The Big Rock Formation is a formation that crops out in the

Tusas Mountains The Tusas Mountains are a mountain range in northern New Mexico, extending slightly into southern Colorado. They are considered the southeasternmost part of the San Juan Mountains. Grouse Mesa, , is the highest peak in the range. The mountains a ...

of northern

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

Detrital zircon geochronology Detrital zircon geochronology is the science of geochronology, analyzing the age of zircons deposited within a specific sedimentary rock, sedimentary unit by examining their inherent radioisotopes, most commonly the uranium–lead dating, uranium ...

gives a maximum age for the formation of 1665 Mya, corresponding to the

Statherian The Statherian Period (; , meaning "stable, firm") is the final geologic period in the Paleoproterozoic Era and lasted from Mya to Mya (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically. Th ...

period Period may refer to: Common uses * Period (punctuation) * Era, a length or span of time *Menstruation, commonly referred to as a "period" Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (o ...

Description

The Big Rock Formation consists of a lower conglomerate member with minor micaceous

quartzite Quartzite is a hard, non- foliated metamorphic rock that was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tecton ...

and an upper quartzite member. The conglomerate is a medium to dark gray

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

pebble

metaconglomerate Metaconglomerate is a rock type which originated from conglomerate after undergoing metamorphism. Conglomerate is easily identifiable by the pebbles or larger clasts in a matrix of sand, silt, or clay. Metaconglomerate looks similar to conglome ...

with minor

beds, interpreted as braided

alluvial Alluvium (, ) is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluvium is also sometimes called alluvial deposit. Alluvium is ...

deposits. The pebbles are mostly of light gray quartz but with some ferruginous chert. Pebble sizes are from 1 to 20 cm and the pebbles are systematically elongated. Crossbedding is pervasive. Flattened

felsic In geology, felsic is a grammatical modifier, modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted ...

clast 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 b ...

s may be metamorphosed

pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of extremely vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicula ...

.Simmons ''et al.'' 2011, p.360 It is exposed in the central Tusas Mountains, where it is distinctive enough to be an important

stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...

marker bed Marker horizons (also referred to as chronohorizons, key beds or marker beds) are stratigraphic units of the same age and of such distinctive composition and appearance, that, despite their presence in separate geographic locations, there is no do ...

. Its thickness is typically .

yields a minimum age for the source regions of 1665 Mya, consistent with the formation being deposited in an extensional basin (the Pilar basin) opened during the transition from the

Yavapai orogeny The Yavapai orogeny was an orogenic (mountain-building) event in what is now the Southwestern United States that occurred between 1710 and 1680 million years ago ( Mya), in the Statherian Period of the Paleoproterozoic. Recorded in the rocks o ...

to the

Mazatzal orogeny The Mazatzal orogeny was an orogenic event in what is now the Southwestern United States from 1650 to 1600 Mya in the Statherian Period of the Paleoproterozoic. Preserved in the rocks of New Mexico and Arizona, it is interpreted as the coll ...

.Jones ''et al.'' 2011 Based on clast deformation, the formation has been compressed 55 percent during folding, while being extended 10 percent parallel to the strike and 65 percent parallel to the dip. This assumes the clasts were not imbricated in the original deposit.Barker 1958, p.70 The Big Rock Formation lies above the undivided lower Vadito Group and below the Burned Mountain Formation. It was long thought to correlate with the

Marquenas Formation The Marquenas Formation is a geological formation that crops out in the Picuris Mountains of northern New Mexico. Detrital zircon geochronology gives it a maximum age of 1435 million years, corresponding to the Calymmian period. Description The ...

, but Marquenas Quartzite has been found via detrital zircon geochronology be much younger. The formation has been interpreted as a beach gravelBarker 1958, p.33 or as a high-energy fluvial deposit associated with the Pilar basin of the

History of investigation

The unit was originally designated as the Big Rock conglomerate member of the Kiawa Mountain Formation by Fred Barker in 1958, during the mapping of the Las Tablas quadrangle.Barker 1958, p.24 It was designated as a formation within the

Vadito Group The Vadito Group is a group of geologic formations that crops out in most of the Precambrian-cored uplifts of northern New Mexico. Detrital zircon geochronology and radiometric dating give a consistent age of 1700 Mya for the group, correspondin ...

by Bauer and Williams in their sweeping revision of northern New Mexico Precambrian stratigraphy in 1989.Bauer and Williams 1989, p.49

Footnotes

References

* * * * * {{cite journal , last1=Simmons , first1=Mary C. , last2=Karlstrom , first2=Karl E. , last3=Williams , first3=Michael W. , last4=Larson , first4=Toti E. , title=Quartz-kyanite pods in the Tusas Mountains, northern New Mexico: A sheared and metamorphosed fossil hydrothermal system in the Vadito Group metarhyolite , journal=New Mexico Geological Society Field Conference Series , date=2011 , volume=62 , pages=359–378 , url=https://nmgs.nmt.edu/publications/guidebooks/downloads/62/62_p0359_p0378.pdf Precambrian formations of New Mexico Quartzite formations