INTRODUCTION

Ordovician strata have long been recognized in two localities in southeastern Arizona, namely, Clifton-Morenci (FOOTNOTE 4) (Lindgren, 1905) and Dos Cabezas (FOOTNOTE 4) (Darton, 1925; Jones and Bacheller, 1953). More recently Sabins (1957) has described them in the northern Chiricahua Mountains. So little has been known about these rocks, however, that both McKee (1951) in his summary of sedimentary basins in Arizona and Gilluly (1956) in his summary of the geology of Cochise County were obliged to dismiss them with no more than a few sentences. The purpose of the present paper is, first, to record the widespread occurrence of lower Ordovician El Paso limestone east of Sulphur Spring Valley, Arizona, and second, to suggest relationships between the Cambrian and Ordovician strata of s utheastern Arizona and southwestern New Mexico. It is hoped that the descriptions and suggestions presented here will stimulate and assist more detailed investigations by paleontologists and geologists interested in these problems in this region.

REGIONAL STRATIGRAPHIC SETTING

The early Paleozoic stratigraphy of southeastern Arizona has been summarized several times (Ransome, 1916; Darton, 1925; Stoyanow, 1936; Gilluly, 1956). Resting on a deeply eroded pre-Cambrian terrane composed of schist and granitic rocks in a quartz arenite of middle Cambrian age, the Bolsa quartzite. This is overlain by slabby siliceous dolomitic limestone and interbedded micaceous shale and siltstone of middle and upper Cambrian age, the Abrigo limestone of Ransome (1904, pp. 30-33). Stoyanow (1936, pp. 466-71) has made a three-fold subdivision

FOOTNOTE 4. Figure 1 shows the general locations of all areas mentioned in this paper.

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of Ransome's original Abrigo limestone into the Cochise formation (middle Cambrian), the Abrigo formation (upper Cambrian), and the Copper Queen limestone (upper Cambrian). Gilluly (1956), like the present writers, favors the terminology of Ransome, and he points out in addition that the Abrigo limestone terminates upward, in most places, in sandstone or quartzite. These strata or their equivalents have been recognized throughout southeastern Arizona west of the Sulphur Spring Valley, where they are overlain directly by fossiliferous upper Devonian beds, the Martin limestone. The contact between Cambrian and Devonian strata is generally smooth and regular and without angular discordance, but it represents a long interval of non-deposition or erosion.

Fig. 1. Sketch map of southeastern Arizona and southwestern New Mexico showing general locations of areas mentioned in text.

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In southwestern New Mexico, there is a somewhat similar sequence in which the Bliss sandstone, of uppermost Cambrian and lowermost Ordovician age (Flower, 1953a; 1955) lies unconformably on pre-Cambrian rocks. The Bliss sandstone grades upward into lower Ordovician El Paso limestone, which has recently been subdivided by Kelley and Silver (1952) into the Sierrite limestone and overlying Bat Cave formation. In the vicinity of Deming (Darton, 1917; Kelley, and Bogart, 1952), near Cooks Peak (Jicha, 1954), and near Silver City (Paige, 1916), the El Paso limestone (Bat Cave formation) is overlain by upper Ordovician and Silurian formations and these, in turn, are overlain by the upper Devonian Percha shale. Farther west, near the Arizona-New Mexico boundary, upper Ordovician and Silurian trata are lacking, and lower Ordovician El Paso limestone is overlain directly by upper Devonian strata. This contact, too, is generally concordant but represents a long period of non-deposition or erosion.

Darton (1925, pp. 50-53) was impressed by the lithologic similarity between the Abrigo and El Paso limestones, and he was inclined to correlate the two. Recently, Kelley and Silver (1952) and Sabins (1957) have described the Bolsa and Abrigo strata in Arizona and the Bliss and El Paso strata, respectively, in New Mexico as lithogenetic equivalents laid down in a sea transgressing from west to east across the pre-Cambrian terrane. This interpretation was completely reasonable in the light of evidence available heretofore, but the present paper presents facts which must be interpreted otherwise. Final solution of such regional stratigraphic problems is elusive, however, because the exposures of Paleozoic strata in this region are discontinuous and widely scattered.

