Changes· ~n· the Nomenclature 

and Sttatigraphy of 

Protertizoic Metamorphic Rocks, 

Tusas Mountains, N orth-Ct?ntral 

New M€xico J 

U.S. GEOLOGICAL SURVEY BULLETIN 1571 

Changes in the Nomenclature 

and Stratigraphy of 

Proterozoic Metamorphic Rocks, 

Tusas Mountains, North-Central 

New Mexico 

By REINHARD A. WOBUS 

U.S. GEOLOGICAL SURVEY BULLETIN 1571 

VNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1985 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of C~ngress Cataloging in Publication Data 

Wobus, Reinhard A. 

Changes in the nomenclature and stratigraphy of Proterozoic metamorphic rocks, Tusas Mountains 

north-central New Mexico. 

iii 19 p. 

(U.S. Geological Survey Bulletin ; 1571) 

Bibliography: p.18--19 

Supt. of Docs. No.: I 19.3:1571 

1. Geology, Stratigraphic-Precambrian. 2. Geology, Stratigraphic-Nomenclature-New Mexico--

Tusas Ridge. 3. Rocks, Metamorphic-Nomenclature. 4. Geology-New Mexico--Tusas Ridge. I. 

Title. II. Series. 

QE75.B9 no. 1571 557.3 s [551.7'15] 84-600387 

[QE653] 

For sale by the Branch of Distribution 

U.S. Geological Survey 

604 South Pickett Street 

Alexandria, VA 22304 
Abstract .. 

Introduction and previous work 

Present study .. 

Recommendations 

CONTENTS 

Conclusions and possible correlations .. 

References cited 

ILLUSTRATIONS 

Page 

2 

5 

9 

15 

18 

Page 

FIGURE 1. Index map of northern New Mexico . . . 3 

2. Generalized geologic map of the Tusas Mountains 6 

3. Cross sections showing different interpretations of Precambrian structure, 

southwestern Tusas Mountains 10 

4. Small-scale geologic map showing trends of major fold axes in Precambrian 

rocks, eastern Aztec 1° x 2° quadrangle 12 

TABLE 

TABLE 1. Comparison of stratigraphic interpretations of layered Precambrian 

metamorphic rocks, Tusas Mountains. 

Page 

16 

III 

CHANGES IN THE NOMENCLATURE 

AND STRATIGRAPHY OF PROTEROZOIC 

METAMORPHIC ROCKS, TUSAS MOUNTAINS, 

NORTH-CENTRAL NEW MEXICO 

By REINHARD A. WoBusl 

ABSTRACT 

Stratigraphic interpretations differ for the Proterozoic metasedimentary and 

metavolcanic rocks of the Tusas Mountains, north-central New Mexico. Quartzite 

and interlayered conglomerate were originally named the Ortega Quartzite by Just 

( 1937) and were considered to be younger than the Hopewell Series and older than 

the Vallecitos Rhyolites. Barker ( 1958) recognized two separate quartzite units, 

with the metavolcanic rocks (which he named the Burned Mountain Metarhyolite 

as a replacement term for the Vallecitos, and the Moppin Metavolcanic Series as a 

replacement term for the Hopewell) in between. Barker believed that the Ortega 

Quartzite was the oldest Precambrian rock, and he defined the Kiawa Mountain 

Formation and its Big Rock Conglomerate and Jawbone Conglomerate Members to 

include what he perceived as the younger quartzite and associated clastic metasedimentary 

rocks. as well as minor amphibolite interlayers. Gresens and Stensrud ( 197 4) found 

that altered felsic metavolcanic rocks formed most of the lower part of Barker's 

Kiawa Mountain Formation; they suggested a return to the single-quartzite model 

and stratigraphic sequence of Just. 

The present study of the Tusas Mountains, part of the mapping of the Aztec 1° x 

2° quadrangle by the U.S. Geological Survey, has provided structural evidence in 

support of the stratigraphic interpretation favored by Gresens and Stensrud and by 

Just. In light of this evidence, it is proposed that the name Kiawa Mountain 

Forn:tation be abandoned and its various units re-alloted. The vitreous ("upper") 

quartzite and Jawbone Conglomerate Member of the Kiawa Mountain are, in fact, 

the same as the Ortega Quartzite and constitute the youngest layered Precambrian 

rock unit of the region. Other "quartzites" within the Kiawa Mountain are largely 

altered felsic metavolcanic rocks; along with the Big Rock Conglomerate Member 

(former alluvial gravels within the metavolcanic sequence), they should be included 

as part of the Burned Mountain Metarhyolite, Barker's term adopted in this report. 

Amphibolites of the Kiawa Mountain Formation are more properly included with 

the Moppin Metavolcanics (name change from Moppin Metavolcanic Series of 

Barker, 1958), which consist of metamorphosed mafic volcanic and volcaniclastic 

rocks. The term Petaca Schist, defined by Just and used by Barker for muscovitic 

metarhyolite and quartzite on La J arita Mesa, has no stratigraphic significance and 

is abandoned. 

!Current address: Department of Geology, Williams College, Williamstown, MA 01267. 
2 PROTEROZOIC METAMORPHIC ROCKS, NEW MEXICO 

These stratigraphic revisions simplify the Precambrian terminology for the 

Tusas Mountains and also assign as the oldest rocks of the region a bimodal 

metavolcanic assemblage, rather than a thick sequence of clean clastic metasedimentary 

rocks without an obvious source area. These revisions are also compatible with 

other recent studies in the Picuris Range and Rio Mora areas to the southeast in 

New Mexico, and in the Needle Mountains to the northwest in Colorado. In all of 

these areas, metavolcanic rocks have been found underlying quartzite and conglomerate (Ortega Quartzite in northern New Mexico, Vallecito Conglomerate in the 

Needle Mountains of southwestern Colorado), thus reversing previous stratigraphic 

interpretations. 

