PRE-PERMIAN ROCKS OF THE SUBSURFACE
METHODS OF STUDY
The following description of pre-Permian rocks is based primarily on the examination of samples and sample logs of material from eight wells drilled in, and adjacent to, the geologic map area. For comparison, logs of wells from nearby areas were examined, and outcrops of pre-Permian rocks in the Franklin Mountains, El Paso County, Tex. (fig. 2), were studied and those in other areas were visited.
The methods of study and the terms used for describing well cuttings differ from the methods of study and terms used for description of rocks on the outcrop. For example, the bedding characteristics of rocks in the subsurface cannot be determined, whereas they are an important attribute of rocks exposed on the surface. On the other hand, the color of the rock is not affected by weathering in cuttings as commonly as it is in outcrops.
In the descriptions of subsurface rocks, the following terms and methods of study are used. The Wentworth (1922) scale is used for grain sizes of clastic rocks. Carbonate rocks are termed "coarse-crystalline" if the crystallinity can be seen with the naked eye, "medium crystalline" if it cannot be seen without a hand lens or low-powered microscope, and "fine-crystalline" if the crystallinity cannot be seen without high-power magnification. Rock colors are generalized from the National Research Council "Rock-Color Chart" (Goddard and others, 1948). The composition of carbonate rocks was estimated by the degree of effervescence detectable under the microscope when cold dilute hydrochloric acid was applied to the sample. If effervescence was extreme, the sample was called limestone; if there was no effervescence, it was called dolomite. Moderate effervescence indicated dolomitic limestone; slight effervescence indicated calcareous dolomite.
Precambrian rocks have been penetrated in three wells drilled in the report area. In the Humble Oil and Refining Co. Huapache Unit 2 (sec. 23, T. 23 S., R. 22 E.) the Precambrian cuttings contain a high percentage of quartz and biotite and are interpreted to represent a granitic rock, possibly a granite pegmatite. Precambrian rock found in the Humble Huapache 1 (sec. 35, T. 23 S., R. 22 E.) was reported to be granite also. Both these wells are near the west margin of the Texas craton as delineated by Flawn (1956, pl. 1) and serve to substantiate the existence of granitic rocks at least as far west as the Guadalupe Mountains. The Precambrian rocks penetrated in the Magnolia Petroleum Co. State "W" 1 (sec. 16, T. 21 S., R. 22 E.) were identified by Flawn (1956, p. 215-216) as chlorite phyllite and metabasalt (?). Apparently all but the northwestern part of the area of this report is underlain by granitic rocks (Flawn, 1956, pl. 1). The highest Precambrian rocks in the northwestern part of the area, according to Flawn (1956, pl. 1), are metamorphosed sedimentary and volcanic rocks.
Exposures of Precambrian rocks nearest to the report area are in the Sierra Diablo, 50 miles to the south (King, P. B., and Knight, J. B., 1944; King, P. B., and Flawn, P. T., 1953, p. 71-89), and probably in the Pump Station Hills, about 40 miles to the southwest (King, P. B., 1949; King, P. B., and Flawn, P. T., 1953, p. 123-124; Masson, 1956, p. 504).
The oldest sedimentary unit overlying the Precambrian crystalline basement in the mapped area has been penetrated in three wells and is correlated with the Bliss Sandstone. The Bliss was originally described by Richardson (1904, p. 27) for exposures in the Franklin Mountains, Tex., about 110 miles to the west-southwest. The exposures of the Bliss nearest to the area of this report are in the Baylor Mountains, Tex., about 70 miles south-southwest (King, P. B., 1940, p. 153; King, P. B., and Knight, J. B., 1944), and in the Sacramento Mountains, N. Mex., about 70 miles west-northwest (Pray, 1954, p. 94; fig. 2).
The Bliss Sandstone ranges in thickness from about 23 feet in the Humble Huapache 1 well (sec. 35, T. 23 S., R. 22 E.) to about 100 feet in the Magnolia State "W" 1 (sec. 16, T. 21 S., R. 22 E.). It consists predominantly of light-gray to white rather poorly sorted coarse quartz sandstone at the base and top, separated by medium-gray fine- to medium-crystalline rather sandy dolomite (pl. 2).