That the general similarity of the early Paleozoic stratigraphic successions in adjacent parts of southern Arizona and New Mexico has been misleading is shown clearly by the section exposed in the northern Swisshelm Mountains, Arizona. There, in a well exposed and fossiliferous sequence of strata, the Abrigo and El Paso limestones occur at distinctly different levels in the same section. Furthermore, this section can be correlated with others nearby, particularly near Bisbee to the southwest, in the Pedregosa and southern Swisshelm mountains on the southeast, and near Dos Cabezas and Blue Mountain on the north and northeast (Figs. 4 and 5). These correlations indicate that most of the Abrigo limestone of Ransome (1904) pinches out in Arizona under the Chiricahua Mountains and is not t be correlated on either biostratigraphic or lithogenetic grounds with the El Paso limestone in New Mexico. They further suggest that the upper Cambrian sandstone at the top of the Abrigo limestone is lithogenetically equivalent to at least a large part of the Bliss sandstone in New Mexico and that the Bolsa quartzite and the Bliss sandstone therefore cannot be regarded as strict lithogenetic equivalents.

Figure 1 indicates localities that are significant in this discussion, and on the following pages reference is made to the early Paleozoic stratigraphy at each of them. Because the primary locus of the present study was the Pedregosa and southern Swisshelm mountains, that area is described first and most fully.

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PEDREGOSA AND SOUTHERN SWISSHELM MOUNTAINS

OCCURRENCE OF EL PASO LIMESTONE

Limestone and dolomite identified as El Paso are to be found throughout the Pedregosa and southern Swisshelm mountains. Exposures of these strata, however, are restricted to several separate localities by faulting and the widespread presence of younger rocks. The most extensive of these outcrops are indicated on the accompanying map (Fig. 2).

Sections at the following three localities are representative of the El Paso formation in this region and are specifically described in this paper. 1. Half a mile east of Deer Mountain, Chiricahua Quadrangle (Sec. 21, T. 20 S., R. 29 E.). Here the section is faulted along both contacts and is not easily accessible, but it is comparatively fossiliferous and is the only Ordovician section exposed in the

Fig. 2. Sketch map of Pedregosa and Swisshelm mountains, Cochise County, Arizona, showing principal outcrops of El Paso limestone. Numbers 1, 2, 3, and 4 refer to measured sections described in text.

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northern Pedregosa Mountains. 2. Boss Ranch, Chiricahua Quadrangle (southwest part of Sec. 29 and northwest part of Sec. 32, T. 21 S., R. 30 E.). This section is complete, well exposed, and easily measured, but it has been dolomitized and nearly all fossils have been destroyed. 3. Leslie Pass area, 1 mile south of Leslie Canyon road, Pearce Quadrangle (Sec. 29, T. 21 S., R. 28 E). This section is complete, easily accessible, and sparsely fossiliferous; typical El Paso limestone predominates although the rocks have been locally dolomitized and silicified.

A fourth locality deserves special mention but is not described here in detail. It lies south of Big Bend Creek, Chiricahua Quadrangle (Secs. 25 and 26, T. 21 S., R. 28 E.). Thickness measurements of the El Paso limestone in this area are unreliable because of complex faulting, but typical El Paso strata are well exposed and fossiliferous and all parts of the formation are visible.

SECTION EAST OF DEER MOUNTAIN

(Loc. 1, Fig. 2; Sec. 21, T. 20 S., R. 29 E.)

Table

SECTION AT BOSS RANCH

(Loc. 2, Fig. 2; Sec. 32, T. 21 S., R. 30 E.)

Table

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SECTION NEAR LESLIE PASS

(Loc. 3, Fig. 2; Secs. 28 and 29, T. 21 S., R. 28 E.)

Table

LITHOLOGY OF EL PASO LIMESTONE

The El Paso limestone consists of slabby gray strata which range from several inches to 2 feet in thickness. Except near the base where it grades into sandstone, the formation is composed entirely of carbonate rocks, limestone or dolomite; and wherever it occurs at the surface it crops out very well.