INTRODUCTION AND PREVIOUS WORK 

Layered Precambrian rocks of late Early Proterozoic age in the Tusas 

Mountains, north-central New Mexico (fig. 1 ), consist of quartzite, 

conglomerate, and bimodal volcanic rocks, all of which have been 

metamorphosed to upper greenschist to middle amphibolite facies. In 

the first reconnaissance study of the area, Just ( 193 7) applied the name 

Ortega Quartzite to all the quartzite and conglomerate exposures from 

Ojo Caliente on the south to Jawbone Mountain, north of Hopewell 

Lake in the Burned Mountain 7%-minute quadrangle. He derived this 

name from the Ortega Mountains, which are immediately southwest of 

the Tusas Mountains and are composed almost entirely of quartzite 

with minor interlayers of pebble conglomerate. 

In the same report, Just named the felsic metavolcanic rocks the 

Vallecitos Rhyolites, and the mafic metavolcanic rocks and associated 

metasedimentary layers the Hopewell Series. He believed the felsic 

metavolcanics were rhyolite and trachyte flows that were interlayered 

with the Hopewell Series and the Ortega Quartzite, and that the Hopewell Series was older than the Ortega. Jahns (1946) restated Just's 

conclusions in his report on the Petaca pegmatite district, which lies 

along the east side of La J arita Mesa south of Las Tablas. 

Barker ( 1958) mapped a large portion of the Tusas Mountains in his 

study of the Las Tablas 15-minute quadrangle (now subdivided into 

the Burned Mountain, Mule Canyon, Ca:iion Plaza, and Las Tablas 

7Yz-minute quadrangles), using methods he stated would now be called 

"semireconnaissance" (Barker and Friedman, 1974, p. 115). He reversed 

Just's stratigraphy and recognized the Ortega Quartzite as the oldest 

unit in the Tusas Mountains, suggesting it was 14,000-20,000 ft ( 4,200-

6,000 m) thick. Barker also believed that the thick section of quartzite 

extending northwesterly from Kiowa Mountain2 was younger than the 

Ortega and was the youngest metasedimentary rock of the region. He 

2The U.S. Forest Service used the spelling "Kiawa" Mountain until the 1950's (at the time Barker did 

his mapping), but the USGS changed the spelling to "Kiowa" on the Las Tablas 7%-minute quadrangle 

published in 1963. 
INTRODUCTION AND PREVIOUS WORK 

BRAZOS PEAK 15' 

QUADRANGLE 

141 ~ Q 

Cu San Pedro ~ 

36o 0 Mountains ~ u ~ ~ 

~- C:J 

~ \ \ 0 Los Alamos 

.. -~VALLES Nac1m1e_nto CALDERA 

Mountams 

SANTA FE 0 

, 

Albuquerque 0 

0 

q;.' 

Sandia 

Mountains 

r----.....--r-r-A_re_,a of figure 1 

NEW MEXICO 

1W 

Las Vegas 0 

35DL_ ____________ L_l_ __________ ~--------------------------~ 

30 MILES 

50 KILOMETERS 

FIGURE 1.-Index map of northern New Mexico showing area of this study. 

Stippled pattern indicates distribution of Precambrian rocks. 

3 

described this supposedly younger quartzite as " ... dense, vitreous, 

light bluish-gray quartzite that contains irregularly distributed pebbly 

layers ... ," noting that it was" ... similar to the Ortega quartzite exposed 

west of the Rio Vallecitos ... "and that" ... indeed, these two quartzites 

are essentially indistinguishable in outcrop" (Barker, 1958, p. 31 ). 

Barker ( 1958) named the strata "overlying" the Ortega Quartzite the 

Kiawa Mountain Formation, with the vitreous quartzite defined as the 

uppermost member that was 5,000-10,000 ft ( 1,500-3,000 m) thick. The 
4 PROTEROZOIC METAMORPHIC ROCKS, NEW MEXICO 

basal member was named the Big Rock Conglomerate Member, a thin, 

discontinuous layer of quartz-pebble conglomerate only 50-200 ft 

( 15-60 m) thick that crops out on La J arita Mesa. A similar but thicker 

conglomerate, with abundant quartzite, underlying Jawbone Mountain 

and supposedly cropping out in Placer Gulch south of Hopewell Lake 

was named the Jawbone Conglomerate Member. In his definition of the 

Jawbone, Barker ( 1958, p. 25) noted that it " ... resembles the quartzpebble beds in the Ortega and Kiawa Mountain quartzites". 

A poorly delineated zone of "vitreous to micaceous quartzite" lying 

south of Kiowa Mountain was informally named the lower quartzite 

member of the Kiawa Mountain Formation, and sporadic thin interlayers 

of amphibolite below the upper vitreous quartzite were collectively 

named the amphibolite member (Barker, 1958 ). The name Kiawa Mountain 

Formation and the names of two of its members, the Big Rock Conglomerate 

and the Jawbone Conglomerate, were adopted by the U.S. Geological 

Survey in a subsequent report (Barker, 1970). 

In his 1958 report, Barker also renamed the Vallecitos Rhyolite of 

Just (1937) the Burned Mountain Metarhyolite after exposures on the 

northwest side of Burned Mountain in sec. 8, T. 28 N., R. 7 E., and he 

renamed the Hopewell Series of Just the Moppin Metavolcanic Series 

after exposures in upper Spring Creek just north of the Moppin Ranch 

(sec. 25, T. 28 N., R. 7 E.). The Moppin, consisting of amphibolite, 

chlorite schist, phyllite, and minor conglomerates and iron-formation, 

was thought to represent metamorphosed mafic volcanic rocks (Barker, 

1958, p. 23 ). Barker observed that the metarhyolite occurs as sills and 

dikes that cut the Moppin, but that some of the metarhyolite also 

represents flows and ash flows. He placed the Moppin stratigraphically 

between his "older" or Ortega Quartzite and the younger Kiawa Mountain 

Formation. Barker continued to apply the name Petaca Schist (Just, 

1937) to highly muscovitized portions of what he considered to be 

Ortega Quartzite, Burned Mountain Metarhyolite, and the lower quartzite 

member of the Kiawa Mountain Formation. 

