157 ProPosed Age And origin of gyPsum needles of CrystAl CrAwl New Mexico Geological Society Guidebook, 65 th Field Conference, Geology of the Sacramento Mountains Region, 2014, p. 157–162 PROPOSED AGE AND ORIGIN OF GYPSUM NEEDLES OF CRYSTAL CRAWL, FORT STANTON CAVE, NEW MEXICO VICTOR J. POLYAK 1 , DONALD G. DAVIS 2 , PAULA P. PROVENCIO 3 , and YEMANE ASMEROM 1 1 Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM, polyak@unm.edu 2 441 South Kearney St., Denver, CO 3 Institute of Meteoritics, University of New Mexico, Albuquerque, NM ABSTRACT—Crystal Crawl in Fort Stanton Cave near Capitan, New Mexico, was once covered with a forest of gypsum needles that has been almost entirely destroyed by over 100 years of frequent visitation and mineral collection. One of those needles was sketched and added to the Great Divide expedition report in 1891. We studied needles from three collections and noted regular-looking growth banding in many of them. The average band width of 10 measured needles (0.16 to 0.38 mm/ band) is similar to growth rates of gypsum needles previously published (0.07 to 0.21 mm/year), supporting our interpreta- tion that the banding was produced from annual growth of the gypsum needle. with that interpretation, we estimate that the individual needles of Crystal Crawl took 100 to 500 years to grow. we also measured the age of one needle based on two uranium-series analyses of needle 2 (3.3 ±2.7 and 4.3 ±3.9 ka) that show that it grew during the Middle or Late Holocene. Our interpretation is that the entire crystal forest grew during this time. It is likely that the needle forest grew during a time when the cave environment shifted to slightly more evaporative conditions, which could have been during onset of drier early and Middle Holocene climate, or initiated due to collapse which created the large entrance of Fort Stanton Cave. Alternatively, the needles could have grown following mobilization of gypsum by the last looding of the cave that reached the Crystal Crawl loor level. This suggested Holocene age and origin for the gypsum forest is also evidence that could support a similar hypoth- esis for deposition of the Snowy River calcite, also in Fort Stanton Cave. INTRODUCTION Fort Stanton Cave, located east of the north end of New Mexico’s Sacramento Mountains (Fig. 1), was the second cave west of the Great Plains in the United States to be surveyed (Davis, 2004). New Mexico was a territory at that time. Twenty years before non-native Americans found Carlsbad Cavern, the Wheeler Survey explored and mapped Fort Stanton Cave. Before that, in 1855, soldiers from newly designated Fort Stanton signed their names in the cave (Davis, 2004). In 1872, Quartermaster Conrad and Lieutenant Boyd, both stationed at nearby Fort Stanton, had a boat built at the fort and used it to explore some of Fort Stanton Cave, which was partially looded at that time (Daw, 1984). It was 1877 when the Wheeler Survey entered and surveyed Fort Stanton Cave (Davis, 2004), and they were some- what disappointed that many grander features reported to be in the cave were not found (Morrison, 1878). Fort Stanton Cave was surveyed and explored as part of the great expeditions of the “West” led by Clarence King, John Wesley Powell, Ferdinand V. Hayden, and George Montague Wheeler, which were terminated soon after this irst survey of Fort Stanton Cave, and reorganized as the U.S. Geological Survey in 1879 (Rabbitt, 1989). Fort Stanton Cave has formed in the thin- to medium-bedded limestone/dolostone of the Permian San Andres Formation, which is gypsum-bearing locally (Kelley, 1972). In contrast to Carlsbad Cavern and other caves farther south in New Mexico, which are complex mazes developed by oxidation and hydrolysis of rising hydrogen sulide, Fort Stanton is a more conventional cave with long horizontal passages developed by epigenic capture of aggressive surface waters (davis and land, 2006). Today, Fort Stanton Cave is known for its beautiful but rare velvety-textured speleothems, the Snowy River calcite chan- nel, and globally important paleoclimate studies. the discovery of Snowy River in 2001 (see Peerman and Bilbo, 2013) has ele- vated the cave’s importance. A section of the cave referred to as the “Crystal Chamber” by the Wheeler expedition (Green, 1891) must have been extraordi- nary. The Crystal Chamber, now called Crystal Crawl (Fig. 1), is almost entirely denuded after more than 100 years of visita- tion and mineral collection. left preserved, it would undoubtedly have been one of the great speleological wonders of the world, as Snowy River is today. Crystal Crawl is 5–10 meters wide and a few hundred meters long, and was loored or at least mostly loored with a “forest” of gypsum needles that grew from clayey cave sediment. green (1891) referred to this crystal forest as follows: “The loor is a sparkling lawn of what appears to be FIGURE 1. General location of Fort Stanton Cave and photograph show- ing Crystal Crawl as it looks today.