Helms Formation

Helms Formation
Stratigraphic range: Serpukhovian PreꞒ Ꞓ O S D C P T J K Pg N
Type	Formation
Underlies	La Tuna Formation
Overlies	Rancheria Formation
Thickness	30 m (98 ft)
Lithology
Primary	Shale
Other	Sandstone, limestone
Location
Coordinates	31.768°N 106.026°W / 31.768; -106.026
Region	Texas, New Mexico
Country	United States
Type section
Named for	Helms West Well
Named by	J.W. Beede
Year defined	1920
Helms Formation (the United States) Show map of the United States Helms Formation (Texas) Show map of Texas

The Helms Formation is a geologic formation in Texas and New Mexico. It preserves fossils dating back to the Chesterian (Serpukhovian) Age of the Carboniferous period.[1]

Description

At its type section in the Hueco Mountains, the formation consists of 30 meters (98 ft) of olive-gray shale, shaly sandstone, and dirty limestone. It is unconformably underlain by the Rancheria Formation[2] and unconformably overlain by the La Tuna Formation. The formation also crops out in the Organ Mountains of New Mexico.[1]

The formation is interpreted as having been deposited during a marine regression (withdrawal of the sea from the continent) to the south.[3]

Fossils

The formation contains fossils characteristic of the Chesterian (Serpukhovian) Age of the Carboniferous period.[2][1] These include abundant specimens of the ostracod Graphiadactyllis arkansana, as well as specimens of the bryozoan Archimedes terebriformis, the echinoderm Pentremites, the brachiopods Composita subquadrata, Spiriferina transversa, Diaphragmus elegans, and several species of Dictyoclostus,[2] and the trilobite Paladin helmensis.[4]

History of investigation

The formation was first designated as the Helms Group by J.W. Beeded in 1920 for outcrops near Helms Well West near the Hueco Mountains.[5] The original definition included the entire set of beds between Silurian and Pennsylvanian in age. In 1949, L.R. Laudon and A.L. Bowsher restricted Helms Formation to only the uppermost beds of the original section.[2]

See also

• List of fossiliferous stratigraphic units in Texas
• Paleontology in Texas

References

1. Lucas, Spencer G.; Krainer, Karl (Fall 2020). "Gallery of Geology: The Pennsylvanian section at Bishop Cap, Doña Ana County, New Mexico" (PDF). New Mexico Geology. 42 (2): 79-81. Retrieved 11 February 2021.
2. Laudon, Lowell R.; Bowsher, Arthur L. (1949). "Mississippian formations of southwestern New Mexico". Geological Society of America Bulletin. 60 (1): 1. doi:10.1130/0016-7606(1949)60[1:MFOSNM]2.0.CO;2.
3. Lane, H. Richard (1974). "Mississippian of Southeastern New Mexico and West Texas--A Wedge-on-Wedge Relation". AAPG Bulletin. 58 (2): 269–282. doi:10.1306/83D913D0-16C7-11D7-8645000102C1865D.
4. Whittington, H.B. (1954). "Two silicified Carboniferous trilobites from west Texas" (PDF). Smithsonian Miscellaneous Collections. 122 (10). Retrieved 13 January 2022.
5. Beede, J.W. (1918). "Notes on the geology and oil possibilities of the northern Diablo Plateau in Texas". University of Texas Bulletin (1852).