Gresens and Stensrud ( 197 4) discovered that Barker's lower quartzite 

member actually consists largely of altered metarhyolite. Some of this 

rock retains remnants of quartz and feldspar phenocrysts and some, 

though chemically identical to the porphyritic variety, is fine grained, 

massive, and may represent metamorphosed silicic tuff. The same 

authors reinforced Barker's suggestion that altered ( muscovitic) metarhyolite 

forms a large part of the Petaca Schist in the central part of La Jarita 

Mesa. They also proposed that it would be more consistent stratigraphically 

to consider the Ortega Quartzite and the vitreous (upper) quartzite of 

the Kiawa Mountain Formation as a single unit, as in Just's usage. 

Moreover, they speculated, as did Just, that this single quartzite and 

conglomerate sequence is younger than the metavolcanic sequence, 
PRESENT STUDY 5 

both in the Thsas Mountains and in the Picuris Range to the southeast 

across the Rio Grande Depression. 

The metavolcanic and metasedimentary rocks of the Thsas Mountains 

were intruded by the Maquinita Granodiorite (a term of Barker, 1958, 

adopted in this report) and the Tres Piedras Granite about 1,700 m.y. 

ago and by the Thsas Mountain Granite about 1,500 m.y. ago. The 

Proterozoic intrusive history of the region has been dealt with in other 

reports and will not be discussed further here (Wobus and Dobson, 

1981; Wobus and Hedge, 1982). 

PRESENT STUDY 

The Precambrian of the Thsas Mountains was re-mapped by the 

author during the summers of 1977 to 1980 as part of the U.S. Geological 

Survey's mapping of the Aztec 1° x 2° quadrangle (fig. 2). This study 

supports the stratigraphic conclusions and speculations of Gresens and 

Stensrud (1974), although it differs with some of their observations. 

Specifically, it provides structural evidence that ( 1) the Ortega Quartzite 

and the vitreous (upper) quartzite of the Kiawa Mountain Formation 

are the same unit, and that ( 2) this quartzite, including its interlayered 

pebble conglomerate but excluding the Big Rock Conglomerate Member, 

overlies the bimodal metavolcanic rocks and their associated 

metasedimentary layers. 

The most critical areas in this structural reinterpretation are the 

Precambrian exposures south and east of the village of Canon Plaza. 

South of the village, Barker ( 1958, pl. 1) shows Ortega Quartzite 

dipping to the northeast away from a ridge of Burned Mountain Metarhyolite 

in sec. 36, T. 27 N., R. 7 E. Barker did not discuss this locality, where his 

mapping actually suggests that Ortega Quartzite is younger than 

metarhyolite. Re-mapping of this area (location 1 in fig. 2) shows that 

the metarhyolite extends northwesterly about one-half mile (0.8 km) 

farther than Barker showed and that vitreous quartzite with wellpreserved crossbedding overlies the metarhyolite along this entire distance. 

Interlayers about 20ft ( 6 m) thick of metarhyolite and quartzite occur 

at the base of the quartzite section. Textures in the metarhyolite suggest 

that it might be a metamorphosed airfall tuff at this locality, because it 

lacks the eutaxitic fabric and abundant euhedral quartz phenocrysts 

observed elsewhere. 

East of Canon Plaza, on the western edge of La Jarita Mesa (location 

2 in fig. 2), vitreous quartzite does indeed occur (contrary to the 

observations of Gresens and Stensrud, 1974, p. 117), although it becomes 

more highly muscovitic to the southwest. Cross beds are well preserved 

in both the altered ( muscovitic) and unaltered quartzites. Along with 

thin conglomeratic marker beds (part of Big Rock Conglomerate Member) 
6 PROTEROZOIC METAMORPHIC ROCKS, NEW MEXICO 

106°15' 

36° 

45' 

36° 

37' 

30" 

OTu 

106°07'30" 

5 KILOMETERS 

106'1 5' 

36'45' 

BURNED 

5 MILES MOUNTAIN 

MF-1409 

CANON 

PLAZA 

36'30' 

106° 

106' 

MULE 

CANYON 

MF-1407 

LAS 

TABLAS 

MF-1408 

LAS TABLAS 15' QUADRANGLE 

FIGURE 2.-Generalized geologic map of the Tusas Mountains, emphasizing Precambrian units and structure. After Barker (1958), Manley and Wobus (1982a), 

and Wobus and Manley ( 1982 ). 
PRESENT STUDY 7 

EXPLANATION 

~ QUATERNARY AND TERTIARY 

UNITS, UNDIVIDED 

MIDDLE PROTEROZOIC 

Tusas Mountain Granite 

EARLY PROTEROZOIC 

Tres Piedras Granite 

Maquinita Granodiorite of Barker, 1958 

Quartzite and pebble conglomerate 

Metavolcanic rocks and associated 

metasedimentary rocks (includes 

Moppin Metavolcanics and Burned 

Mountain Metarhyolite). Xbc , Big 

Rock Conglomerate Member of 

Burned Mountain 

CONTACT 

FAULT -Bar and ball on downthrown 

side where relative displacement 

known 

4 ANTICLINE- Showing trace of crestal 

plane and direction of plunge. 