The formation can usually be identified, even in the absence of diagnostic fossils, by the distinctive character of its limestone strata. Many of these are a type of intraformational conglomerate and consist of angular to subrounded fragments of dense limestone, from several millimeters to a centimeter or two in diameter, set in a matrix of fine-grained calcite. The clastic texture of these beds is revealed by etching and is clearly evident in the field on most weathered surfaces. Indeed, this texture is one of the characteristic and distinctive features of the formation (Pls. 1 and 2a). Interbedded with the coarsely clastic limestones are other equally numerous strata which are uniformly aphanitic or fine-grained. Some of these are silty and finely laminated or cross-laminated, and a few of them can be seen to end by breaking into small slabs and thus changing into intraformational conglomerate.

Weathering of typical El Paso limestone produces a light gray surface having a distinctly bluish cast. But most beds are also abundantly marked by dolomitic reticulations that weather to an orange-brown color and contrast strikingly with the blue-gray of the weathered limestone matrix. These brown markings are spongy masses of fine-grained dolomite and, in some beds, cryptocrystalline

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silica. They are oval or irregular in shape but always tend to have their maximum dimensions in the plane of the stratification. Individual markings range from ¼ to 1 inch in length and are frequently interconnected by a network of thin dolomitic stringers (Pl. 2b, c). Dolomitic mottling of very similar appearance has been recently described by Beales (1953) from Devonian limestone in Alberta.

In general, the El Paso limestone is not cherty, but lenses and irregular thin layers of white or brown chert oriented parallel with the bedding are numerous and characteristic at several horizons in the formation. These are probably primary features of the deposits. Thin stringers of chert and irregular masses of

Click to view image in GIF format. PL. 1. [Grey Scale] Coarse clastic texture in El Paso limestone.

1.A. Intraformational conglomerate consisting of limestone fragments cemented by fine-grained calcite as seen on bedding surface. Scale in centimeters.

1.B. Edge of same specimen (A) showing irregular contact between intraformational conglomerate above and laminated fine-grained limestone below. Fragments in conglomerate are composed of limestone generally similar to that in lower layer of specimen.

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Click to view image in GIF format. PL. 2. [Grey Scale] Character of El Paso limestone.

2.A. Fine-grained laminated limestone interbedded with intraformational conglomerate. Knife equals 3½ inches.

2.B. Bedding surface of limestone etched by weathering; dolomitic reticulations stand out in relief.

2.C. Detail of dolomitic mottling in El Paso limestone; light-colored patches are dolomite. Scale in centimeters.

2.D. Surface of thin bed containing very abundant low-spired gastropods. Scale in centimeters.

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silicified limestone cutting across the stratification are common locally and were probably formed later by processes unrelated to sedimentation.

The upper Cambrian Abrigo limestone in southern Arizona has often been compared or correlated with the El Paso limestone on lithologic grounds, for it too is characterized by thin-bedded clastic limestones mottled by siliceous dolomite. But whereas all of the El Paso strata are carbonate rocks, the Abrigo limestone is abundantly interstratified with micaceous siliceous shale and micaceous siltstone. These peculiarities differentiate the two formations and it is particularly striking where both occur in the same stratigraphic section, as they do in the northern Swisshelm Mountains.

FOSSILS AND AGE OF EL PASO LIMESTONE

El Paso limestone is locally very fossiliferous, and some thin beds are largely composed of low-spired gastropods (Pl. 2d). The fauna consists chiefly of these gastropods together with less numerous straight cephalopods. Identifiable specimens have been obtained wherever non-dolomitized El Paso limestone is exposed. All of these collections are generally similar and indicate a lower Ordovician age. The following have been identified.

