BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA Vol. 32, pp. 417-428 December 1 , 1921

SOLITARIO UPLIFT, PRESIDIO-BREWSTER- COUNTIES, TEXAS 1

BY SIDNEY POWERS

(Read before the Society December 29, 1920)

CONTENTS P ag e Introduction...... 417 Summary...... 4 1 9 Regional geology 4 2 1 Detailed geology. 42 2 Stratigraphy. 42 2 Structure___ 4 2 5 Origin...... 4 2 6

I ntroduction

The Solitario is a circular dome 5 miles in inside and 7 miles in out­ side diameter, situated 60 miles south of Marfa, west Texas, in both Presidio and Brewster counties (figure 1). It is in the trans-Pecos sec­ tion, 9 miles in a straight line from the Rio Grande, on the western side of the “Big Bend” in the river and 25 miles east of Presidio. Occasional ranches are scattered through this barren mesquite desert, but few are near the highways. Military roads have been constructed from Marfa to Lajitas past the Solitario and to other outposts. There is a wagon road from the Lajitas road past McMahon’s ranch into the north end of the uplift. Although the Solitario is shown on the topographic sheet of the Ter- lingua quadrangle, United States Geological Survey (figure 1), and al­ though it has been visited by several geologists, including G. K. Gilbert, J. A. Udden, and R. T. Hill, nothing has ever been written about it.2

1 Manuscript received by the Secretary of the Society May 17, 1921. 2 It is shown on the “Geological map of a portion of west Texas,” by B. F. Hill and J. A. Udden, University of Texas Mineralogicai Survey, 1904 References to the Big Bend country are: J. A. Udden, A sketch of the geology of the Chisos country, Brewster (417)

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/32/4/417/3414294/BUL32-0417.pdf by guest on 29 September 2021 418 SIDNEY POWERS— THE SOLITAltlO UPLIFT

LEGEND E 3 S Igneous Rock H--- 1 Upper Cretaceous 1 I Lower Cretaceous Pennsylvanian pnvT] Devonian [?]. , iL-*------1 (arrows show dips] Ordovician [?]. 1-SoIifario ---- Sections ito4 I- Marathon 7-Ouachita Mb 3- Shaft er 8-Muckle Mti.' K Silver Prospect 4 - Delaware Mb. 9-Wichita Mis. 5-Uano-Bumet 10-Amarillo Granite' Abandoned- 6-dend Arch11-Anton Chico Granite 12-Rid River Uplift '« 5 Miles

1-20 1______2 3 ______4. ■*> Kilnmjytei-»

Contour in terval lOO feet Datum is rnmxn sea level. F ig u r e 1 .— Geological Map of the Solitario Uplift, Presldio-Brewster Counties, Texas The accompanying outline map of Texas shows the relative position of the uplift to other areas of folding.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/32/4/417/3414294/BUL32-0417.pdf by guest on 29 September 2021 INTRODUCTION 419

The most remarkably symmetrical dome attracted the attention of the writer in 1919, in passing, and caused him to spend several days within it in 1920 in part in the company of Wallace E. Pratt, to whom he is indebted for many suggestions. Mr. W. S. Adkins and others of the Bureau of Economic Geology, Austin, Texas, have studied the area in detail, under the direction of Dr. TJdden, and the present brief descrip­ tion is preliminary to their report. *

Summary

Geologically, the Solitario consists of an unroofed, steep-sided, flat- topped dome formed of Lower Cretaceous limestones, within which there is an eccentric dome of steeply tilted Ordovician cherts, limestones, and shales, Devonian (?) novaculite, Pennsylvanian shales and sandstones, and Permian limestones. A portion of the Cretaceous cover is preserved in the center of the dome on a high ridge which runs from east to west. South of this ridge the Pennsylvanian shales and sandstones on the south side of the Ordovician dome are cut and in part covered or replaced by volcanic breccias and intrusives (figure 2). Volcanic activity is every­ where visible in dikes, sills, and laccoliths, the most extensive sill being within 20 to 100 feet of the base of the Cretaceous and extending around a great part of the periphery of the dome, as well as through the inlier in the center. Solitario Peak is an igneous mass, probably a volcanic plug, and an unnamed hill on the north is a laccolith, but no attempt is made to map or describe the other igneous masses. The Solitario and the Marathon uplift, 60 miles northeast, are part of an area mountain-built in Permian time and later buried. The broad and gentle uplift of the latter after Cretaceous deposition is believed to have resulted from compression at right angles to the direction of the earlier forces. The very abrupt and symmetrical doming of the Solitario may be accounted for in the same manner or possibly by a laccolithic intrusion in basal Paleozoic strata. The Cretaceous cover of the Solitario

