NEWMEXICOGEOLOGY ISSN 0196-948X Volume 22, No. 4, November 2000 Science and Service Geologic history, stratigraphy, and paleontology of SAM Cave, north-central New Mexico by Karel L. Rogers, Biology Department, Grand Valley State University, Allendale, MI 49401, Charles A. Repenning, Denver Museum of Natural History, Denver, CO 80205, Fred G. Luiszer, Department of Geological Science, University of Colorado, Boulder, CO 80309, and Richard D. Benson, University of Minnesota, St. Paul, MN 55108 Abstract San Antonio Mountain (SAM) Cave is one of the oldest lava tube caves in North America. Its walls, part of the Servilleta Basalt located north of San Antonio Mountain, New Mexico, date between 3.4 and 3.9 Ma. Well- explored parts of the cave are more than 170 m (558 ft) long, and some rooms are over 12 m (40 ft) high. Fifteen fossil localities have been identified within the cave ranging in age from ~1 Ma to younger than 0.74 Ma. Sediments contain evidence of the Brun- hes–Matuyama paleomagnetic reversal, and faunal analysis provides evidence of warmer, more equable climates than charac- teristic of the region today. Deep sea Oxygen Isotope Stages 22–18 (core V28–239) glacia- tions were represented in the region by cli- mates that sustained forests. Analysis is aided by the nearby, contemporaneous, and well-dated Hansen Bluff, Colorado, locality. The paleofauna includes many mammals and a few birds, reptiles, amphibians, fish, and molluscs. Biochronologic implications of evolutionary stages of Lemmiscus, Microtus, and Allophaiomys are discussed based on SAM Cave fossils (Bot 4) and other localities. The site contains the oldest record of Clethri- onomys rutilus in North America. Introduction SAM Cave (Fig. 1) is located approximate- ly 10 km (6.2 mi) northwest of San Antonio Mountain in Rio Arriba County, New Mexico, and it is approximately 4 km (2.5 mi) south of the Colorado–New Mexico State line in the San Luis Valley (Fig. 2). Originally discovered and explored in the early 1950s by the late Fidel Z. Cisneros of Also in this issue Oil and gas activities in 1999 p. 101 NMGS Spring Meeting call for papers p. 107 Manzano Mountains State Park p. 108 Upcoming meetings p. 117 Index to volume 22 p. 118 Coming soon Arsenic in ground water in the Socorro Basin FIGURE 1—Before the excavations of Fidel Cisneros in the 1950s, the modern entrance to SAM Cave 40Ar/39Ar geochronology of the Ocate volcanic field was a slight grassy depression in the sagebrush-covered hillside. San Antonio Mountain can be seen Living Desert Zoo and Gardens State Park in the distance south of the SAM Cave entrance. Los Pinos 7.5-min quadrangle pass in the cave because old lightning T31N R8E strike areas distort readings. The measured part of the lava tube extends 30 m (98 ft) 8900 south-southwest and 130 m (427 ft) north- northeast from the entrance (Fig. 3). It is 9000 unlikely that any part of the cave is far COLORADO below ground surface because the gentle x 8995 27 slope of the cave floor approximates the 28 * SAM Cave slope of the ground surface. 8,980 Approximately 60–70 m ( 197–230 ft) of Dry Lake core * lava tube not pictured in Figure 2 extends NEW MEXICO 9060 x 9062 farther north, but those passages are x 9021 beyond a 15-cm-high (6-in.) opening. Most of the unpictured tunnel is nearly plugged Volcanic cones with ceiling fall, and no fossiliferous sedi- 142 ment has been found in these relatively unexplored parts of the cave. It is possible 17 that the lava tube extends beyond the 9000 COLORADO 33 unmapped section just described because NEW MEXICO Rio Grande cool, fresh air pours into the tube from SAM Cave Taos 522 8900 between the blockage at the farthest point Chama * County 34 San Antonio Mtn.* of the cave. It is exceptionally difficult to 285 x 8982 move rocks to open more passages because 64 Tres 38 Piedras the spaces are small and the rocks are cemented together with calcium carbonate. Base from U.S. Geological Survey, 1995 Rio Arriba 84 County 0 0.5 mi Taos 0 10 20 30 mi 0 0.5 1km NEW MEXICO 0 10 20 30 40 50 km GEOLOGY FIGURE 2—SAM Cave is located in north-central New Mexico, about 4 km south of the Colorado– Science and Service New Mexico State line in the San Luis Valley. The modern entrance is slightly downhill to the north- east of the ancient volcano that gave rise to the lave tube. The cave is at 2,737 m (8,980 ft) elevation. ISSN 0196-948X Volume 22, No. 4, November 2000 Antonito, Colorado, SAM Cave is an approximately 24 m (79 ft) higher in eleva- Editors: Jane C. Love, Nancy S. Gilson, ancient lava tube in the 2–4.5 Ma Servilleta tion than the