Vaginoceras sp.
Endoceroid sp.
Piloceroid sp.
Ophileta sp.
Pachendoceras sp.
Low-spired gastropods
Straight cephalopods
Trilobite cephalon and fragments
Endoceroid fragments

Regarding this assemblage, R. L. Langenheim (FOOTNOTE 5) concludes:

The collection is definitely of early Canadian age and is probably biostratigraphically equivalent to the Roubidoux formation. According to Twenhofel et al. (1954), piloceroid cephalopods range through the biostratigraphic equivalents of the Roubidoux, Jefferson City, Cotter-Smithville, and possibly higher formations of the Ozark Mountains section. Pachendoceras s. l., however, is known only from biostratigraphic equivalents of the Gasconade and Roubidoux formations (Ulrich et al., 1944). Ophileta ranges through biostratigraphic equivalents of the Gasconade to Cotter-Smithville formations (Twenhofel et al., 1954).

The stratigraphic positions of fossil specimens are recorded on preceding pages in the descriptions of measured sections and also graphically in Figures 4 and 5. It is particularly noteworthy that one of the critical specimens, Pachendoceras, was found in El Paso limestone south of Big Bend Creek (Sec. 26, T. 21 S., R. 28 E.) only 25 feet stratigraphically below the contact with overlying Devonian strata. Since this genus is known only from equivalents of the Gasconade and Roubidoux formations, its occurrence here suggests that the El Paso limestone in the Pedregosa and Swisshelm mountains is equivalent, in a biostratigraphic sense, only to the lower part of the type El Paso in the Franklin Mountains of Texas (Cloud and Barnes, 1948).

STRATA UNDERLYING EL PASO LIMESTONE

Finely crystalline, buff-weathering, gray dolomite lies beneath typical El Paso limestone in the Pedregosa and Swisshelm mountains. This, in turn, grades

FOOTNOTE 5. Department of Paleontology, University of California, Berkeley.

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downward through sandy dolomite into cross-bedded dolomitic quartz sandstone and orthoquartzite. The dolomite horizon between typical fossiliferous El Paso limestone and underlying sandstone is approximately 200 feet thick and forms a distinctive lithologic unit. It is widespread and uniform and is not to be confused with locally dolomitized El Paso limestone. The latter has a similar superficial appearance, as at Boss Ranch and also in the northern Chiricahua Mountains but occurs at a higher stratigraphic horizon; and on close inspection, one can see the clastic texture and mottled structure typical of El Paso limestone, together with traces of gastropods and cephalopods, faintly preserved in a few beds of the dolomitized rock. The lower dolomite, on the other hand, has a uniformly f ne granular texture, varied in some strata only by chert lenticles or numerous grains of quartz sand.

In the Pedregosa and southern Swisshelm mountains, the dolomite strata below El Paso limestone have not yielded a single fossil, but this same dolomite unit can be recognized in the northern Swisshelm Mountains and at Black Gap near Bisbee, where Billingsella spp. occurs near the base of the unit. Sabins (1957) has also reported Billingsella from the base of a similar dolomite at Blue Mountain in the northern Chiricahua Mountains. These occurrences suggest that the brachiopod Billingsella defines a regional zonule which is included in the uppermost Abrigo limestone at Bisbee (Ransome, 1904) and lies at the base of, or beneath, the El Paso limestone (Fig. 4). Billingsella signifies for this dolomite a very late Cambrian age (Franconian or Trempealeauan). A similar age has been assigned to the lower part of the Bliss sandstone in south-central New Mexico (Flower, 1953a; 1955).

Nowhere in the Pedregosa or southern Swisshelm mountains is the base of the Paleozoic section exposed. The oldest visible formation is non-fossiliferous quartz sandstone or orthoquartzite which, on lithologic grounds alone, might be correlated either with the Bliss sandstone in New Mexico or with the Bolsa quartzite in Arizona. However, its gradation upward into the dolomite which elsewhere contains the Billingsella zonule identifies it with similar sandstones which overlie Abrigo limestone in the northern Swisshelm Mountains and at Black Gap. Since the Bolsa quartzite is older than the Abrigo limestone, this sandstone cannot be Bolsa and is more logically considered the lithogenetic equivalent of at least part of the Bliss sandstone in New Mexico. These relationships are evident wher ver the Abrigo limestone is present between the upper Cambrian sandstone and the underlying Bolsa quartzite, as at Black Gap and in the northern Swisshelm Mountains; but elsewhere, as at Dos Cabezas and the sections in the northern Chiricahua Mountains, a single thick sandstone occurs at the base of the Paleozoic sequence, and its relationships are ambiguous. No correlation of these latter sandstones can be demonstrated beyond doubt, because fossils are lacking and exposures are discontinuous, but the present writers believe that they are mostly, if not entirely, Bliss equivalents, although they have been called Bolsa in the past. Our interpretations are shown graphically in Figures 4 and 5.

