Bulletin 45—Precambrian and Tertiary geology of Las Tablas quadrangle, New Mexico

By F. B. Barker, 1958, 104 pp., 14 tables, 3 figs., 13 plates, 1 index.

Las Tablas quadrangle is in Rio Arriba County, northern NM. Its center is about 33 mi west-northwest of Taos and about 65 mi 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 Rio Tusas and Rio Vallecitos, 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 Precambrian rocks, and the other areas mainly by Tertiary rocks.

The oldest exposed rocks in the quadrangle are Precambrian metasedimentary and metavolcanic rocks that comprise the Ortega quartzite, of which a 14,000–20,000 ft section is exposed; the Moppin metavolcanic 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 from 50 to 100 ft thick and overlies the Ortega quartzite; the Jawbone conglomerate, which overlies the Moppin series in the northwestern part of the area, and is more than 1,000 ft in maximum thickness, and pinches out to the southeast; the lower quartzite member, which overlies the Big Rock conglomerate member, and is several hundred feet thick; the amphibolite member, which consists of one to seven thin layers of amphibolite and intercalated quartzite, and is from 35 to 2,000 ft thick, and the upper quartzite member, which is from 4,000 to 8,000 ft thick and is the youngest Precambrian rock in this area.

These strata were compressed during Precambrian 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 intersected 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 Precambrian metarhyolite were injected into the sedimentary rocks prior to 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 Precambrian 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; to chlorite-biotite-albite greenschist; to chlorite-biotite-oligoclase greenschist; to oligoclase-biotite-hornblende amphibolite; and finally to hornblende-andesine amphibolite. These rocks represent the greenschist and amphibolite 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 of nuclei.

Kyanite is present in all the vitreous quartzite, and is associated with both metamorphic facies. It occurs along bedding planes, in hema - tite-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 the quartzose and feldspathic rocks have been muscovitized, and amphibolite has been partly converted to chlorite and quartz. Locally the amphibolite has been wholly replaced by mus covite, biotite, garnet, epidote, and quartz. The net material changes in this process have been addition of K, Al, and H2O, 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: Several thin beds with the cherty, feldspathic, quartzose sandstone lithology of the Santa 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 eolian origin underlies parts of the upper Tusas Valley.

Fault zones extend along the Rio Tusas and Rio Vallecitos valleys. The Tusas zone consists of northwest trending main normal faults connected by crossfaults that are subnormal to the main 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 crossfaults; 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.