The La Pasada Formation is a geologic formation in the southern

Sangre de Cristo Mountains ) , country= United States , subdivision1_type= States , subdivision1= , parent= Rocky Mountains , geology= , orogeny= , area_mi2= 17193 , range_coordinates= , length_mi= 242 , length_orientation= north-south , width_mi= 120 , w ...

of New Mexico. It preserves

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...

dating back to the

early Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * Early ...

middle Pennsylvanian The Pennsylvanian ( , also known as Upper Carboniferous or Late Carboniferous) is, in the ICS geologic timescale, the younger of two subperiods (or upper of two subsystems) of the Carboniferous Period. It lasted from roughly . As with most o ...

Description

The formation is a cyclic carbonate consisting of alternating limestone and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

with some thin sandstones. Total thickness is . The formation is more

clastic Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...

towards its base (50% shale and

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

) 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. 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. 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.

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 ''Ditomopyge'' is an extinct genus of trilobite belonging to the family Proetidae. It was extant during the Carboniferous and Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of t ...

scitula'' and ''

Ameura ''Ameura'' is an extinct genus of trilobite belonging to the family Proetidae. Fossils from the genus have been found in late Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The nam ...

missouriensis''.

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 The Madera Group is a group of geologic formations in northern New Mexico. Its fossil assemblage dates the formation to the middle to late Pennsylvanian period. Description The group consists primarily of marine limestones, and it is exposed in ...

. 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.

Spencer G. Lucas Spencer George Lucas is an American paleontologist and stratigrapher, and curator of paleontology at the New Mexico Museum of Natural History and Science. His main areas of study are late Paleozoic, Mesozoic and early Cenozoic vertebrate fossils ...

and coinvestigators also exclude the La Pasada Formation from the Madera Group.

References

{{Reflist Carboniferous formations of New Mexico Carboniferous southern paleotropical deposits

Description

Fossils

History of investigation

See also

References