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From comparison of the sequences exposed in the northern and southern parts of the Swisshelm Mountains, a considerable thickness of Abrigo limestone is presumed to lie buried beneath the exposed Paleozoic strata in the southern Swisshelm Mountains (Fig. 5). Under the Pedregosa Mountains at Boss Ranch, the Abrigo limestone should be thinner, or even lacking, for the formation thins markedly toward the east.

NORTHERN SWISSHELM MOUNTAINS

For the regional stratigraphy of lower Paleozoic rocks, the northern part of the Swisshelm Mountains is one of the most significant areas in southeastern Arizona. Here, a stratigraphic succession beginning with pre-Cambrian granitic rock continues unbroken through Bolsa quartzite, Abrigo limestone, upper Cambrian sandstone and dolomite, and El Paso limestone, and these, in turn, are overlain by fossiliferous upper Devonian beds. The present authors have investigated the northern Swisshelm Mountains only in reconnaissance fashion, but Ordovician rocks are well exposed and easily studied in the area shown on the accompanying sketch map and sections (Fig. 3). Although this area is broken by numerous faults, the lithologic sequence is distinctive and fossils are sufficiently abundant to e tablish the stratigraphic succession here described.

SECTION EXPOSED IN SEC. 2 (UNSURVEYED), T. 20 S., R. 27 E.

(Loc. 4, Fig. 2)

Table

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Fig. 3. Sketch map and cross sections of part of northern Swisshelm Mountains (Loc. 4, Fig. 2) showing relations of early Paleozoic strata. Upper Cambrian dolomite containing Billingsella has been included in base of El Paso limestone.

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Table

The general lithologic characteristics of El Paso strata are similar in both northern and southern parts of the Swisshelm Mountains and in the Pedregosa Mountains. The formation is thinner in the northern Swisshelms, but its physical features are so distinctive that, even without fossils, the field geologist would not be likely to doubt its identity. The upper Cambrian sandstone below the El Paso limestone in the northern Swisshelms, however, is much more dolomitic and less quartzitic than it is farther south.

Fossils collected from the El Paso limestone in the northern Swisshelm Mountains consist of low-spired gastropods and less numerous straight cephalopods, including Ophileta sp., Vaginoceras sp., piloceroid cephalopods, and undescribed genus of Protocameroceratidae, and several unidentified straight cephalopods. These are similar to those collected in the southern Swisshelm and Pedregosa mountains, and no difference in age is indicated.

In sandy dolomite 12 feet above the upper Cambrian sandstone in the northern Swisshelm Mountains, a single thin but persistent bed contains numerous small silicified brachiopods, Billingsella spp., indicating uppermost Cambrian age. These brachiopods are identical as to form and mode of occurrence with others in a similar lithologic unit at Black Gap (Bisbee District) about 35 miles southwest; and recently Billingsella sp. has been reported from the base of a similar dolomite on Blue Mountain about 30 miles northeast (Sabins, 1957). This brachiopod seems to define a regional zonule present in the uppermost Abrigo limestone at Bisbee and at the base of the El Paso limestone elsewhere.