The age of the Bliss Sandstone is Early Ordovician. Richardson (1904, p. 27) tentatively assigned a Cambrian age to the formation in the Franklin Mountains. P. B. King (1940, p. 155), on the basis of fossils found near Van Horn and elsewhere, classified the Bliss as Ordovician. Kelley (1951, p. 2203-2205) restated the case for a Late Cambrian age, and Flower (1953, p. 2054-2055) presented new evidence for an Ordovician age of part of the Bliss and concluded that it may range from Late Cambrian to Early Ordovician in age. However, according to Flower (1959, p. 158), only Ordovician fossils have been found in its easternmost outcrops; so it is very likely that the Bliss in the subsurface of the report area is entirely Ordovician in age.
EL PASO FORMATION
Overlying the Bliss Sandstone apparently without angular discordance is a sequence of rocks correlated with the El Paso Formation of Early Ordovician age. The El Paso was originally defined and later redefined by Richardson (1904, p. 29; 1908, p. 476-479) on the basis of exposures in the Franklin and Hueco Mountains, El Paso County, Tex. The exposures of El Paso Formation nearest to wells in the area of this report are in the Sierra Diablo, Tex. (King, P. B., and Knight, J. B., 1944), about 55 miles to the south-southwest (fig. 2). Some workers, especially those whose experience has been primarily with subsurface work in Texas, have used the term Ellenburger Group for the rocks herein referred to as El Paso; but because of the proximity of surface outcrops of the El Paso and close lithologic resemblances, the term El Paso is used in this area. Kelley and Silver (1952, p. 42) proposed raising the El Paso Formation to a group consisting of the Sierrite Limestone at the base and the overlying Bat Cave Limestone at the top. In the mapped area, however, this subdivision cannot be made with any certainty.
The thickness of the El Paso Formation in the report area ranges from 520 feet in the Magnolia State "W" 1 (sec. 16, T. 21 S., R. 22 E.) to 550 feet in the Humble Huapache 2 (sec. 23, T. 23 S., R. 22 E.). The formation consists predominantly of medium-gray fine- to medium-crystalline generally siliceous dolomite. Slightly sandy dolomite is common especially near the base and top. White to light-gray aphanitic chert is common but not abundant (pl. 2).
ROCKS CORRELATED WITH THE SIMPSON GROUP
Overlying the El Paso Formation in some wells is dark-gray or dark-greenish-gray shale and light-gray, coarse- to fine-grained sandstone, which is tentatively correlated with the Simpson Group of Oklahoma of Middle Ordovician age. The Simpson was originally named by Taff (1902) for exposures in south-central Oklahoma. The term is now widely used for subsurface equivalents in western Texas and southeastern New Mexico (Jones, T. S., 1953, p. 7-10). The nearest exposures of rocks that have been referred to the Simpson are in the Sierra Diablo (Jones, T. S., 1953, p. 7) and Baylor Mountains (West Texas Geol. Soc., 1949, p. 20; Howe, 1959, p. 2289) about 70 and 85 miles to the south, respectively (fig. 2).
In the Humble Huapache 2 well (sec. 23, T. 23 S., R. 22 E.) the rocks tentatively correlated with the Simpson Group are about 40 feet thick (pl. 2), and in the Humble Huapache 4-1 (sec. 14, T. 24 S., R. 22 E.) the group is reported to be about 70 feet thick. In the Humble Huapache 3 (sec. 22, T. 24 S., R. 22 E.) equivalent rocks are absent, and they are apparently absent also in the Continental Oil Co-Standard Oil Co. of Texas Bass 1 (sec. 5, T. 22 S. R. 21 E.). The Simpson equivalent is reported to be 186 feet thick in the Superior Oil Co. Federal 1-134 (sec. 12, T. 26 S., R. 24 E.) in the southeastern part of the report area. About 1 mile east of the report area, in the Humble Wiggs 1 (sec. 31, T. 24 S., R. 27 E.) about 250 feet of rocks are tentatively correlated with the Simpson Group. The western wedge edge of the sequence apparently underlies the report area, and the sequence thickens in an easterly direction at an average rate of nearly 10 feet per mile.