Dotted where concealed 

4 • t '-._/ OVERTURNED ANTICLINE-Showing 

direction of dip of limbs and direction 

of plunge 

4 SYNCLINE-Showing trace of trough 

plane and direction of plunge. Dashed where approximately located; 

u dotted where concealed 

4 OVERTURNED SYNCLINE-Showing 

direction of dip of limbs and direction 

of plunge 

MAJOR FOLDS IN PRECAMBRIAN ROCKS 

Jawbone syncline 

II Hopewell anticline 1 

III Kiowa syncline 1 

IV Poso anticline 1 

v Big Rock syncline 

VI La Jarita anticline 

VII Canon Plaza syncline 

VIII Vallecitos anticline 

1

Named by Gresens and Stensrud (19741 

•' LOCATION REFERRED TO IN TEXT 

A A, LINE OF CROSS SECTION SHOWN IN 

FIGURE 3 
8 PROTEROZOIC METAMORPHIC ROCKS, NEW MEXICO 

in altered metarhyolite, the cross beds help to define a northwest-plunging 

faulted syncline and an adjacent overturned anticline that were not 

recognized by any previous workers. (For map details, see Manley and 

Wobus, 1982a). Another pair of previously unmapped faulted folds on 

the same scale is indicated by bedding in quartzite and layering in 

metarhyolite immediately southwest of La J arita Mesa, along the 

valley southeast of Canon Plaza. 

Figure 3 shows in restored cross section the differences in structural 

and stratigraphic interpretation between the present mapping and 

Barker's mapping ( 1958 ). Figure 3A (from pl. 1, Barker, 1958) indicates 

that for a horizontal distance of more than 4 mi (6.5 km), steeply 

dipping Ortega Quartzite, with two thick interlayers of Burned Mountain 

Metarhyolite, is part of a large syncline, with its axis on La J arita Mesa 

and with the Big Rock Conglomerate Member in its trough. The Ortega 

would have to be more than 20,000 ft ( 6,000 m) thick by this interpretation 

and would indeed be the oldest unit of the Proterozoic layered sequence. 

The present interpretation (fig. 3B) shows five folds over the same 

4-mi ( 6.5-km) distance, with Ortega Quartzite dipping to the northeast 

and southwest off a metavolcanics-cored anticline just east of the Rio 

Vallecitos. The Ortega is obviously younger than the metavolcanics in 

this interpretation, and structural restoration across La Jarita Mesa 

indicates that there is only a single quartzite unit. 

The thickness of the Ortega Quartzite is still a problem, however. The 

top of the quartzite is apparently not exposed east of the Rio Vallecitos, 

and intense deformation of former crossbedding in the quartzite west of 

the Rio Vallecitos (including the Ortega Mountains) has rendered these 

primary structures useless for stratigraphic and structural purposes 

(as recognized by Bingler, 1965). There are probably several folds 

within the quartzite of the Ortega Mountains on the same scale as those 

to the northeast in the Tusas Mountains, but transposition of bedding 

due to shearing and lack of continuous marker horizons have precluded 

their delineation and, hence, the determination of the thickness of 

the Ortega. 

In summary, the most significant features of this re-study of the 

Tusas Mountains, as shown in figure 3, include: 

1. Vitreous quartzite of the upper part of Barker's Kiawa Mountain 

Formation, as exposed in the syncline at Kiowa Mountain (northeast end of cross sections in fig. 3 ), is repeated in a series of folds 

to the southwest across La J arita Mesa, connecting with quartzite previously mapped as Ortega Quartzite. The "two" quartzites 

are thus the same unit. 

2. An assemblage of metarhyolite, minor amphibolite (metabasalt), and 

associated volcaniclastic metasedimentary rocks everywhere 

underlies the quartzite. Much of the felsic metavolcanic rock of 
RECOMMENDATIONS 9 

this assemblage was incorrectly mapped by Barker (1958) as 

Ortega Quartzite and lower quartzite member of the Kiawa 

Mountain Formation, as pointed out by Gresens and Stensrud 

( 197 4) and confirmed in the present study. 

3. The Big Rock Conglomerate Member of Barker ( 1958, 1970) consists 

of conglomeratic layers and lenses that are wholly within the 

metavolcanic-metasedimentary section, not at the base of the 

Kiawa Mountain Formation as defined by Barker ( 1970). These 

conglomerate beds are locally highly deformed but contain recognizable cobbles and pebbles of metarhyolite; they were probably deposited in a braided alluvial environment ( K. A. Eriksson, 

pers. commun., 1982). 

In addition to the changes discussed above, there are differences 

between this report and Barker ( 1958, 1970) concerning mapping and 

interpretation of the Jawbone Conglomerate Member of the Kiawa 

Mountain Formation. As mentioned previously, Barker defined this 

unit for quartz-pebble conglomerate beds in vitreous quartzite at Jawbone 

Mountain. Re-mapping of the type locality (Wobus and Manley, 1982) 

shows that most of the rock there is quartzite showing crossbedding 

attitudes that define a northwest-plunging syncline. This structure is 

another one of a series of folds that repeat quartzite, with minor 

conglomerate interbeds, from the Ortega and La Madera Mountains in 

the southeast to the Brazos Box and Canones Box (Brazos Peak 

15-minute quadrangle) in the northwest (fig. 4). The Jawbone Conglomerate Member is thus part of the Ortega Quartzite. The only other 

"outcrop" of Jawbone outside the type locality supposedly extends 

across Placer Gulch about 1.5 mi (2.4 km) southwest of Hopewell Lake 

(Barker, 1958). This area was found to be composed of unconsolidated 

rounded cobbles of quartzite and other Precambrian rocks that are part 

of the Eocene and Oligocene El Rito Formation of Smith ( 1938) (Wobus 

and Manley, 1982). 

