VINCENT C. KELLEY JAMES W. FURLOW

LOWER PALEOZOIC WEDGE EDGES IN SOUTH-CENTRAL NEW MEXICO: NEW CONTROL

Abstract: A thin sequence of and Ordo- known occurrence of these rocks in south-central vician beds lying on Precambrian and over- New Mexico. The occurrence suggests that Cam- lain by Pennsylvanian Magdalena beds exists at the brian- strata originally may have ex- Eaton ranch locality along the southeastern edge tended more directly northwesterly toward similar of the San Mateo Mountains. The section includes rocks in northeastern Arizona and northwestern representatives that are widespread in southern New Mexico. The position of the Montoya on very New Mexico: the Bliss formation, El Paso group, low beds of the El Paso emphasizes the mid- and Montoya group. This is the northernmost Ordovician in the region.

mile southwest of Eaton ranch at the northern Introduction edge of the Buck Hills. These hills, about 1-2 During the course of mapping along the miles east of the San Mateo Mountains, consist southeastern foothills of the San Mateo Moun- of a thick section of southward-dipping Penn- tains in Socorro County, a section consisting of sylvanian and shale of the Magdalena Bliss, El Paso, and Montoya formations was Group as well as a thin section of red found resting on Precambrian granite and over- beds of the Abo formation overlain uncon- lain by Pennsylvanian beds at the Eaton ranch formably by Tertiary felsitic volcanic rocks. locality (Fig. 1). The entire marine section of Figure 2 shows the distribution of the lower some 2600 feet was mapped in reconnaissance Paleozoic rocks beneath the Magdalena at the by J. H. Schilling and M. E. Willard (Dane northern edge of the hills and in the slightly and Bachman, 1961) and in some detail by dissected pediment to the north. The outcrop F. E. Kottlowski (I960, p. 37-43,161). Neither belt is disconnected by faulting and alluvial party recognized the lower Paleozoic rocks cover but over-all is about 4000 feet long. It which comprise nearly 300 feet of the base of trends generally east-west in the eastern part the section although Kottlowski mentioned and turns sharply to the northwest in the that the lower resembled those of western part. The eastern exposures dip south- the Bliss. ward at angles from 45 to 90 degrees. The In south-central New Mexico, lower Paleo- western exposures are steeply overturned to zoic rocks occur as a southward-thickening steeply upright, with the principal overturning wedge beneath the more extensive overlying occurring near a probable east-west wrench upper Paleozoic rocks; the lower Paleozoic se- fault inferred to lie beneath the alluvium of quence reaches nearly 3000 feet in thickness in Crawford Hollow. the southern part of the state. Northward these formations are thinner, partly because of depo- Stratigraphy sition, but probably because of interdeposi- To understand the regional implication of tional periods of epeirogenic uplift and strip- these exposures, it is necessary to know the ping. Studies in the San Andres, Fra Cristobal, general stratigraphy of south-central New and (Fig. 1) show that the Mexico south of the edges of wedging where all episodes of uplift and stripping which gave rise or parts of certain formations have been re- to the total wedge belt occurred in Middle moved. Therefore, Table 1 gives the stratig- Ordovician, Late to Middle , raphy of the principal Paleozoic units. and Late Mississippian to Early Pennsylvanian The only sedimentary units present at the times (Kelley and Silver, 1952, p. 53, 133, 224, Eaton ranch locality are, in ascending order, 228-229). Bliss formation, Sierrite limestone, Cable Can- The lower Paleozoic exposures lie about 1 yon , Upham dolomite, and Magda-

Geological Society of America Bulletin;, v. 76, p. 689-694, 3 figs., 2 pis., June 1965 689

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/6/689/3417354/i0016-7606-76-6-689.pdf by guest on 01 October 2021 690 KELLEY AND FURLOW—LOWER PALEOZOIC WEDGE EDGES, NEW MEXICO

106?

r*9^

33°.

32" • " FRANKLIN I v MTS. M E X I C O n in ?r> *rj an "so MM»<:

Figure 1. Index map showing the northern wedge edges of several Paleozoic units in south- central New Mexico

lena formation. The rest, with the possible ex- The Eaton ranch Cambrian-Ordovician ex- ception of some of the lithologically variable posures range from about 295 feet thick at the units of the Devonian and Mississippian, were eastern end to as little as 100 feet thick in the once present in the area and were removed by southern exposures. Most of this difference in pre-Pennsylvanian intervals of uplift and thickness appears to be in the Bliss. East of the road (Fig. 2), a thickness of about 145 feet of

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/6/689/3417354/i0016-7606-76-6-689.pdf by guest on 01 October 2021 Figure 1. Bedding surface on brown oolitic hematite of the Bliss formation