Rocks that are here correlated with the Montoya Dolomite of Late Ordovician age overlie either the El Paso Formation or the rocks correlated with the Simpson Group and have been completely penetrated by six or possibly seven wells in the mapped area. The Montoya was originally named Montoya Limestone by Richardson (1908, p. 476, 478-479) for exposures in the Franklin Mountains, Tex. (fig. 2). On the basis of work in the Caballo Mountains, N. Mex., the Montoya was considered a group by Kelley and Silver (1952, p. 57). The divisions of Kelley and Silver have since been described by Kottlowski and others (1956, p. 23) as members in the San Andres Mountains, N. Mex., about 110 miles to the west. More recently, Howe (1959) has recognized and described several of the subdivisions of the Montoya even closer to the present area in the Sacramento, Hueco, and Baylor Mountains. The nearest known exposures of the Montoya to the mapped area are in the Sierra Diablo, Tex. (King, P. B., and Knight, J. B., 1944) about 55 miles to the south-southwest (fig. 2).
The basal member of the Montoya in the Caballo Mountains is the Cable Canyon Sandstone (Kelley and Silver, 1952, p. 58-59). This member is apparently absent in the report area; however, some of the coarse sandstone at the top of the sequenceherein tentatively correlated with the underlying Simpson Group equivalentmay represent an eastern remnant of the Cable Canyon.
The oldest rocks underlying the mapped area that are definitely a part of the Montoya Dolomite are here correlated with the Upham Dolomite (Kelley and Silver, 1952, p. 59-60), which is now considered a member of the Montoya. Where rocks correlated with the Simpson Group are absent, the Upham Member lies directly on the El Paso Formation and the contact is difficult to place. Because of this and because there are apparent structural complications in or near this interval in several wells, the thickness of the Upham can be determined in the Humble Huapache 2 (sec. 23, T. 23 S., R. 22 E.) only. Here it consists of about 60 feet of medium- to light-gray fine- to medium-crystalline calcareous dolomite (pl. 2). In the Humble Wiggs 1 (sec. 31, T. 24 S., R. 27 E.), just east of the mapped area, the Upham is about 80 feet thick.
Overlying the Upham Member is a distinctive cherty unit correlated with the Aleman Cherty Member of the Montoya Dolomite, formerly called the Aleman Formation (Kelley and Silver, 1952, p. 60-62). The Aleman ranges in thickness from 60 feet to possibly as much as 100 feet and consists of medium to light-gray fine-crystalline dolomite containing 10 to 50 percent or more of white to very light gray porcelaneous chert (pl. 2).
In the Franklin Mountains, Tex., the Aleman Member is overlain by a sequence of rocks which Richardson (1908, p. 476) may or may not have included as a part of the Montoya, and which Darton (1917, p. 43) included in the overlying Fusselman. Kelley and Silver (1952, p. 62-64) named this unit the Cutter Formation and included it in their Montoya Group. The Cutter is now considered a member of the Montoya Dolomite. The rocks in the mapped area that are correlated with the Cutter Member range in thickness from 95 feet in the Continental Oil Co. East Texas Hill 1 well (sec. 1, T. 22 S., R. 21 E.) to more than 140 feet in the Humble Huapache 2 (sec. 23, T. 23 S., R. 22 E.) and consist of medium-gray fine-crystalline dolomite (pl. 2).
Rocks here correlated with the Fusselman Dolomite of Silurian age overlie the Cutter Member of the Montoya Dolomite with a fairly sharp lithologic break. The Fusselman was named by Richardson (1908, p. 478-480; 1909, p. 4) for exposures in the Franklin Mountains, Tex. The surface exposures of the Fusselman closest to the report area are in the Sierra Diablo, Tex., about 55 miles to the west-southwest (King, P. B., and Knight, J. B., 1944).
