Geology of Anthony quadrangle, Doña Ana County, New Mexico Geologic Map 54 By S. Kelley and J. P. Matheny, 1983 Abstract the Aleman Dolomite Member, a fine-grained, dark-gray dolomite with conspicuous, abundant The northernmost extension of the Franklin lenses and nodules of medium- to light-gray Mountains, a tilted block of Paleozoic (Ordovi­ chert. Overlying the Aleman is the Cutter Dolo­ cian- Permian) carbonates and shales, is the dom­ mite Member, a fine-grained, dark-gray dolomite inant topographic feature of the Anthony 7 Yz ­ that weathers to light gray. The Upham Member min quadrangle in southeast Dof\a Ana County, (Middle Ordovician) and the Aleman and Cutter New Mexico. Rocks of the northern Franklin Members (Upper Ordovician) (Kottlowski and Mountains have been subjected to early Tertiary others, 1956; Howe, 1959) contain a few brachio­ (?) folding, possibly related to the Laramide pods, corals, and gastropods and abundant, orogeny, and late Tertiary to Quaternary faulting dolomitized fossil debris. Thickness of the Mon­ associated with the Rio Grande rift. The most toya ranges from 116m (380ft), at a point 4.8 km unusual structures in the quadrangle are north­ (3.0 mi) south of the New Mexico-Texas state line trending, low-angle, normal faults. Apparently, (Harbour, 1972), to 130 m (426 ft) at Bishop Cap these low-angle fault planes originally were steep (Kramer, 1970). The Upham, Aleman, and Cut­ fractures that rotated to low-angle attitudes as the ter Members of the Montoya are 27 m (89 ft), 44 Franklin Mountains block tilted west. As rotation m (144 ft), and 52 m (170 ft) thick, respectively continued, movement along the low-angle faults (Harbour, 1972); total thickness of the Montoya became inefficient; consequently, high-angle, Dolomite in this area is 123m (403 ft). normal faults developed in the Hueco Bolson, 7 The Fusselman Dolomite (Silurian), which east of the Franklin Mountains. The Camp Rice was defined by Richardson (1908), disconform­ Formation (late Tertiary to early Quaternary) ably overlies the Montoya Dolomite. Rugged, and middle to late Quaternary fan and eolian complexly jointed, light-gray cliffs formed by the deposits cover large portions of the map area. Fusselman crop out along the entire east side of Three distinct facies of the Camp Rice Formation the northern Franklin Mountains, except at the are present in the Anthony quadrangle: 1) a flu­ extreme northern end. The Fusselman is a pure, vial facies composed of channel sand and thick-bedded, light-gray dolomite with minor floodplain clay, 2) an eolian facies consisting of amounts of chert and limestone. Richardson loamy sand, and 3) a piedmont-slope facies com­ (1909) and Pray (1958) both assigned a Middle posed of fan material from the adjacent moun­ Silurian age to the Fusselman on the basis of tains. These Quaternary deposits indicate that brachiopods, corals, and gastropods found during the Pleistocene a substantial amount of within the formation. Erosion prior to the Middle water from the ancestral Rio Grande flowed east Devonian caused the formation to thin from 185 through Fillmore Pass into the Hueco Bolson. m (607 ft) in the southern part of the area to 150 Less water flowing west of the mountains into the m (492 ft) at the north end of the Franklin Moun­ Mesilla Valley led to the development of a tains (Harbour, 1972). Silicified zones containing relatively stagnant, deltaic environment, south­ barite, fluorite, and limonite mineralization are west of Fillmore Pass. During the Pleistocene, the common in the Fusselman near its unconform­ prevailing westerly wind blew across the broad able upper contact with the Canutillo Formation. floodplain in the Mesilla Valley and piled fluvial Mineralizing fluids probably migrated upward sand into eolian dunes against alluvial fans that through extensive fractures in the Fusselman and were forming adjacent to the rising Franklin spread laterally along the unconformity upon en­ Mountains. countering the impermeable Canutillo Formation and Percha Shale. 8 The Canutillo Formation overlies the INTRODUCTION Fusselman with slight angular discordance caused by minor erosion following tilting of the Fussel­ 1 The Anthony 7 !12-min quadrangle is in man and older rocks. Nelson (1940) included all southeast Dof\a Ana County, New Mexico, just Devonian rocks in the Franklin Mountains in the north of the New Mexico-Texas state line (fig. !). Canutillo Formation; however, Laudon and A tilted block of folded and faulted Paleozoic Bowsher (1949) later realized that the upper part carbonates and shales forms a low mountain of Nelson's Canutillo Formation .is actually cor­ ridge in the central portion of the map area. This relative with the Percha Shale farther north. As a block, which is the northernmost extension of the result, the name "Canutillo Formation" is ap­ Franklin Mountains, is