County, Texas, University of Texas, Bulletin 93, 1907 ; J. A. Udden, Notes on the geology of the Glass M ountains; and C. L. Baker and W. F. Bowman, Geologic exploration of the southeastern Front Range of trans-Pecos, Texas,.University of Texas, Bulletin 1753, 1917; J. A. Udden, C. L. Baker, and E. Böse, Review of the geology of Texas, University of Texas, Bulletin 44, 1916 ; R. A. Liddle, The Marathon fold and its influence on petro­ leum accumulation, University of Texas, Bulletin 1847, 1918 ; B. F. Hill, The Terlingua quicksilver deposits, Brewster CGunty, University of Texas, Bulletin 15, 1902 ; T. W. Vaughan, Reconnaissance in the Rio Grande coal fields of Texas, U. S. Geological Survey, Bulletin 164, 1900; -T. A. Udden, The anticlinal theory as applied to some quicksilver deposits (Terlingua District, Brewster County), University of Texas, Bulletin 1822, 1918.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/32/4/417/3414294/BUL32-0417.pdf by guest on 29 September 2021 on 29 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/32/4/417/3414294/BUL32-0417.pdf 420 I NY OES------TE OIAI UPLIFT SOLITARIO POWERS------THE SIDNEY

X'Xn WNE0U5 POCK ] LOWER CPETACEOUS | . ; . ;.} DEVONIAN (?) NOVACULLTE UPPER CRETACEOUS \ PENN-LIMESTONE & SHALE [ : r r ^ ORDOVIOAN C?J LIMESTONE & SHALE

F ig u r e 2.—Cross-sections of the Solitario Uplift GEOLOGY OF THE REGION 421

has been fractured by faults and by volcanic intrusions and extrusions and largely eroded.

R e g i o n a l G e o l o g y

The Big Bend country and the area to the northward comprise the volcanic plateau of western Texas. A vast region is covered by lava flows and ash beds or by bolson deposits which conceal the bedrock. Cretaceous rocks are exposed at the edges of, and occasionally within, the volcanic area. That volcanic activity began during the Upper Cretaceous and con­ tinued into the Tertiary is shown by plant remains.3 The lavas are ande- sitic and rhyolitic in character and they are described by Dr. Whitman Cross in an unpublished manuscript of the United States Geological Sur­ vey by Mr. E. T. H ill on the Big Bend country. Both flows and ash beds can be traced for many miles in mesas, and the good exposures invite careful mapping and study. Structure of the Big Bend region must be considered for the Paleozoic and Cretaceous terranes separately. The younger, as shown to the writer by Mr. H ill and later observed in the field, forms a high plateau 100 miles in width, broken on either side by fault blocks tilted toward the plateau.4 The faults on the east are in the Boquillas (Carmen) Moun­ tains; those on the west are in the Sierra Vieja Mountains. Many other faults with profound escarpments, like those south of Lajitas and Ter- lingua, add complexity to the structure. In spite of these fractures, the Rio Grande has maintained its course developed during the igneous activity, and the only sedimentary records of the canyon-cutting stage, as interpreted by the writer, are part of the very thick bolson deposits between Presidio and El Paso. In Mexico, the Burro Mountains south­ east of the Big Bend represent block-faulted Cretaceous mountains cut by intrusions. Paleozoic rocks outcrop at only four localities in the Big Bend region— Marathon, the Solitario, Shafter, and west of the Chinati Mountains, in Pinto Canyon * (undescribed). The nearest Paleozoic areas are the Guadalupe and Delaware Mountains to the north (figure 1) and the Llano-Burnet (Central Mineral) region to the east. The former is