In the area of this report the Fusselman consists almost entirely of white to light-gray coarse- to medium crystalline dolomite which contrasts rather sharply with darker colored and finer grained dolomite of the underlying Cutter Member of the Montoya Dolomite. The Fusselman has been completely penetrated by 8 wells in the mapped area and ranges in thickness from about 580 feet in the Continental-Standard of Texas Bass 1 (sec. 5, T.22 S., R. 21 E.) to about 740 feet in the Humble Huapache 2 (sec. 23, T. 23 S., R. 22 E.). Just east of the report area in the Humble Wiggs 1 (sec. 31, T. 24 S., R. 27 E.), the Fusselman is apparently 945 feet thick.
The Fusselman Dolomite is abruptly overlain in the report area by a sequence of dark-gray, locally silty, shale less than 100 feet thick, which at most place contains a few feet of dark- or medium-gray chert at the base. On the bases of stratigraphic position and lithology the shale of this unit is tentatively correlated with the Percha Shale of Late Devonian age but some shale of Mississippian age also may be included. The name Percha was given by Gordon (1907, P. 60-62) to exposures along Percha Creek in Sierra County, N. Mex. He assigned a Devonian age to the unit. The name Percha was used by most workers to include all Devonian and possibly then basal Mississippian strata of southern New Mexico, until Stevenson (1945) subdivided the strata into several formations and members and restricted the name Percha to the upper part of the original stratigraphic unit. The dark-gray shale in the subsurface of this area is almost certainly equivalent to some part of the unrestricted Percha as first designated and is more similar lithologically to the restricted Percha as differentiated by Stevenson (1945, p. 241-244) than to any of his older Devonian formations. The chert that is generally present at the base of the dark-gray shale is similar in stratigraphic position to a cherty unit of presumed Devonian age of the Sierra Diablo region, Texas (King, P. B., and Knight, J. B., 1944), the Hueco Mountains, Tex. (King, P. B., King, R. E. and Knight, J. B., 1945), and the Franklin Mountains, Tex. (Laudon and Bowsher, 1949, p. 36). Lloyd (1949, p. 46-52) applied the term Woodford to a similar shale and chert sequence found in the subsurface of southeastern New Mexico and correlated it with the Ready Pay Member of the Percha as defined by Stevenson (1945, p. 241-243). Because of the great distance between this report area and the type Woodford Chert of Oklahoma, the term Woodford is not used here.
The Percha (?) Shale and associated chert have been penetrated by 10 wells in the report area; they range in thickness from about 20 feet in the Continental-Standard of Texas Bass 1 (sec. 5, T. 22 S., R. 21 E.) to about 60 feet in the Humble Huapache 3 (sec. 22, T. 24 S., R. 22 E.). The Percha(?) Shale has a reported thickness of about 100 feet in the Gulf Oil Corp. Estill 1AD (sec. 29, T. 24 S., R. 26 E.). Just east of the report area the Percha (?) is about 70 feet thick in the Humble Wiggs 1 (sec. 31, T. 24 S., R. 27 E.).
Above the Percha (?) Shale is a sequence of rocks assigned to the Mississippian System. This sequence can be subdivided into two general units of about equal thickness, a lower cherty limestone unit and an upper shaly unit (pl. 2).
The lower cherty limestone unit consists predominantly of dark- to light-gray fine-crystalline limestone and abundant dark- to medium-gray chert. In most wells minor amounts of dark-gray silty shale and medium- to light-gray calcareous siltstone are present in the unit. The thickness of the unit ranges from about 250 to about 365 feet in the 8 wells in the area which have penetrated it. The unit cannot be definitely related to any known surface formation. It could be equivalent to any or several limestone units of preChester Mississippian age of south-central New Mexico or Trans-Pecos Texas and is here referred to informally as the limestone unit of Mississippian age.