part of the continuous, plied to only the lower portion of Nelson's Canu­ north-trending San Andres-Organ-Franklin tillo Formation. fault block of south-central New Mexico and west 9 The Canutillo crops out in talus-covered Texas. The mountains are flanked on the east by slopes along the eastern escarpment of the range, the Hueco Bolson and on the west by the Mesilla except at the north end of the Franklin Moun­ Valley. Although large portions of the valley are tains. The lower two-thirds of the formation is covered by recent eolian and alluvial-colluvial primarily lenses of chert and marl; the upper sediments, entrenchment of the Mesilla Valley by third consists of interbedded, dark-gray shale and the Rio Grande and Quaternary movement along siltstone. Because the Canutillo is composed of the Artillery Range fault in the Hueco Bolson incompetent strata, small, local, disharmonic have provided good exposures of Pleistocene to folds are common within the formation. The only Holocene deposits in the area. significant fossils found in the Canutillo Forma­ 2 Previous workers in this quadrangle were tion (late Middle Devonian) are brachiopods and concerned with either bedrock geology or ground conodonts (Harbour, 1972). This formation thins water and geomorphic studies. A geologic map of northward from at least 30m (97 ft) at North An­ the southern third of the Anthony quadrangle thony's Nose (Harbour, 1972) to approximately was published by Harbour ( 1972) as part of his 15m (49ft) near Webb Gap. map of the northern Franklin Mountains in Texas 10 Conformably overlying the Canutillo For­ and New Mexico. Several minor modifications of mation is the Percha Shale, a fissile, black shale, his map are made in this report. The stratigraphy distinguished mainly by its stratigraphic position and microfacies of the Pennsylvanian formations between thick limestone or dolomite units. This exposed south and west of Anthony Gap have shale was correlated by Laudon and Bowsher been described by Allouani (1976), El Foul (1949) with the Percha Shale of southwest New (1976), and Hair (1976, 1977). Figuers (personal Mexico. Like the Canutillo, the Percha forms a communication, 1979) is currently analyzing the talus-covered slope on the east face of the north­ ·structural geology of the "pipeline complex" ern Franklin Mountains. The Percha pinches out southeast of Anthony Gap. Knowles and Ken­ just north of North Anthony's Nose and appears nedy (1958), Leggat and others (1962), and King to be absent between North Anthony's Nose and and others (1971) discussed the lithology and Anthony Gap. A relatively thin (12-15 m; 39-49 groundwater conditions surrounding several ft) Percha section crops out in the mountains wells located in or near the map area. In addition, south of Anthony Gap. The exact age and corre­ Hawley (unpublished data, 1969) has done de­ lation of the Percha in this area are uncertain, tailed work on the soils and geomorphic surfaces because no fossils have been found in this shale. near Anthony Gap in the southern part of the The average thickness of the Percha in the Frank­ area. In this report, we have combined the results lin Mountains north of North Anthony's Nose is of previous efforts with our own observations in approximately 18- 21 m (60- 70 ft). describing the geology of the Anthony 7 Y2 -min 11 The Las Cruces Limestone (Mississip­ quadrangle. pian), named by Laudon and Bowsher (1949) for A CKNOWLEDGMENTS- This map is the result of exposures in Vinton Canyon, 3.5 km (2 .I mi) work done for our undergraduate field geology south of the New Mexico- Texas state line, uncon­ class project at New Mexico State University formably overlies Devonian rocks. The Las Cruces under the direction of Dr. William R. Seager. We forms distinctive, light-gray, evenly bedded (0.3- would like to thank Dr. Seager and Dr. John W. 0.6-m-thick beds), !edgy cliffs along the east side Hawley of the New Mexico Bureau of Mines and of the Franklin Mountains. In general, the Las Mineral Resources for suggesting this project, Cruces is a dense, brittle, primarily chert-free spending time with us in the field, answering micrite. Small chert lenses and thin (0.5- 1.0 em), questions concerning the structural and Quater­ bioturbated, sandy layers are interbedded with nary geology of the area, and reviewing the map limestone in the upper 4 m (13 ft) of the section, and text. We also thank Sandy Figuers of the Uni­ north of Webb Gap. Small chert lenses are more versity of Texas at El Paso for allowing us to use common throughout the Las Cruces section, a simplified version of his geologic map of the south of Webb Gap. Although the Las Cruces "pipeline complex" in our report. We greatly ap­ contains very few fossils, Laudon and Bowsher preciate Richard E. Kelley's assistance in the (1949) considered the formation to be of Mera­ fi eld.