RECOMMENDATIONS 

In light of the substantial changes indicated by this most recent 

mapping of the Tusas Mountains and by the previous interpretations 

and conclusions of Gresens and Stensrud ( 197 4 ), the following proposals 

are made, with a brief rationale for each: 

1. The name Kiawa Mountain Formation is abandoned. The vitreous 

quartzite at Kiowa Mountain has been shown to be the same as 

the Ortega Quartzite to the southwest, and "Ortega," defined by 

Just in 1937, has precedence. The various members of the Kiawa 

Mountain Formation, as defined by Barker ( 1958, 1970), will be 

discussed individually below. 
A 

FEET 

9000 ---, on 

BODO 

7000 

6000 

5000 

A 

Valley of 

Rio Vallecitos Canon Plaza 

- I 

~ 

G 

QUATERNARY 

Alluvium 

TERTIARY 

Ritito conglomerate 

PRECAMBRIAN 

Tres Piedras granite 

LA JAR IT A MESA 

0 1 2 MILES 

0 1 2 KILOMETERS 

EXPLANATION 

Petaca schist-Muscovitized quartzite 

and metavolcanics 

Kiawa Mountain formation 

Upper quartzite member 

Amphibolite member 

Lower quartzite member 

Big Rock conglomerate member 

Burned Mountain metarhyolite Ortega quartzite 

I-' 

0 

A' 

METERS 

3000 '"d 

~ 

0 

2500 >-3 

trl 

~ 

0 2000 N 

0 

Q 

~ 

trl 

>-3 

>-

~ 

0 

~ 

'"d 

::c: 

Q 

~ 

0 

0 

~ 

sn 

z 

trl 

~ 

~ 

trl 

>< 

..... 

0 

0 
A 

FEET 

9000 

8000 

7000 

6000 

5000 

B 

CANON PLAZA 

SYNCLINE 

VALLECITOS 

ANTICLINE 

~ 

~ 

QUATERNARY 

Alluvium 

Gravel 

TERTIARY 

LA JARITA 

ANTICLINE 

BIG ROCK 

SYNCLINE 

0 1 2 KILOMETERS 

EXPLANATION 

2 MILES 

POSO 

ANTICLINE 

KIOWA 

SYNCLINE 

PRECAMBRIAN 

Tres Piedras Granite 

Quartzite and pebble conglomerate 

~ El Rito Formation of Smith (1938) 

Metavolcanic rocks and associated 

metasedimentary rocks. Xbc, interlayers of Big Rock Conglomerate 

Member of Burned Mountain Metarhyolite 

A' 

METERS 

3000 

2500 

2000 

FIGURE 3.-Cross sections along A-A' of figure 2 showing differences in interpretation of Precambrian structure and 

stratigraphy in southwestern part of Tusas Mountains. Opposed arrows show relative displacement of faults. A, 

From Barker ( 1958, pl. 1 ). Solid lines represent gradational contacts, dashed where position uncertain. B, From 

Manley and Wobus ( 1982a ). Dashed lines represent trend of sedimentary bedding or volcanic layering. 

:::0 

trl 

(") 

0 

~ 

~ 

trl 

z 

tj 

> >-3 

0 

z 

UJ 

1--' 

1--' 
12 PROTEROZOIC METAMORPHIC ROCKS, NEW MEXICO 

37° COLORADO -- ----------~---NEW MEXICO-------

Canones --.;;:U/ <rx.h',>-,.. 

Box 

0 Tierra Amarilla 

&f9 

®~@ 

0 0 

() 

10 MILES 

10 KILOMETERS 

tJ 

San Antonio 

Mountain.c,. 

36°15' c__ _____________ _j 

FIGURE 4.-Small-scale geologic map showing trends of major fold axes in 

Precambrian rocks of eastern part of Aztec 1° x 2° quadrangle, New Mexico. 

(Modified from Manley and others, 1978 ). Only the most continuous faults 

that cut Precambrian rocks are shown. 
RECOMMENDATIONS 

EXPLANATION D PHANEROZOIC ROCKS. UNDIVIDED 

u 

u 

PROTEROZOIC 

Intrusive rocks 

Quartzite and conglomerate 

Metavolcanic and associated 

supracrustal metasedimentary rocks 

Higher-grade metasedimentary rocks 

CONTACT 

FAULT 

ANTICLINE-Showing trace of crestal 

plane and direction of plunge. 

Dotted where concealed 

OVERTURNED ANTICLINE-Showing 

direction of dip of limbs and direction 

of plunge 

SYNCLINE-Showing trace of trough 

plane and direction of plunge. Dotted where concealed 

OVERTURNED SYNCLINE-Showing 

direction of dip of limbs and direction 

of plunge 

MAJOR FOLDS IN PRECAMBRIAN ROCKS 

Canones syncline 

II Brazos anticline 

III Jawbone syncline 

IV Hopewell anticline 1 

V Kiowa syncline 1 

VI Poso anticline 1 

VII Canon Plaza syncline 

VIII Vallecitos anticline 

1

Named by Gresens and Stensrud (1974) 

13 
14 PROTEROZOIC METAMORPHIC ROCKS, NEW MEXICO 

2. The name Ortega Quartzite should be applied to all the quartzite and 

quartz-pebble conglomerate horizons that are repeated by folding 

through the region shown in figure 4. Just (1937, p. 43) did not 

define a specific type locality for the Ortega Quartzite, naming it 

for exposures throughout the Ortega Mountains. The type locality 

selected by Barker ( 1970, p. A21) in the Las Tablas 15-minute 

quadrangle (sec. 25, T. 27. N., R. 8 E.) is inappropriate, as his 

map shows that entire section to be underlain by Petaca Schist. 

A principal reference locality in the Ortega Mountains is selected 

for the excellent exposures in a canyon of the Rio Vallecitos in 

the east-central part of sec. 34, T. 26 N., R. 8 E. of the La Madera 

7%-minute quadrangle, about 3.5 mi (2.1 km) south-southeast 

of the village of Vallecitos. 