Figure 2. Sierrite limestone near its base. Note typical ragged chert development (lower left) OUTCROPS OF BLISS AND SIERRITE FORMATIONS IN THE EASTERN EXPOSURES, NEW MEXICO

KELLEY AND FURLOW, PLATE 1 Geological Society of America Bulletin, volume 76

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/6/689/3417354/i0016-7606-76-6-689.pdf by guest on 01 October 2021 Figure 1. Typical white, pocked surface of Cable Canyon sandstone at the southwestern exposures

Figure 2. Outcrop of Cable Canyon sandstone (left) in vertical contact with (Jpham dolomite (right) in the southern exposures. Stereoscope is aligned with the strike ot the formations OUTCROPS OF CABLE CANYON SANDSTONE AND LTHAM DOLOMITE, NEW MEXICO

KELLEY AND FURLOW, PLATE 2 Geological Society of America Bulletin, volume 76

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/6/689/3417354/i0016-7606-76-6-689.pdf by guest on 01 October 2021 SHORT NOTES 691

Bliss is fairly well exposed. It consists of buff- to ticulations on the bedding although massive light-tan arkosic sandstone and conglomerate thick units are present. With the poor ex- in the lower 40-50 feet; hematitic, coarse- posures at the Eaton ranch locality, however, grained sandstone with subrounded grains and it is not surprising that the thin-bedded aspect with oolitic seams up to 6 inches thick in a is not more apparent. Only scattered small out- middle 30-40 feet (PI. 1, fig. 1); and an upper crops of limestone are exposed, and these are incompletely to poorly exposed part consisting the thicker ledges. In some of these, irregular of occasional thin-bedded, dense hematitic brown-weathering chert lenses are present, and sandstone ledges interspersed with thin-bedded the lithology is clearly that of the El Paso

EXPLANATION •37°37'30" Oal Valley alluvium N. Pediment gravel Pm Pennsylvanian Magdalena Group T. es. Om Upper Ordovician Uphom dolomite of the R.4W. Montoya group wifh Coble Canyon sand- Quarter corner stone at r elev. 5630 feel Oe Lower Ordovician Sierrite limestone 8 of the El Paso group •Cb Upper Cambrianl?) Bliss formation p€ Precombrian granite

Figure 2. Geologic map of the Eaton ranch locality showing lower Paleozoic out- crops, New Mexico

siltstone and limestone that is 60-70 feet thick. group and probably the Sierrite limestone (PI. Here, the Bliss rests in well-exposed contact on 1, fig. 2). The interval is fairly well delineated Precambrian granite, and the upper contact at the eastern end of the belt along the edge of with the overlying El Paso Limestone beds can the pediment where a thickness of 75 feet was be narrowed between scattered exposures to measured. The upper unconformable contact about 10 feet. In the western part of the area, with the Cable Canyon sandstone is sharp, and the Bliss is much thinner where it appears to it is in the upper 5 feet that the best exposures range from about 30-85 feet. The differences in of the typical-gray to light-gray Sierrite may thickness of the Bliss are probably due to broad be seen. channels filling swales in the Precambrian The Cable Canyon is white to light gray, rather than erosion at the base of the El Paso. coarse-grained, angular to subrounded cjuartz The El Paso beds are inferred, on the basis sandstone which ranges to granule conglomer- of regional relationships, to be the Sierrite, for ate in many exposures. The unit is 10-14 feet it appears to be widely true that the Cable thick and crops out prominently owing to its Canyon sandstone truncates downward toward firm cementation and partial silicifkation. the north into successively older beds of the El Throughout the region, the clean, clear color- Paso group. In other areas, the Sierrite beds are less to bluish quartz sand and granules and typically slabby with chert laminations or re- calcitic dolomite cement are distinguishing

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/6/689/3417354/i0016-7606-76-6-689.pdf by guest on 01 October 2021 692 KELLEY AND FURLOW—LOWER PALEOZOIC WEDGE EDGES, NEW MEXICO

features of the unit. However, the ferruginous and massively bedded, and locally, it contains weathering common elsewhere is not present scattered grains and thin seams of chert. at this locality. At the Eaton ranch locality, the No banded chert layers, typical of the Ale- sandstone is silicified except for remnant nod- man formation which normally overlies the ules that contain the original carbonate cement. Upham, were observed in the area. A few These nodules weather into cavities that com- crinoid stem fragments 1-3 mm in diameter monly give the ledge a coarsely pocked appear- were observed, but no other were found ance (PI. 2, fig. 1). The distinctive Cable Can- here or in other parts of the Cambrian-Ordo- yon exposures and the oolitic hematite seams vician section at the Eaton ranch locality.