The Cambrian strata in the northern Swisshelm Mountains were measured and briefly described by Gilluly (1956, p. 19 and Pl. 8) at a locality which is probably

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the same as the one described in the present paper.(FOOTNOTE 6) The top of Gilluly's section is in the dolomite beneath the El Paso limestone at a horizon approximately 100 feet stratigraphically above the bed containing Billingsella (writers' B-4101; USGS No. 749). That these strata represent ages from middle through upper Cambrian is clearly established by the fauna collected from them and described by Palmer (Gilluly, 1956, pp. 20-24). Gilluly applied the name Abrigo to all of these Cambrian strata above the Bolsa quartzite, and the writers agree that they correlate with the Abrigo limestone of Ransome (1904) at Bisbee. However, the upper part of the Cambrian sequence in the Swisshelm Mountains, consisting of cross-bedded dolomitic sandstone and overlying sandy dolomite, can also b correlated in a lithogenetic sense with at least part of the Bliss sandstone and lowermost El Paso limestone in New Mexico (Fig. 4).

BLACK GAP AREA

Black Gap is in the Bisbee District directly south of the town of Warren and about 3½ miles southeast of the center of Bisbee itself. Ransome (1904, Pl. 1) showed correctly the general geologic setting at this locality, but he gave no description of the stratigraphic section exposed there. On the hill just east of Black Gap, shaly siliceous thin-bedded limestone typical of the Abrigo formation is overlain conformably by coarse-grained cross-bedded dolomitic sandstone which weathers dark reddish brown and forms a prominent cliff. This sandstone is about 100 feet thick and is very similar, except that some beds are glauconitic, to the upper Cambrian sandstone in the northern Swisshelm Mountains. Above it is a series of sandy dolomite strata about 130 feet thick, including near the ase a thin layer which contains abundant silicified brachiopods identified by R. L. Langenheim as Billingsella spp. (B-4095). Billingsella has also been reported by Stoyanow (1936, p. 470) from the base of his Copper Queen limestone (uppermost Abrigo of Ransome, 1904) on the slopes of Mt. Martin about 4 miles northwest; and specimens of Billingsella (B-4101) identical with those at Black Gap have been collected by the present writers from dolomite a few feet above the upper Cambrian sandstone in the northern Swisshelm Mountains. At Black Gap, the El Paso limestone is lacking, and fossiliferous Martin limestone (Upper Devonian) directly overlies the sandy dolomite unit in which Billingsella occurs.

It is strange that specific mention of this section, both well exposed and easily accessible, has never appeared in the literature, especially in view of its difference from the classic section on Escabrosa Ridge directly west of Bisbee. In the latter area, thin-bedded siliceous Abrigo limestone and shale are overlain by about 100 feet of light gray, cliff-forming sandy limestone (the Copper Queen limestone of Stoyanow, 1936) topped by a white quartzite several feet thick. All of these beds were included by Ransome (1904, pp. 30-33) in his original description of the

FOOTNOTE 6. Personal communication. Our locality in Sec. 2 (unsurveyed), T. 20 S., R. 27 E., Pearce Quadrangle, is about 3 miles south-southeast of Whitehead Ranch as designated by Gilluly on Pl. 8 of his report. The section measured by Gilluly was approximately along line AB in the writers' Figure 3, and here the fossiliferous Ordovician limestone is missing.

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Abrigo limestone, and he recognized the fact that the strata near the top of the Abrigo (including Stoyanow's Copper Queen limestone) become more sandy and dolomitic to the southeast, in the direction of Black Gap. From his description, however, one could not anticipate the marked difference to be seen there. In fact, the upper Cambrian section at Black Gap is more closely comparable in lithologic features with the section in the northern Swisshelm Mountains, and correlation of the two is clearly indicated (Fig. 4).

SUGGESTED CORRELATIONS

The lithologic characteristics, stratigraphic succession, and fossils of the early Paleozoic strata in southeastern Cochise County, which have been briefly described on preceding pages, are sufficiently distinctive to suggest general correlations. The broader regional relationships of these strata are suggested by comparison of the stratigraphy in southeastern Cochise County with that in other localities farther north and east. Several of these latter localities are briefly described and the suggested correlations and relationships are summarized graphically in Figures 4 and 5 by stratigraphic columns arranged along northeast-southwest and north-south lines through southeastern Arizona and southwestern New Mexico.