3. The name Jawbone Conglomerate Member of the Kiawa Mountain 

Formation (Barker, 1970) is abandoned. The quartz-pebble 

conglomerate at Jawbone Mountain is part of a larger section of 

Ortega Quartzite that contains numerous interbeds of conglomerate 

throughout its extent. Just ( 1937, p. 43) recognized that 

" ... conglomeratic phases of Ortega occur on the east end of 

Jawbone Mountain". There never was a need for a separate term 

for the conglomerate-quartzite sequence at Jawbone Mountain; 

bedding symbols (based on crossbed orientations) from Barker's 

map ( 1958, pl. 1) can be used to show that this sequence of clean 

clastics occupies the same stratigraphic position relative to the 

underlying Moppin and Burned Mountain metavolcanic units as 

does the quartzite-conglomerate section at Kiowa Mountain. 

4. The informal lower quartzite and amphibolite members of the Kiawa 

Mountain Formation should be included with the appropriate 

previously recognized metavolcanic units of the Tusas Mountains. 

It has been shown (Gresens and Stensrud, 1974) and confirmed 

in this study that the lower quartzite consists largely of altered 

metarhyolite (flows and tuffs) with intercalated volcaniclastic 

sedimentary rocks, such as feldspathic sandstones. It would 

seem sensible to include these lithologies as part of the Burned 

Mountain Metarhyolite, a term Barker ( 1958) applied to all the 

silicic metavolcanic rocks he recognized in the Las Tablas 15-

minute quadrangle. The amphibolite of the Kiawa Mountain 

Formation should be considered part of the Moppin Meta volcanics, 

applied by Barker to the mafic metavolcanic rocks ( amphibolites) 

and associated metasedimentary layers (chlorite schist, phyllite, 

sericite schist) of the Tusas Mountains. 

5. The Moppin Metavolcanic Series of Barker ( 1958) is adopted as the 

Moppin Metavolcanics. The term "series" is not used as a rank 
CONCLUSIONS AND POSSIBLE CORRELATIONS 15 

term for a lithostratigraphic unit (North American Commission 

on Stratigraphic Nomenclature, 1983, Article 73 ). 

6. The name Big Rock Conglomerate Member is retained, though not as 

a member of the Kiawa Mountain Formation. This distinctive 

conglomerate on La J arita Mesa forms important marker beds 

(former stream channel deposits) in an otherwise monotonous 

terrane of muscovitized felsic metavolcanic rocks. The Big Rock 

would most appropriately be considered here as a member of the 

Burned Mountain Metarhyolite in its expanded usage, as 

recommended in the preceding section. Barker's description ( 1958, 

1970) of the Big Rock Conglomerate Member is adequate, but 

his type locality is incorrectly located; no conglomerate occurs in 

the SE1/4, sec. 27, T. 27 N., R. 8 E. The unit is well exposed, 

however, in the NW1/4 of that section, and a principal reference 

section is defined as the northwest-trending ridge in the NW1/ 4, 

sec. 27, T. 27 N., R. 8 E., southeast of elevation 8,715 ft. This 

location is about 1.2 mi ( 2 km) southwest of the prominence 

known as Big Rock. 

7. The name Petaca Schist is abandoned. This term was defined by Just 

( 1937) as a quartz-muscovite schist phase of the Ortega Quartzite, 

which was developed only in pegmatite-rich areas of La Jarita 

Mesa, presumably due to alteration by volatile-rich granitic 

intrusions. According to Just (1937, p. 43), it was not to be 

regarded as a separate member of the Ortega for purposes of 

correlation. Barker ( 1958, p. 34) continued to use the term for 

highly altered (muscovitic) parts of" ... the Ortega quartzite, 

part of the lower Kiawa Mountain formation, and many layers of 

Burned Mountain Metarhyolite". Most areas mapped by Barker 

as Petaca Schist retain original volcanic textures, especially 

quartz phenocrysts (many doubly terminated), and are better 

mapped as felsic metavolcanic rocks (Burned Mountain 

Metarhyolite); a stippled overprint has been used in mapping to 

indicate areas of most intense alteration (Manley and Wobus, 

1982a). As the Petaca Schist has never held any particular 

stratigraphic significance, its continued use seems superfluous. 

CONCLUSIONS AND POSSIBLE CORRELATIONS 

The numerous changes in Proterozoic nomenclature and stratigraphy 

proposed here are summarized in table 1. In addition to the specific 

needs met by this re-ordering, as explained above, the revised stratigraphy 

also assigns as the oldest Precambrian units in the Tusas Mountains a 

bimodal metavolcanic assemblage, rather than a thick section of clean 

clastic metasedimentary rocks without an obvious source area. The 
TABLE 1.-Comparison of stratigraphic interpretations of layered Proterozoic metamorphic rocks, Tusas Mountains, New !vfexico 

Just (1937) 

Vallecitos Rhyolites 

Ortega Quartzite 

(includes Petaca 

Schist phase) 

Hopewell Series 

Barker (1958) 

Burned Mountain Metarhyolite 

Petaca Schist (includes 

muscovitized parts of 

Ortega Quartzite and 

Kiawa Mountain Formation) 

Kiawa Mountain Formation2 : 

Upper quartzite 

Jawbone Conglomerate Member2 

Amphibolite 

Lower quartzite 

Big Rock Conglomerate Member2 

Moppin Metavolcanic Series 

Ortega Quartzite 

~ 

~ 

~ 

~ 

~ 

~ 

~ 

~ 

~ 

~ 

c 

~ 

Wobus (this report)l 

Ortega Quartzite 

(includes upper quartzite 

member formerly assigned 

to Kiawa Hountain Formation) 

Burned Mountain Metarhyolite 

(includes lower quartzite 

member and Big Rock Conglomerate 

Member formerly assigned to 

Kiawa Mountain Formation) 

~ Moppin Metavolcanics 

~ (includes amphibolite 

~ formerly assigned to 

j Kiawa Mountain Formation) 

1Kiawa Mountain, Jawbone, and Petaca are abandoned in this report. 