modern entrance (Fig. 2). Susan Voss, and Beth Campbell Basalt (Lipman and Mehnert, 1979; Eruptions from the volcanic cone gave rise Cartographers: Kathryn E. Glesener and Leo Gabaldon Dungan et al., 1984). The entrance to the to sheets of lava that cooled at the surface EDITORIAL BOARD James M. Barker, NMBMMR, Chair lava tube is on a basalt hillside at an eleva- but continued to flow underneath forming Steve M. Cather, NMBMMR tion of 2,737 m (8,980 ft). Fossiliferous sed- the lava tube (Waters et al., 1990). Layers of Thomas Giordano, NMSU Laurel B. Goodwin, NMIMT iments in the lava tube were excavated for basalt inside the cave indicate subsequent Barry S. Kues, UNM more than 30 yrs by Fidel Cisneros under a flows through the tube occurred at least Larry Crumpler, NMMNHS mining permit through Carson National seven times. From the volcanic cones, there Published quarterly by Forest. Thus, the cave is known as the is a relatively gentle downhill trend in a New Mexico Bureau of Mines and Mineral Resources Cisneros mine in official documents. north-northwesterly direction with the a division of New Mexico Institute of Entrance to the cave is by means of a 12-m- slope dropping approximately 24.4 m (80 Mining and Technology diameter (40-ft) section where the lava ft) per half kilometer (1,640 ft). In general, BOARD OF REGENTS Ex-Officio tube collapsed during this century. The the lava tube is aligned with the slope of Gary Johnson, Governor of New Mexico entrance was a low, rock-filled depression the hill downward. Michael S. Davis, Superintendent of Public Instruction in the ground surface when Mr. Cisneros SAM Cave may be the oldest known Appointed Randall E. Horn, Pres., 1997–2003, Albuquerque started mining and, according to Mr. lava tube in the western United States. The Kathryn E. Wavrik, Student Mem., Sec./Treas., Cisneros, was a shallow rock shelter with basalt walls of the lava tube are magneti- 1999–2000, Socorro Ann Murphy Daily, 1999–2004, Santa Fe Native American artifacts before the roof cally reversed (E. Larson, pers. comm. Sidney M. Gutierrez, 1997–2001, Albuquerque caved in. In the 1950s, Mr. Cisneros 1990) with a K–Ar age of 3.5 ± 0.4 Ma (E. Robert E. Taylor, 1997–2003, Silver City removed tons of rock and fossil-bearing Larson, pers. comm. 1990). The K–Ar age is New Mexico Institute of Mining and Technology matrix and, in doing so, left an excellent constrained by the end of the reversed President . Daniel H. López outcrop of sediment layers as he exposed Gilbert magnetic chron, so the cave was New Mexico Bureau of Mines and Mineral Resources passageways into an extensive and ancient formed between ~3.4 and ~3.9 Ma (E. Director and State Geologist . Peter A. Scholle lava tube. The cave is located far from Larson, pers. comm. 1990). Because de- Subscriptions: Issued quarterly, February, May, August, November; roads, so it is nearly pristine. posits older than about 1.0 Ma were not subscription price $10.00/calendar year. Editorial Matter: Articles submitted for publication should be in the found in the cave, it appears the cave was editor’s hands a minimum of five (5) months before date of pub- sealed for ~ 2 m.y. after it formed. lication (February, May, August, or November) and should be no longer than 20 typewritten, double-spaced pages. All scientific Geology papers will be reviewed by at least two people in the appropriate field of study. Address inquiries to Jane C. Love, Editor of New The lava tube most likely originated from Explored passages Mexico Geology, New Mexico Bureau of Mines and Mineral one of two ancient volcanic cones repre- Resources, Socorro, New Mexico 87801-4796. Published as public domain, therefore reproducible without permis- sented today as loess-filled, grassy depres- Most of the lava tube was mapped using sion. Source credit requested. sions uphill and south-southeast of the triangulation and measuring tape during Circulation: 1,000 modern entrance. These volcanic cones are July 1990. It is not possible to use a com- Printer: University of New Mexico Printing Services 90 NEW MEXICO GEOLOGY November 2000 opening into the cave may have formed Tight Spot ~2 m.y. after the cave’s formation in the Collapsed to surface wall of the volcanic cone that is today a grassy depression southwest of the mod- Drop to lower floor ern opening. Regardless of its location, Sediment floodwater from this earliest opening deposited sediment at the four oldest local- Cross section of passage ities in the cave. In order from oldest to Collapse rubble youngest they are Under Arch, Bot 4, Pink Solid, and Tight Spot.