Near Dos Cabezas, in northern Cochise County, Arizona, early Ordovician limestone has been reported by Darton (1925), by Stoyanow (1936), and by Jones and Bacheller (1953). Here, beneath upper Devonian calcareous siltstone and shale, there are approximately 100 feet of thin-bedded, blue-gray, dolomitic limestone containing early Ordovician gastropods and endoceroid siphuncles. These strata are considered by Flower (1953b) to be of Roubidoux age and biostratigraphically equivalent to the lowest part of the Bat Cave formation (El Paso) in New Mexico. They are also lithologically and faunally similar to the El Paso limestone in the Pedregosa and Swisshelm mountains, which Langenheim considers to be of similar age. The Ordovician limestone at Dos Cabezas overlies about 250 feet of gray an buff dolomite and sandy dolomite which is strikingly similar to that beneath the El Paso limestone in the northern Swisshelm Mountains. The oldest unit in the Paleozoic sequence at Dos Cabezas consists of about 400 feet of cross-bedded dolomitic sandstone and orthoquartzite resting on pre-Cambrian schist.

The close lithologic similarity of the El Paso limestone and uppermost Cambrian strata in the Swisshelm Mountains with the pre-Devonian Paleozoic succession at Dos Cabezas is reasonable basis for their correlation (Fig. 5). With this assumption, the name Abrigo is no longer warranted for the fossiliferous Ordovician limestone beneath the Devonian beds at Dos Cabezas, although the sandy dolomite beneath this limestone is probably equivalent to the uppermost part of the Abrigo limestone at Bisbee (Ransome, 1904). Correlation of the Ordovician beds at Dos Cabezas with the Longfellow limestone at Morenci (Lindgren, 1905) has been suggested by Darton (1925) and by Jones and Bacheller

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Fig. 4. Suggested correlations and relations of early Paleozoic strata between Bisbee, Arizona, and Silver City, New Mexico.

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(1953) and is indicated in Figure 5. As suggested by Sabins (1957), however, the more widely used term El Paso is a more suitable designation for these strata.

In the northern Chiricahua Mountains, Sabins (1957) has described the equivalent early Paleozoic stratigraphic succession, at Apache Pass about 12 miles southeast of Dos Cabezas and also farther southeast at Blue Mountain. In both of these localities, there is a basal Paleozoic orthoquartzite, which Sabins calls Bolsa, and an overlying pre-Devonian dolomite several hundred feet thick, which Sabins calls El Paso. Designation of the latter strata as El Paso is based on the presence of a middle Canadian nautiloid in the strata at Apache Pass and on the general similarity of the lithologic sequence with that a few miles east of Blue Mountain in the Peloncillo Mountains of New Mexico, where Darton (1928) and Quaide (1953) have described El Paso strata. At Blue Mountain, beds of sandy dolom te overlying the basal quartzite are uppermost Cambrian in age, for they contain Billingsella sp. (Sabins, 1957).

Near Portal, about 7 miles southeast of Blue Mountain, the early Paleozoic rocks are thicker but otherwise similar to those at Blue Mountain (Raydon, 1953; Sabins, 1957). Here, the basal quartzite is overlain by a series of tan and gray, locally cherty, dolomite beds. Raydon (1953) reports poorly preserved gastropods and nautiloid siphuncles in the upper half of this dolomite, and its correlation with the El Paso limestone is established by the occurrence in one locality north of Portal of non-dolomitized fossiliferous strata characteristic in every way of El Paso limestones elsewhere. These strata are surely to be correlated with the dolomite at Blue Mountain (Sabins, 1957), where the presence of Billingsella sp.

Fig. 5. Suggested correlations and relations of early Paleozoic strata between Swisshelm Mountains and Morenci, southeastern Arizona. For legend, see Figure 4.