2nenotes names formally adopted by the U.S. Geological Survey in Barker (1970). 

~ 

~ 

"0 

::0 

0 

--3 

tr:l 

::0 

0 

N 

0 

(1 """" 

~ 

tr:l 

~ ~ 

0 

::0 

"0 

::c: 

c=s 

::0 

0 

(') 

~ 

r.J) 

z 

tr:l 

~ 

~ 

tr:l 

~ 

(') """" 

0 
CONCLUSIONS AND POSSIBLE CORRELATIONS 17 

radiometric age of the Burned Mountain Metarhyolite is 1,725-1,775 

m.y., as reported by L. T. Silver in Barker and Friedman (1974), with 

corrections for revised U-Pb decay constants; rocks of significantly 

greater age than this are not known anywhere in southern Colorado or 

in New Mexico. If the Ortega Quartzite were the oldest Precambrian 

unit of the region, the source of its clean quartzose sediment would 

indeed be problematic. Instead, pebbles and cobbles of milky quartz, 

ferruginous chert, and iron-formation described by all previous workers 

in conglomeratic beds of the Ortega could well have been derived from 

the Moppin Metavolcanics, which contain layers of iron-formation and 

are locally laced with veins of white (hydrothermal) quartz. 

The foregoing stratigraphic revisions for the Tusas Mountains are 

compatible with recent reinterpretations of Precambrian stratigraphy 

in several other nearby parts of New Mexico and Colorado. In the 

Picuris Range, 30 mi ( 48 km) southeast of the Tusas Mountains, Ortega 

Quartzite was for many years considered to be the oldest Precambrian 

unit, overlain by rocks of a volcanic character in the Vadito Formation 

(Montgomery, 1953; Miller and others, 1963; Long, 1976). Schistose 

layers in the lower Ortega (also called Rio Pueblo Schist by Miller and 

others, 1963) were, however, found to be altered metarhyolite upon 

which the quartzites rest (Gresens and Stensrud, 1974). More recently, 

Holcombe and Callender ( 1982) have concluded that the Vadito Formation 

of the Picuris Range, consisting largely of amphibolite and felsite with 

associated metasedimentary rocks, probably lies beneath the Ortega. 

About 25 mi ( 40 km) southeast of the Picuris Range in the Rio Mora 

portion of the Sangre de Cristo Range, Grambling and Codding ( 1982) 

have used crossbedding in quartzite to establish that the Vadito Formation, 

including metarhyolite and amphibolite, is older than the Ortega Quartzite. 

This once again represents a reversal of previous stratigraphic interpretation 

(Miller and others, 1963; Moench and Robertson, 1980). 

The nearest Precambrian exposures northwest of the Tusas Mountains 

are about 80 mi ( 128 km) away in the Needle Mountains of southwestern 

Colorado. Barker ( 1969) had reversed the earlier stratigraphic sequence 

proposed by Cross and others (1905) by recognizing the Vallecito 

Conglomerate (a thick section of quartzite and conglomerate) as the 

oldest rock of that region. He believed the Vallecito to be overlain by the 

Irving Formation (amphibolite, some with pillow structures; iron-formation; 

and volcaniclastic metasedimentary rocks), the Twilight Gneiss (composed 

largely of felsic metavolcanic rocks), and the Uncompahgre Formation 

(thick quartzite units with minor meta-conglomerate and slate or phyllite). 

Burns and others ( 1980) have recently shown that cross beds in the 

Vallecito point away from a scoured contact with the Irving and that 

cobbles in some of the conglomeratic beds of the Vallecito could well 

have been derived from unusual lithologies within the Irving. 
18 PROTEROZOIC METAMORPHIC ROCKS, NEW MEXICO 

Thus it appears that in the Needle Mountains, metavolcanic rocks of 

the Irving Formation are unconformably overlain by quartzite and 

conglomerate of the Vallecito Conglomerate, in general agreement with 

Cross and others ( 1905) but the opposite of the stratigraphic conclusions 

of Barker ( 1969 ). In spite of the higher metamorphic grade and structural 

complexity of the Needle Mountains rocks, general similarities of premetamorphic lithology, radiometric age, and revised stratigraphic sequence 

allow a cautious comparison with the Proterozoic section of the Tusas 

Mountains (Burns and Wobus, 1983). 

On a regional scale in northern New Mexico and southwestern Colorado, 

late Early Proterozoic supracrustal assemblages appear to grade upward 

from a bimodal metavolcanic sequence to one composed largely of 

quartzite and conglomerate. Though not referring specifically to the 

Tusas or Needle Mountains sections, Condie (1981, 1982) recognized 

this same general trend in rocks of similar Proterozoic age throughout 

the southwestern United States. In terms of the plate-tectonics model, 

Condie ( 1982) suggested that this suite of rocks may have been generated 

by the opening and closing of successive back-arc basins underlain by 

continental crust. 

REFERENCES CITED 

Barker, Fred, 1958, Precambrian and Tertiary geology of Las Tablas quadrangle, 

New Mexico: New Mexico Bureau of Mines and Mineral Resources Bulletin 

45, 105 p. 

___ 1969, Precambrian geology of the Needle Mountains, southwestern Colorado: 

U.S. Geological Survey Professional Paper 644-A, 35 p. 

___ 1970, Ortega Quartzite and the Big Rock and Jawbone Conglomerate 

Members of the Kiawa Mountain Formation, Tusas Mountains, New Mexico: 

U.S. Geological Survey Bulletin 1294-A, p. A21-A22. 