3 E. W. Berry : An Eocene flora from trans-Pecos, Texas. U. S. Geol. Survey, Profes­ sional Paper 125A, 1919. 4 C. L. Baker described this plateau as a block relatively less elevated than any other part of the trans-Pecos (Review of the Geology of Texas, University of Texas, Bulletin 44, 1916, p. 15).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/32/4/417/3414294/BUL32-0417.pdf by guest on 29 September 2021 422 SIDNEY POWERS----THE SOLITARIO UPLIFT

clearly a part of the Cordilleran region. The latter is a block-faulted uplift comparable to a horst, which shows very gentle folding. It forms the southern end of the Bend arch. The Paleozoic rocks in the Solitario and Marathon areas show the same northeast-southwest strike, but the continuation of this line of folding has not been traced beneath the younger sediments. On the north side of the Marathon area the Glass Mountains, composed of a great thickness of Permian limestones, mark, locally, the southern limit of a series of Permian sediments unconform- able with the earlier Permian, which extend into New Mexico with abrupt lithologic change. Somewhere east of the truncated edges of these Per­ mian sediments another Bend arch may be found extending toward the Wichita Mountains. Buried granite knobs have been found to extend through Beckham County, Oklahoma; Amarillo, Texas, and Anton Chico, New Mexico, and this evidence may be interpreted as indicating a lack of connection between the Arbuckle and Wichita Mountains and the Bend arch-Marathon-Solitario folding. All this folding is, however, Appa­ lachian, as long ago pointed out by E. T. Hill,5 because the Cosdilleran folding began with the Laramie revolution.6 Contrasting the Cretaceous Cordilleran and Paleozoic Appalachian structures, the strike of the former is northwest-southeast, the latter at right angles. Undoubtedly the combination of these lines of folding has been an important, if not a decisive, factor in the formation of domes such as the Solitario and Marathon. The exposures of Pennsylvanian and Permian rocks at Shafter and probably west of the Chinati Mountains are accidental, because they are apparently the tilted edges of east-west fault blocks exposed in the deep dissection of the volcanics by Cibolo Creek. Similar exposures in Mexico are reported to the writer by Dr. Udden.

D etailed Geology

STRATIGRAPHY Fossils in pre-Cretaceous rocks are difficult to find, and the age deter­ minations are made by analogy with the Marathon region, where excellent

5 Physical geography of the Texas region. U. S. Geol. Survey, topographic atlas of the United States, Folio 3, 1900, p. 4. 6 Willis T. Lee disagrees with this reasoning in his paper on “Granite in eastern New Mexico wells” (Bull. Amer. Assn. Pet. Geol., vol. 5, no. 2, 1921, p. 165), where he states: “The Pennsylvanian sedimentation was brought to a close in New Mexico by the eleva­ tion of mountains which the writer has termed the Ancestral Rocky Mountains. The Pennsylvanian strata were upturned in these mountains, eroded, and probably in some places entirely removed. . . . It seems appropriate to regard [the post-Pennsylvanian unconformity] as a major unconformity.”

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/32/4/417/3414294/BUL32-0417.pdf by guest on 29 September 2021 DETAILED GEOLOGY 423

collections have been made. Cambrian fossils have been identified in an anticlinal fold south of Marathon, but not at the Solitario. The white novaculite, which forms conspicuous ridges in the northern half of the uplift, is the most conspicuous Paleozoic formation, and it is clearly the Caballos novaculite which is possibly of Lower Devonian age. Beneath it and within the novaculite rim in the northern half are the black cherts and silicified shales, which belong to the Maravillas formation of Middle and Upper Ordovician age and contain graptolites at Marathon.7 The older blue and gray limestone and black shale are probably the Marathon formation of Lower and Middle Ordovician age. Overlying the novacu­ lite and structurally conformable with it is the Tesnus formation, of Pennsylvanian age.8 The Tesnus formation is exposed in the south part of the Solitario and in the synclines in the northeast and southwest parts of the novaculite. Plant remains collected 225 feet from the top of the Tesnus formation at Marathon (latitude, 30° 5%'; longitude, 103° 19') were identified by Mr. David White as probably of Upper Pottsville age, to be correlated with the base of the Magdalena group of New Mexico and with either the Strawn or Bend formations of west central Texas. Uplift and erosion everywhere followed the deposition of these and accompany­ ing formations. Pennsylvanian or Permian (?) limestones are exposed on either side of the wagon road in the north end of the Solitario, between the basal Cretaceous conglomerate and hills of igneous rock. The hills seem to represent an intrusion which came up along the fault separating these limestones from those of Ordovician age. The limestone is bright yellow in color, the appearance being similar to that of the Permian limestone in Shackelford County. It contains crinoids and other poorly preserved fossils, which are of Upper Paleozoic age. The only Pennsylvanian lime­ stone with which it can be correlated is the Dimple at Marathon, but there are many Permian limestones. Lower Cretaceous sedimentation followed a period of orogeny and ero­ sion. The basal Trinity conglomerate, 100 feet or more in thickness, consists of well rounded pebbles. The absence of lithologic breaks in the overlying limestone series and the thinness of the basal conglomerate

7 Dr. Kudolf Ruedemann identified the graptolites as of Trenton Normanskill age. Collections from the Viola limestone in the Criner I-Iills, Oklahoma, indicate a slightly younger Trenton horizon, 8 This formation was measured along San Francisco Creek parallel to the Southern Pacific Railroad east of Marathon and found to be 7 ,5 9 0 feet thick, subject to duplica­ tions, but only two, apparently minor faults, were observed. Baker and Bowman meas­ ured the thickness as 3 ,3 7 0 feet. The overlying Dimple formation was determined to be 882 feet thick.

