Pre-Cambrian and Tertiary geology of the Las Tablas Quadrangle, New Mexico
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The Las Tablas quadrangle is in Rio Arriba County, northern New Mexico, and lies between north latitudes 36[degrees] 30' and 36[degrees] 45' and west longitudes 106[degrees...
| Summary: | NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
The Las Tablas quadrangle is in Rio Arriba County, northern New Mexico, and lies between north latitudes 36[degrees] 30' and 36[degrees] 45' and west longitudes 106[degrees] 00' and 106[degrees] 15'. Its center is about 33 miles west-northwest of Taos and about 65 miles north and slightly west of Santa Fe. Its principal geographic features include the Jawbone Mountain-La Jarita Mesa highland, which trends diagonally from northwest to southeast across the area; the valleys of the southeastward-flowing Tusas and Vallecitos creeks, which flank the highland area; a northeastern area that slopes gently eastward and is a part of the Taos Plateau; and a southwestern area that also slopes gently eastward and is a part of the highland that lies east of the Chama River Valley. The Jawbone Mountain-La Jarita Mesa highland is underlain chiefly by pre-Cambrian rocks, and the other areas mainly by Tertiary rocks.
The oldest rocks exposed in the quadrangle are pre-Cambrian metasedimentary and metavolcanic types that comprise the Ortega quartzite, of which a 14,000- to 20,000-foot section is exposed; the Moppin meta-volcanic series, which consists of metamorphosed basaltic rocks with minor intercalated metasedimentary rocks and is from 1,000 to several thousand feet thick; and the Kiawa Mountain formation, which is composed of five members. These members are the Big Rock conglomerate, which is 50 to 100 feet thick and overlies the Ortega quartzite; the Jawbone conglomerate, which overlies the Moppin series in the northwestern part of the area, is more than 1,000 feet in maximum thickness, and pinches out to the southeast; the lower quartzite member, which overlies the Big Rock conglomerate member, the amphibolite member, which consists of one to seven thin layers of amphibolite and intercalated beds of quartzite, and is from 35 to 2,000 feet thick; and the upper quartzite member, which is 5,000 to 10,000 feet thick and is the youngest pre-Cambrian rock in this area.
These strata were compressed during pre-Cambrian time into two large overturned folds that trend and plunge northwest. These are the Hopewell anticline and the Kiawa syncline. Two subsidiary and similarly oriented folds, the Poso anticline and the Big Rock syncline, lie on the southwest flank of the Kiawa syncline. Numerous internested minor folds, ranging from a fraction of an inch to several thousand feet in flank-to-flank dimension, occur on the larger folds.
Sills of pre-Cambrian metarhyolite were injected into the sedimentary rocks prior to the folding. Three plutons of granodiorite were emplaced during the folding, and four bodies of granite were intruded during a late stage of the folding or after the folding ceased. Many bodies of granitic pegmatite, at least 36 of which are of some commercial importance, lie in muscovitized quartzite and metarhyolite on La Jarita Mesa.
Regional metamorphism was essentially synchronous with the folding of the pre-Cambrian rocks. Basalt, the only widespread rock in the area that is sensitive to changes in metamorphic grade, was progressively metamorphosed to chlorite-muscovite-albite greenschist, chlorite-biotite-albite greenschist, chlorite-biotite-oligoclase greenschist, oligoclase-biotite-hornblende amphibolite, and finally to hornblende-andesine amphibolite. These rocks represent the typical greenschist and amphibolite metamorphic facies. The regional metamorphism has involved breakdown of unstable minerals, migration of atoms along grain boundaries, nucleation of new phases by statistical fluctuations of concentration, and grain growth by accretion of atoms to surfaces to nuclei.
Kyanite is present in all of the vitreous quartzite, and is associated with both metamorphic facies. It occurs along bedding planes, in hematite-rich laminae and in quartzose veins. Bodies of quartz-kyanite rock, which are oval in plan, occur in quartzite and metarhyolite at and near Big Rock on La Jarita Mesa.
The La Jarita pegmatites are surrounded by an aureole of pegmatitic-hydrothermal metamorphism that postdates the regional metamorphism. In this aureole, quartzofeldspathic rocks have been muscovitized, and amphibolite has been partly converted to chlorite and quartz. Locally the amphibolite has been wholly replaced by muscovite, biotite, garnet, epidote, and quartz. The net material changes in this process have been addition of K, A1, and H[subscript 2]O, and loss of Ca, Mg, Si, and a little Na. Fluids from pegmatitic magmas undergoing second-boiling are believed to have caused the metasomatism.
Terrestrial sedimentary and volcanic rocks of Tertiary age underlie much of the quadrangle. The general stratigraphic relations are summarized in the following table:
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Several thin beds with the cherty, feldspathic, quartzose sandstone lithology of the Sante Fe formation are present within the Cordito member of the Los Pinos formation.
Quaternary alluvium lies along the bottoms of the larger creeks, and some material of probable aeolian origin underlies parts of the upper Tusas Valley.
Fault zones extend along the valleys of the Tusas and Vallecitos creeks. The Tusas zone consists of northwest-trending main normal faults that are connected by subparallel cross faults; the general displacement is east-side-down on the main faults. The Vallecitos zone is defined by northwest- to west-trending main faults, only some of which are joined by cross faults; rocks west of the fault zone have been lowered relative to the rocks that lie east of the fault zone. Thus the Jawbone Mountain-La Jarita Mesa highland has been elevated relative to the Tertiary rocks to the southwest, and depressed relative to the tertiary rocks that are part of the Taos Plateau on the northeast. |
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