Helms Formation
{{Short description|Geologic formation in Texas and New Mexico, US}}
{{Infobox rockunit
| name = Helms Formation
| image =
| caption =
| period = Serpukhovian
| type = Formation
| age = {{Geological range|Serpukhovian}}
| prilithology = Shale
| otherlithology = Sandstone, limestone
| namedfor = Helms West Well
| namedby = J.W. Beede
| year_ts = 1920
| region = Texas, New Mexico
| country = United States
| coordinates = {{coord|31.768|N|106.026|W|display=inline}}
| unitof =
| subunits =
| underlies = La Tuna Formation
| overlies = Rancheria Formation
| thickness = {{convert|98|feet|m|order=flip|abbr=on}}
| extent =
| area =
| map = {{Location map+ | United States#Texas
| relief = 1
| width = 250
| float = center
| places =
{{Location map~ | United States#Texas
| lat_deg = 31.768
| lon_deg = -106.026
| mark = Green pog.svg
| marksize = 12
}}
}}
| map_caption =
}}
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.{{cite journal |last1=Lucas |first1=Spencer G. |author1-link=Spencer G. Lucas |last2=Krainer |first2=Karl |title=Gallery of Geology: The Pennsylvanian section at Bishop Cap, Doña Ana County, New Mexico |journal=New Mexico Geology |date=Fall 2020 |volume=42 |issue=2 |page=79-81 |url=https://geoinfo.nmt.edu/publications/periodicals/nmg/42/n2/nmg_v42_n2.pdf |access-date=11 February 2021}}
Description
At its type section in the Hueco Mountains, the formation consists of {{convert|98|feet|m|order=flip|sp=us}} of olive-gray shale, shaly sandstone, and dirty limestone. It is unconformably underlain by the Rancheria Formation and unconformably overlain by the La Tuna Formation. The formation also crops out in the Organ Mountains of New Mexico.
The formation is interpreted as having been deposited during a marine regression (withdrawal of the sea from the continent) to the south.{{cite journal |last1=Lane |first1=H. Richard |title=Mississippian of Southeastern New Mexico and West Texas--A Wedge-on-Wedge Relation |journal=AAPG Bulletin |date=1974 |volume=58 |issue=2 |pages=269-282 |doi=10.1306/83D913D0-16C7-11D7-8645000102C1865D}}
Fossils
The formation contains fossils characteristic of the Chesterian (Serpukhovian) Age of the Carboniferous period. 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, and the trilobite Paladin helmensis.{{cite journal |last1=Whittington |first1=H.B. |year=1954 |title=Two silicified Carboniferous trilobites from west Texas |journal=Smithsonian Miscellaneous Collections |volume=122 |number=10 |url=https://repository.si.edu/bitstream/handle/10088/22896/SMC_122_Whittington_1954_10_1-16.pdf |access-date=13 January 2022}}
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.{{cite journal |last1=Beede |first1=J.W. |year=1918 |title=Notes on the geology and oil possibilities of the northern Diablo Plateau in Texas |journal=University of Texas Bulletin |number=1852}} 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.{{cite journal |last1=Laudon |first1=Lowell R. |last2=Bowsher |first2=Arthur L. |title=Mississippian formations of southwestern New Mexico |journal=Geological Society of America Bulletin |date=1949 |volume=60 |issue=1 |pages=1 |doi=10.1130/0016-7606(1949)60[1:MFOSNM]2.0.CO;2}}
See also
{{Portal |Earth sciences|Texas|Paleontology}}
References
{{reflist}}
Category:Carboniferous formations of New Mexico
Category:Carboniferous southern paleotropical deposits
Category:Shale formations of the United States
Category:Sandstone formations of the United States
Category:Limestone formations of the United States
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