XXX—B u l l . G e o l . S o c . A m ., V o l . 3 2 , 1 9 2 0

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/32/4/417/3414294/BUL32-0417.pdf by guest on 29 September 2021 on 29 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/32/4/417/3414294/BUL32-0417.pdf 424 INY OES------H SLTRO UPLIFT SOLITARIO POWERS------THE SIDNEY

F ig u r e 3.— Panoramic Vieivs of the Exterior of Solitario Uplift The upper picture shows the rim of Lower Cretaceous limestone as seen from the southwest. The lower picture shows the Lower Cretaceous mountains on the left, the Upper Cretaceous hills in the foreground, and the lava-capped mesas on the right. DETAILED GEOLOGY 425

indicate rapid subsidence of the region and relatively great distance from a shore. Most remarkable exposures of Lower Cretaceous limestone form the rim of the Solitario one mile in width on the north, east, and west sides, dipping 30 degrees to 50 degrees (figure 3). More gentle dips on the southeast extend these limestones 5 miles from the rim. The thickness of the limestone must be over 3,000 feet. Above it and in the valleys surrounding the rim the Del Eio clay and the Buda limestone may both be recognized as at Del Rio. The latter is, in places, very white. Upper Cretaceous sediments, except the Eagle Ford flags, are lacking. These flags consist of bluish arenaceous shale and of sandstone, which weather dark yellow in color. They compose the low hills on the outer side of the circular drainage system and they also compose the low hills south and southwest of the dome, where they become chalky. Lava flows and ash beds capping the volcanic plateau overlie the Eagle Ford flags, but the folding in the latter is so gentle that the structural relations with the flows were not observed. Elsewhere the Austin chalk and Taylor marl or their equivalents have been found in western Texas, but it is doubtful if they were ever deposited here. Uplift and volcanic activity began after the Eagle Ford sedimentation.

STRUCTURE

The structure of the Solitario is shown in the cross-sections presented in figure 2. The Paleozoic rocks are closely compressed with folds, some of which are isoclinal or even overturned. These folds are best seen in the hills on the east and west—on the east where a creek cuts through the novaculite and winds between two anticlines of novaculite, and on the west in the novaculite ridges which finger out toward Burnt Camp into anticlines which disappear beneath the Cretaceous. Another outcrop of novaculite south of McGuirk’s tanks is overlain by Cretaceous limestone. When the outcrops of novaculite are considered together, the contin­ uous and curved ridge on the east shows a pronounced dip outward from the central plain; the ridge on the south shows tilting to the south and the ridge on the west also shows a southern dip. On this evidence the plain which forms the northern half of the uplift is believed to be anti- clinorial, and the novaculite, which would normally outcrop in parts of this area, is believed to have been eroded from an older arch in pre- Cretaceous time. Subtracting the dips of the Cretaceous rocks from those of the Paleo­

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/32/4/417/3414294/BUL32-0417.pdf by guest on 29 September 2021 426 SIDNEY POWERS----THE SOLITARIO UPLIFT

