{{Short description|Geologic formation in New Mexico, US}} {{Infobox rockunit | name = La Pasada Formation | image = La pasada formation daltons bluff.jpg | caption = La Pasada Formation at its type section at Dalton's Bluff | type = Formation | age = {{Geological range|Moscovian|Bashkirian}} | period = Pennsylvanian | prilithology = Limestone | otherlithology = Shale | namedfor = Upper La Pasada (abandoned Spanish settlement) | namedby = P.K. Sutherland | year_ts = 1963 | region = New Mexico | country = United States | coordinates = {{coord|35.662|N|105.693|W|display=inline}} | unitof = | subunits = | underlies = Alamitos Formation | overlies = Tererro Formation | thickness = {{convert|973|feet|meters|abbr=on}} | extent = | area = | map ={{Location map+ | United States#New Mexico | relief = 1 | width = 250 | float = center | places = {{Location map~ | United States#New Mexico | lat_deg = 35.662 | lon_deg = -105.693 | mark = Lightgreen pog.svg | marksize = 12 }} }} | map_caption = }}
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.<ref name="sutherland-1963">{{cite encyclopedia |last1=Sutherland |first1=P.K. |year=1963 |title=Paleozoic rocks |editor1-last=Miller |editor1-first=J.P. |editor2-last=Montgomery |editor2-first=Arthur |editor3-last=Sutherland |editor3-first=P.K. |encyclopedia=Geology of part of the southern Sangre de Cristo Mountains, New Mexico |publisher=New Mexico Bureau of Mines and Mineral Resources Memoir 11 |pages=22–44 |url=https://geoinfo.nmt.edu/publications/monographs/memoirs/11/ |accessdate=15 December 2021}}</ref>
==Description== The formation is a cyclic carbonate consisting of alternating limestone and shale with some thin sandstones. Total thickness is {{convert|973-1002|feet|meters|abbr=on}}. The formation is more clastic towards its base (50% shale and siltstone) than towards its upper portion (24% shale and siltstone). 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.<ref name="sutherland-1963"/>
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.<ref name="sutherland-1963"/> <gallery> File:Pasada massive bed.jpg|Massive limestone bed of La Pasada Formation at Dalton's Bluff. File:La Pasada Formation road cut.jpg|La Pasada Formation in road cut north of Pecos. </gallery>
== Fossils == The formation contains abundant fossils of ''Mesolobus'' and other brachiopods, fenestrate bryozoans, crinoid fragments, and less common pectinid bivalves, as well as small numbers of trilobites, including ''Ditomopyge scitula'' and ''Ameura missouriensis''.<ref name="Kues2004">{{cite journal |last1=Kues |first1=B.S. |title=Pennsylvanian trilobites from the Sangre de Cristo and Jemez Mountains, north-central New Mexico |journal=New Mexico Geological Society Field Conference Series |date=2004 |volume=55 |pages=326–334 |url=https://nmgs.nmt.edu/publications/guidebooks/downloads/55/55_p0326_p0334.pdf |access-date=15 December 2021}}</ref>
==History of investigation== The formation was first defined in 1963 by Patrick K. Sutherland, who considered it correlative with the lower part of the Madera Formation.<ref name="sutherland-1963"/> However, in 2004, Barry Kues and Katherine 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.<ref name="kues-giles-2004-100">{{cite encyclopedia |last1=Kues |first1=B.S. |last2=Giles |first2=K.A. |year=2004 |title=The late Paleozoic Ancestral Rocky Mountain system in New Mexico |editor1-last=Mack |editor1-first=G.H. |editor2-last=Giles |editor2-first=K.A. |encyclopedia=The geology of New Mexico. A geologic history (Special Volume 11) |publisher=New Mexico Geological Society |pages=95–136}}</ref> Spencer G. Lucas and coinvestigators also exclude the La Pasada Formation from the Madera Group.<ref name="lucas-etal-2016">{{cite journal |last1=Lucas |first1=Spencer G. |last2=Krainer |first2=Karl |last3=Vachard |first3=Daniel |title=The Pennsylvanian section at Priest Canyon, southern Manzano Mountains, New Mexico |journal=New Mexico Geological Society Field Conference Series |date=2016 |volume=67 |url=https://nmgs.nmt.edu/publications/guidebooks/downloads/67/67_p0275_p0301.pdf |accessdate=11 June 2020}}</ref>
==See also== {{Portal|Earth sciences|Paleontology}} * List of fossiliferous stratigraphic units in New Mexico * Paleontology in New Mexico
==References== {{Reflist}}
Category:Carboniferous formations of New Mexico Category:Carboniferous southern paleotropical deposits Category:Limestone formations of the United States Category:Shale formations of the United States