TABLE 1. PRINCIPAL PALEOZOIC UNITS, SOUTH-CENTRAL NEW MEXICO

System Contact Unit

Pennsylvania!! Magdalena Group Unconformity Helms Formation Rancheria formation Las Cruces formation Mississippian Kelly formation Lake Valley formation Caballero formation Percha Shale Devonian Sly Gap formation Onate formation Unconformity Silurian Fusselman dolomite f Cutter formation Ordovician „ , Aleman formation Montoya group Upnam aolormte [ Cable Canyon sandstone Unconformity Bat Cave formation Cambrian El Paso group Sierrite limestone Bliss formation Nonconformity Precambrian Granite

in the Bliss are the features which most readily distinguish the lower Paleozoic section. Regional Interpretations The Upham dolomite of the Montoya group At the Eaton ranch locality, the Bliss thick- conformably or gradationally overlies the Cable ness of 30-145 feet compares with thicknesses Canyon sandstone, and it is unconformably of 75-161 feet in the Fra Cristobal Mountains; overlain by the Magdalena Group. It ranges 144 feet in the Mud Springs Mountains; and from about 63 feet thick at the eastern end of 20-30 feet in the northern end of the San the area to about 40 feet at the northwestern Andres Mountains. It is locally variable here end. The exposures are meager along the belt, as well as elsewhere in its full thickness, owing but locally, as at the eastern end and in the to channels of thicker sandstone or conglom- interfluve between the two washes in the west- erate at the base and to thinning over low hills ern part, the upper contact position can be on the Precambrian surface. readily narrowed to a space of 2 or 3 feet. The The Sierrite limestone at the Eaton ranch basal contact is completely exposed in numer- consists of only the lower 75 feet of the forma- ous places. It is usually sharp although the tion; whereas, it is about 150 feet thick in the lower foot or so of the dolomite commonly has Caballo Mountains; 158 feet in the Mud numerous "floating" coarse angular grains of Springs Mountains; and 65-124 feet in the Fra quartz, a feature generally typical in the region Cristobal Mountains. Prior to deposition of the (PI. 2, fig. 2). The dolomite is gray, crystalline, Cable Canyon and younger Ordovician forma-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/6/689/3417354/i0016-7606-76-6-689.pdf by guest on 01 October 2021 SHORT NOTES 693

tions, erosion had removed all the Bat Cave the Fra Cristobal Mountains, the Cable Canyon formation, which is 216-305 feet thick in the was truncated by the Magdalena. From the ranges to the south, as well as part of the relationships depicted in Figure 3, it appears Sierrite. that the Cable Canyon may have originally Kelley and Silver (1952, p. 53, 228-229, fig. stepped all the way down onto the Bliss and 3) showed that similar northward thinning and Precambrian rocks. This did not occur in the wedging out occurred along both the San immediate vicinity as evidenced by the clean- Andres-Oscura uplifts and the Caballo-Fra ness of the sandstone and the absence in it of Cristobal uplifts. Uplift and southward epeiro- hematitic debris characteristic of the Bliss. genic tilting at the end of Early Ordovician The regional relationships supported by the time caused this northward thinning and unique situation at the Eaton ranch locality

T. 7 S. T. 10 S. T. 13 S. T. 16 S. T. 22 S. T. 29 S. w i

a. IA I c s o 5 c* aO. LJS2 Bose Sof UPPER ORDOVICIAN/ (Montoyo) LOWER ORDOVICIAN (El

Vertical Scale Figure 3. Stratigraphic diagram showing El Paso-Bliss wedge beneath the truncating Cable Canyon sandstone, New Mexico. Dashed lines at Fra Cristobal latitude show alternate profile along these mountains as compared with that to the west.