zoic strata gives approximately the dips of the older series in pre-Cre­ taceous time, except for the foreshortening taken up in metamorphism. Therefore the dips must have been 5 degrees to 50 degrees. At Marathon the Cretaceous now dips at angles of 3 degrees to 10 degrees over trun­ cated folds which are in part isoclinal. This relation indicates that the major folding in the Big Bend region was pre-Cretaceous. The Cretaceous rocks are more gently and regularly folded on the sides of the uplift, the steepest dips being on the north, northeast, and west. The eroded edges of these limestones at the basal contact with the older rocks around the periphery of the dome dip steeply, but on the western side the contact is still preserved, extending within the dome and rising to the top of a ridge of novaculite. Here the degree of dip in the lime­ stone diminishes rapidly and becomes gentle and almost flat above the novaculite. This relation suggests that a relatively flat arch of limestone existed over the uplift before the dome was disrupted by igneous intru­ sion accompanied by faulting. It further suggests that the post-Cre­ taceous doming can be compared with the punching upward of a circular core by igneous intrusion. Inliers of Cretaceous limestone cap several hills in the volcanic area at the south end of the uplift. Still other fragments of limestone are found •in the volcanic breccia, into which they have evidently fallen with the erupted material. The volcanic disturbances have depressed the inliers below the base of the limestone in the Solitario rim. The principal inlier is in the center of the dome. The central hills consist of Cretaceous limestone with a sill at the base resting on novaculite and cut off or overlain by volcanic breccias on the east and south. A dike separates the limestone from the novaculite on the west. The elevation of the base of the limestone on the north is approximately the same as that of the basal contact around the dome, but the dip of the inlier is to the south. Such relationship may be explained in at least two ways: The folding in the limestone roof may originally have conformed to rela­ tive competence of formations accentuated by folding and the inlier may be part of an original syncline. On the other hand, this inlier more probably owes its present position to faulting.

O r i g i n

The problem is to explain the origin of a circular, flat-topped dome 5 miles in inside and 8 miles in outside diameter, uplifted one mile in a horizontal distance of 2 miles. The Marathon dome is broad, with gentle

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/32/4/417/3414294/BUL32-0417.pdf by guest on 29 September 2021 ORIGIN OF THE UPLIFT 427

slopes. It has a diameter of approximately 36 miles, with an uplift of about one mile in 10 miles or more. In both areas there were two periods of major folding, with compressive forces of the second period acting at right angles to those of the first period. At the Solitario there was in­ tense volcanic actipn manifested as intrusives and extrusives, while at Marathon there was practically no volcanic action. Intrusion of a laccolith or other igneous body into the lowest Paleozoic strata at the Solitario may explain the abruptness of the uplift. The sills, dikes, and volcanic breccia would be connected with such a body. Six or more laccoliths in and near the Chisos Mountains southeast of the Solitario have been described by Dr. Udden.9 The Christmas Mountains, near by, are a faulted dome in Cretaceous limestone, one and one-half miles in diameter, uplifted 3,500 feet, and are probably a laccolith, as yet unroofed. Other similar intrusive uplifts are found farther south in the Burro Mountains of Mexico. This hypothesis would explain the circular outline, the steep sides, and apparently flat top of the dome. Furthermore, some intrusive body or bodies must underlie the dome, because of the concentration of dikes, plugs, and sills and because of the vast amount of volcanic breccia which has come from some intrusive which broke through to the surface. It is, however, difficult to conceive of the form and size of an intrusive, even of an intraformational laccolith, in complexly folded rocks which would fulfill the requirements. The laccolithic intrusions above cited seem to be confined to Cretaceous limestones. Another hypothesis is that the doming can be explained by tectonic forces without igneous intrusion. An analogous uplift in Tennessee has been called to the attention of the writer by W. A. Nelson, the State Geologist. This dome, the Wells Creek Basin, in Stewart County, near Cumberland City, is two miles in outside diameter and shows an uplift of about one-half mile, with dips around the edges “at high angles and at some points even vertical.” 10 This uplift of Upper Cambrian into' Mississippian limestone is 70 miles from the nearest dike in Kentucky. An unconformity is shown in the section at the dome, but not elsewhere, and therefore the dome is a rejuvenation of an older uplift. The doming is clearly the result of tectonic movements free from igneous action.

0 University of Texas, Bulletin 93, 1507 ; 1758, 1917, p. 148. Also, U. S. Geol. Survey, topographic atlas, Chisos Mountains quadrangle. 10 J. M. Safford : Geology of Tennessee, 1869, p. 147, par. 364-5. J. J. Galloway: Geology of natural resources of Rutherford County, Tennessee. Geological Survey of Tennessee, Bulletin 22, 1919.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/32/4/417/3414294/BUL32-0417.pdf by guest on 29 September 2021 428 SIDNEY POWERS— THE SOLITARIO UPLIFT

From the analogy of the Wells Creek Basin it is believed that the original doming of the Solitario was caused principally by tectonic forces. It is further believed that the symmetry of the dome is related to the preexisting structure in the older rocks, which was probably anti- clinorial, this structure forming a locus for the concentration of stresses developed during the folding of the Cretaceous rocks.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/32/4/417/3414294/BUL32-0417.pdf by guest on 29 September 2021