Barker, Fred, and Friedman, Irving, 1974, Precambrian metavolcanic rocks of 

the Tusas Mountains, New Mexico: New Mexico Geological Society, 25th, 

Guidebook, Ghost Ranch, p. 115-117. 

Bingler, E. C., 1965, Precambrian geology of La Madera quadrangle, Rio Arriba 

County, New Mexico: New Mexico Bureau of Mines and Mineral Resources 

Bulletin 80, 132 p. 

Burns, L. K., Ethridge, F. G., Tyler, Noel, Gross, A. S., and Campo, A. M., 1980, 

Geology and uranium evolution of the Precambrian quartz-pebble conglomerates of the Needle Mountains, southwest Colorado: Grand Junction, Colo., 

Bendix Field Engineering, contract No. DE-AC13-76GJ01664, U.S. Department of Energy, 161 p. 

Burns, L. K., and Wobus, R. A., 1983, Correlations and revisions of Precambrian 

stratigraphy, Needle Mountains, southwest Colorado, and Tusas Mountains, 

north-central New Mexico: Geological Society of America Abstracts with Programs, v. 15, no. 5, p. 424. 

Condie, K. C., 1981, Evolving parallel tectonic styles in adjacent Proterozoic 

crustal provinces of the southwestern United States: Geological Society of 

America Abstracts with Programs, v. 13, no. 2, p. 50. 

___ 1982, Plate-tectonics model for continental accretion in the southwestern 

United States: Geology, v. 10, p. 37-42. 
REFERENCES CITED 19 

Cross, Whitman, Howe, Ernest, Irving, J.D., and Emmons, W. H., 1905, Description of the Needle Mountains quadrangle, Colorado: U.S. Geological Survey 

Geologic Atlas Folio 131, 14 p. 

Grambling, J. A., and Codding, D. B., 1982, Stratigraphic and structural relationships of multiply-deformed Precambrian metamorphic rocks in the Rio Mora 

area, New Mexico: Geological Society of America Bulletin, v. 93, p. 127-137. 

Gresens, R. L .. and Stensrud, H. L., 1974, Recognition of more metarhyolite 

occurrences in northern New Mexico and a possible Precambrian stratigraphy: 

Mountain Geologist, v. 11, p. 109-124. 

Holcombe, R. J., and Callender, J. F., 1982, Structural analysis and stratigraphic 

problems of Precambrian rocks of the Picuris Range, New Mexico: Geological 

Society of America Bulletin, v. 93, p. 138-149. 

Jahns, R. H., 1946, Mica deposits of the Petaca district, Rio Arriba County, New 

Mexico: New Mexico Bureau of Mines and Mineral Resources Bulletin 25, 294 p. 

Just, Evan, 1937, Geology and economic features of the pegmatites of Taos and 

Rio Arriba Counties, New Mexico: New Mexico School of Mines Bulletin 13, 

73 p. 

Long, P. E., 1976, Precambrian granitic rocks of the Dixon-Penasco area, northern 

New Mexico: Stanford, Calif., Stanford University, Ph.D. thesis, 533 p. 

Manley, Kim, Scott, G. R., and Wobus, R. A., 1978, Preliminary geologic map 

of the Aztec 1° x 2° quadrangle, northwestern New Mexico: U.S. Geological 

Survey Open-File Report 78-466, 7 p. 

Manley, Kim, and Wobus, R. A .. 1982a, Reconnaissance geologic map of the Las 

Tablas quadrangle, Rio Arriba County, New Mexico: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1408, scale 1:24,000. 

___ 1982b, Reconnaissance geologic map of the Mule Canyon quadrangle, Rio 

Arriba County, New Mexico: U.S. Geological Survey Miscellaneous Field 

Studies Map MF-1407, scale 1:24,000. 

Miller, J. P., Montgomery, Arthur, and Sutherland, P. K., 1963, Geology of part 

of the southern Sangre de Cristo Mountains, New Mexico: New Mexico Bureau 

of Mines and Mineral Resources Memoir 11, 106 p. 

Moench, R. H .. and Robertson, J. M., 1980, Mineral resources of the Pecos Wilderness and adjacent areas, Santa Fe, San Miguel, Mora, Rio Arriba, and Taos 

Counties. New Mexico: U.S. Geological Survey Open-File Report 80-382, 

Chapter A, 41 p. 

Montgomery, Arthur, 1953, Precambrian geology of the Picuris Range, northcentral New Mexico: New Mexico Bureau of Mines and Mineral Resources 

Bulletin 30, 89 p. 

North American Commission on Stratigraphic Nomenclature, 1983, North American Stratigraphic Code: American Association of Petroleum Geologists, v. 67, 

no. 5, p. 841-875. 

Smith, H. T. U .. 1938, Tertiary geology of the Abiquiu quadrangle, New Mexico: 

Journal of Geology, v. 46, no. 7, p. 933-965. 

Wobus, R. A., and Dobson, P. F., 1981, Proterozoic intrusive history of the Tusas 

Mountains, north-central New Mexico: Geological Society of America Abstracts 

with Programs, v. 13, no. 4, p. 230. 

Wobus, R. A., and Hedge, C. E., 1982, Redefinition of the Precambrian Tusas 

Mountain and Tres Piedras Granites, north-central New Mexico: Mountain 

Geologist. v. 19, p. 105-114. 

Wobus, R. A., and Manley, Kim, 1982, Reconnaissance geologic map of the Burned 

Mountain quadrangle, Rio Arriba County, New Mexico: U.S. Geological Survey Miscellaneous Field Studies Map MF-1409, scale 1:24,000 . 

.,. U. S. <DVERNMENl' PRINriNG OFFICE 1985/576-049/20.031