wedging out. The El Paso and Bliss were emphasize the magnitude of the unconformity eroded to a low, stripped surface over which between the Upper and Lower Ordovician. the Middle to Upper Ordovician Montoya Apparently, some 300 feet of El Paso beds may beds were later spread. A low Precambrian have been removed in the process of eroding granitic source far to the north is indicated at the surface upon which the Cable Canyon the beginning of this transgression, and the sandstone rests. Cable Canyon sandstone may represent the It is furthermore worthy of note that the residuum on a broad pediplain of Middle Or- Sierrite limestone preserved here contains no dovician age. Figure 3 illustrates this progres- lithology indicative of a nearby shore line. sive downward truncation of the El Paso at Thus, it appears that no nearby positive area the Montoya base. The similarity of this strati- existed, and the El Paso may have continued graphic diagram with the one drawn by Kelley many miles to the north if not across the entire and Silver (1952, p. 53) for the longitude of state. the San Andres Mountains as of prime sig- The 30-60 feet of Upham dolomite preserved nificance to the regional paleogeographic and beneath the Pennsylvanian Magdalena Group paleotectonic interpretations. compares with a full section of 65-78 feet in The Eaton ranch stratigraphy is additionally the Caballo Mountains; 51 feet in the Mud significant, for it represents the lowest trunca- Springs Mountains; and 80-90 feet in the Sun tion by the Cable Canyon sandstone (Mo- #1 well east of the Fra Cristobal Mountains hawkian?) that has been found. In the San (Kelley and Furlovv, this paper, fig. 1; Kott- Andres Mountains, the Cable Canyon either lowskij and others, 1956, p. 78). Thus, it ap- pinched out or possibly was beveled out by the pears that a nearly full section is preserved, Upham (Bachman, 1962, p. 121-122), and in especially at the eastern end of the outcrops.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/6/689/3417354/i0016-7606-76-6-689.pdf by guest on 01 October 2021 694 KELLEY AND FURLOW—LOWER PALEOZOIC WEDGE EDGES, NEW MEXICO

However, a thickness of some 250 feet of the ward convexity of the Cambrian-Ordovician remainder of the Montoya that is present in wedge-edge lines (Fig. 1) in the Fra Cristobal the Mud Springs Mountains to the south is Mountains supports this concept further. missing. This part of the Ordovician was most The principal conclusions pertaining to the likely removed in Late Mississippian or Early depositional and deformational history of Pennsylvanian time although the higher part Cambrian-Ordovician time in southern New might have been removed in Silurian and Mexico or along the southern side of the Texas- Devonian times. In any event, little or nothing New Mexico arch (Eardley, 1962, Pis. 3, 4) are in the Upham lithology indicates proximity to as follows: (1) Cambrian-Ordovician sedimen- an actively eroding area, and the Upham as tation extended far to the north; (2) Middle well as the rest of the Montoya, in all proba- Ordovician activity of the Texas-New Mexico bility, originally extended many miles to the arch is emphasized by the "stepping" of the north. Cable Canyon sandstone well down into the Laudon and Bowsher (1949, p. 21) first pos- lower part of the El Paso group; (3) The tulated a north-south arch along the Caballo curvature of the lower Paleozoic wedge-edge axis that was active in Mississippian time. Kel- belt from the Oscura Mountains through the ley and Silver (1952, p. 88, 134) further de- Fra Cristobal Mountains and northward to tailed the axis on the basis of thin to missing Eaton ranch supports the Mississippian wedge- Mississippian strata and to the local truncation edge configuration that indicates north-south by Magdalena beds well down into the Ordo- arching following the Caballo-Fra Cristobal vician. The Mississippian studies of Armstrong trend. (1962, p. 26) support this also, and the south-

References Cited Armstrong, A. K., 1962, Stratigraphy and paleontology of the Mississippian system in southwestern New Mexico and adjacent southeastern Arizona: New Mexico Bur. Mines and Min. Res. Memoir 8, 99 p. Bachman, G. O., 1962, Pre-Pennsylvanian Paleozoic stratigraphy, Mockingbird Gap Quadrangle, New Mexico: U. S. Geol. Survey Prof. Paper 424-B, p. 119-122 Dane, C. H., and Bachman, G. O., 1961, Preliminary geologic map of the southwestern part of New Mexico: U. S. Geol. Survey Misc. Geol. Inv. Map 1-344 Eardley, A. J., 1962, Structural geology of North America: New York, Harper & Row Publishers Inc., 743 p. Kelley, V. C., and Silver, Caswell, 1952, Geology of the Caballo Mountains, with special reference to regional stratigraphy and structure and to mineral resources including oil and gas: Albuquerque, Univ. New Mexico Pub. in Geol. No. 4, 286 p. Kottlowski, F. E., and others, 1956, Stratigraphic studies of the San Andres Mountains, New Mexico: New Mexico Bur. Mines and Min. Res. Memoir 1, 132 p. —•— 1960, Summary of Pennsylvanian sections in southwestern New Mexico and southeastern Arizona: New Mexico Bur. Mines and Min. Res. Bull. 66, 187 p. Laudon, L. R., and Bowsher, A. L., 1949, Mississippian formations of southwestern New Mexico: Geol. Soc. America Bull, v. 60, p. 1-88

DEPT. GEOLOGY, UNIVERSITY OF NEW MEXICO, ALBUQUERQUE, N. MEX. MANUSCRIPT RECEIVED BY THE SOCIETY DECEMBER 7, 1964

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/6/689/3417354/i0016-7606-76-6-689.pdf by guest on 01 October 2021