstone (35%), with a lesser proportion of siltstone (20%). The color is reddish brown to brown and the sandstone is thinly bedded and ripple laminated.

The Painted Desert Member is up to 176 meters (577 feet) thick and is primarily reddish brown bentonitic mudstone. The name was originally applied to the beds above the Sonsela Sandstone Bed in Petrified Forest National Park in eastern Arizona. Correlative beds of similar lithology are found throughout west-central and central New Mexico and the Four Corners. The Black Forest Bed within this member has a maximum age of 213 ± 1.7 Ma based on detrital zircon geochronology, with an actual age estimated as 209 Ma. This places the upper part of the Painted Desert Member in the latest Norian age.^[4]

Fossils

The Snyder quarry, discovered by amateur fossil hunter Mark Snyder near the town of Abiquiu, New Mexico, in 1998, is an unusually productive and diverse bone bed in the Petrified Forest Formation. Abundant charcoal suggests a mass kill due to a paleowildfire. Fossils include archosaurs, procolophonids, metoposaurid amphibians, semionotid fish, a decapod, a conchostracan, and unionid bivalves.^[5]

See also

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

Footnotes

1. Gregory 1950. sfn error: no target: CITEREFGregory1950 (help)
2. GEOLEX 2020.
3. Lucas et al. 2005.
4. Riggs et al. 2003.
5. Zeigler, Heckert & Lucas 2005.

References

• "Geolex Petrified Forest Member entry". ngmdb.usgs.gov. U.S. Geological Survey. 24 Sep 2018. Retrieved 27 January 2019.
• Gregory, H.E. (1950). "Geology and geography of the Zion Park region, Utah and Arizona" (PDF). Geological Survey Professional Paper. doi:10.3133/PP220. ISSN 0096-0446. Wikidata Q61046031.
• Lucas, Spencer G.; Zeigler, Kate E.; Heckert, Andrew B.; Hunt, Adrian P. (2005). "Review of Upper Triassic stratigraphy and biostratigraphy in the Chama Basin, northern New Mexico" (PDF). New Mexico Geological Society Field Conference Series. 56: 170–181. Retrieved 29 April 2020.
• Riggs, N.R.; Ash, S.R.; Barth, A.P.; Gehrels, G.E.; Wooden, J.L. (2003). "Isotopic age of the Black Forest Bed, Petrified Forest Member, Chinle Formation, Arizona: An example of dating a continental sandstone". Geological Society of America Bulletin. 115 (11): 1315. doi:10.1130/B25254.1.
• Zeigler, Kate E.; Heckert, Andrew B.; Lucas, Spencer G. (2005). "Taphonomic analysis of a fire-related Upper Triassic vertebrate fossilassemblage from north-central New Mexico" (PDF). New Mexico Geological Society Field Conference Series. 56: 341–354. Retrieved 25 May 2020.