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shows that the lowest dolomite beds are of uppermost Cambrian age. Raydon (1953) applied the name Abrigo to the lower portion of the dolomite sequence near Portal, and the occurrence of Billingsella at this horizon on Blue Mountain now justifies the biostratigraphic correlation of these beds with the uppermost Abrigo strata at Bisbee. From a lithogenetic standpoint, however, as indicated by Sabins (1957), this entire dolomite sequence is more appropriately called El Paso. Southwestward, as shown in Figure 4, these beds can be reasonably correlated with the dolomite and limestone overlying the upper Cambrian sandstone in the northern Swisshelm Mountains.

In the northern Chiricahua Mountains, general dolomitization of El Paso limestone is characteristic, with consequent destruction of most of its fossils. In this respect, these strata are very similar to the El Paso of the Pedregosa Mountains near Boss Ranch (loc. 2, Fig. 2).

In the central Peloncillo Mountains, on the western border of New Mexico southwest of Lordsburg, the following early Paleozoic stratigraphic succession has been described by Quaide (1953).

Table

The El Paso limestone of the central Peloncillo Mountains is similar both lithologically and faunally to the limestones underlying Devonian strata in the Pedregosa, Swisshelm, and northern Chiricahua mountains, and correlation of all these occurrences is suggested. Eastward, it is reasonable to correlate this limestone with El Paso strata near Silver City (Paige, 1916), near Cooks Peak (Jicha, 1954), and in the Florida Mountains (Darton, 1917).

REGIONAL PALEOGEOGRAPHY

Judged by the distribution and thickness of Cambrian strata (McKee, 1951), the first Paleozoic basin of deposition in this region included most of southeastern Arizona and had its deepest part southwest of Bisbee. Middle Cambrian seas covered most of Cochise County, but not until uppermost Cambrian time did marine deposition extend eastward into southwestern New Mexico. Thereafter, the site of maximum deposition shifted eastward, and thick accumulations of

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Ordovician and Silurian carbonate rocks were laid down in southern New Mexico. There is no evidence, other than the relatively thin lower Ordovician strata mentioned in this paper, to suggest that similar deposits were laid down in Arizona (Gilluly, 1956, p. 25). Alternatively, Kelley and Silver (1952) have suggested that the thinness or absence of Ordovician and Silurian rocks in southern Arizona may be due to widespread pre-Devonian erosion resulting from epeirogenic tilting of eastern Arizona toward the east and southeast. Such widespread tilting as would be required for complete removal of these strata, however, would be expected to result in stripping of Cambrian strata also, especially farther west, with the consequence that Devonian beds should in some places rest directly on t e pre-Cambrian basement. Actually upper Cambrian strata constitute a continuous rather uniform blanket beneath Devonian rocks in all exposed sections in southern Arizona for more than 100 miles west of the New Mexico boundary.

In his summary of the geology of Cochise County, Gilluly (1956, p. 25) emphasizes that the uppermost strata of the Abrigo limestone generally are sandstones or quartzites, so that the Cambrian limestone is both underlain and overlain by sandstone. He considers that these strata represent a single sedimentary cycle, with the Bolsa quartzite representing a transgressive phase of marine invasion from the west or southwest and the sandstones at the top of the Abrigo representing the regressive phase.

To the writers it appears that the area including what is now Sulphur Spring Valley and the boundary between southern Arizona and New Mexico was the site of minimum deposition in this general region during early Paleozoic time. The total of accumulated early Paleozoic strata within this area is less than it is either east or west. Southwestward, subsidence was pronounced and thick deposits accumulated during the Cambrian period; eastward and southeastward, subsidence and deposition were pronounced during the Ordovician and Silurian periods. The eastern margin of southern Arizona was thus a sort of hinge area. It probably remained relatively high throughout early Paleozoic time, although it was submerged by shallow seas at least during late Cambrian and early Ordovician times.

Subsequently the late Devonian and Mississippian seas spread across the entire region, but the deposits of these younger periods also seem to reflect the continued influence of this hinge area. Here, there is a general change in the Devonian lithofacies from east to west (Epis, Gilbert, and Langenheim, 1957); the Mississippian Escabrosa limestone becomes very much thinner westward across the area; and within it occurs the western limit of the upper Mississippian Paradise formation.