The upper shaly unit of the Mississippian sequence is about half dark-gray shale, much of which is silty; the rest of the unit is mostly medium- to light-gray fine-crystalline limestone, and some medium- to light-gray calcareous very fine grained sandstone and siltstone. It is about 200 feet thick in the Humble Huapache 1 well (sec. 35, T. 23 S., R. 22 E.) and Union Oil Co. of California White 1 (sec. 17, T. 24 S., R. 22 E.); it is about 254 feet thick in the Humble Huapache 2 (sec. 23, T. 23 S., R. 22 E.). This shaly unit is correlated with the Upper Mississippian Helms Formation, as restricted by Laudon and Bowsher (1949, p. 19) in the Hueco Mountains, Tex. (fig. 2). The nearest known exposures of the Helms Formation are in the Sacramento Mountains, about 70 miles west-northwest (Pray, 1954, p. 99).
Pennsylvanian rocks in the report area range in thickness from 0 to more than 4,000 feet. The lithologic characteristics are variable from place to place. These variations are apparently related to tectonic features of Pennsylvanian age which are discussed in more detail on pages 40-42. Because of vertical repetitions in lithology and lateral variations in lithology and thickness, the Pennsylvanian sequence is not easily divisible on a lithologic basis. However, three major subdivisions with indefinite boundaries can be recognized.
The Pennsylvanian strata of the report area are comparable in a general way to those of northern New Mexico which Read and Wood (1947, p. 223) recognized as constituting a major sedimentary cycle consisting of
Where Pennsylvanian strata are present in the mapped area, the basal part, ranging in thickness from about 230 feet in the Humble Huapache 1 well (sec. 35, T. 23 S., R. 22 E.) to more than 400 feet in the Humble Huapache 2 (sec. 23, T. 23 S., R. 22 E.), consists of approximately equal amounts of fine- to coarse-grained poorly sorted locally conglomeratic quartz sandstone; mottled gray oolitic limestone; and medium- to dark-gray shale (pl. 2). These strata are similar in stratigraphic position and lithology to the Sandia Formation of Early Pennsylvanian age of central and northern New Mexico (Herrick, 1900, p. 114-115) and to the lower part of the Gobbler Formation of Pray (1954, p. 93) of the Sacramento Mountains, N. Mex.
Lying gradationally above the basal Pennsylvanian rocks is a sequence of Pennsylvanian strata that varies considerably in thickness and lithology. In general, these rocks consist of varying amounts of medium-gray fine-crystalline locally argillaceous or siliceous limestone and light- to dark-gray locally calcareous or silty shale, minor amounts of dark- to light-gray chert, and a few thin intervals of poorly sorted sandstone (pl. 2). Except in the Continental-Standard of Texas Bass 1 (sec. 5, T. 22 S., R. 21 E.) where strata of Pennsylvanian age are absent, this unit apparently ranges in thickness from less than 500 feet in the Union White 1 (sec. 17, T. 24 S., R. 22 E.) to more than 3,500 feet in the Humble Huapache 2 (sec. 23, T. 23 S., R. 22 E.).
In most wells in the western part of the area the uppermost strata of Pennsylvanian age are similar to those lying below them, but they contain varying amounts of reddish-brown and greenish-gray shale of possible continental origin. These strata may represent the third phase of the major sedimentary cycle described by Read and Wood (1947, p. 223). They are similar in age and lithology to the Holder Formation of Pray (1954, p. 93; 1959, p. 115-117). Because of Late Pennsylvanian uplift and erosion or nondeposition, these uppermost beds of mixed continental (?) and marine origin are absent in the Humble Huapache 1 well (sec. 35, T. 23 S., R. 22 E.), the Continental-Standard of Texas Bass 1 (sec. 5, T. 22 S., R. 21 E.), and the wells in the Delaware basin part of the area. Where reddish-brown and greenish-gray shales are present in Upper Pennsylvanian rocks, the systemic boundary between the Pennsylvanian and Permian as based on fusulinid studies is not generally marked by a sharp lithologic change (pl. 2). This situation pertains in the Magnolia State "W" 1 (sec. 16, T. 21 S., R. 22 E.) and in exposures 65 miles to the west near Sand Canyon in the Sacramento Mountains (Bachman and Hayes, 1958, p. 691).
Last Updated: 13-Feb-2008