CALCITE CAVE SPAR AND ITS RELATIONSHIP TO THE LANDSCAPE EVOLUTION OF THE GUADALUPE MOUNTAINS, SOUTHWESTERN UNITED STATES

The timing of the rise of the Guadalupe Mountains is not precisely known. One model suggests that Rio Grande rifting was responsible for ~1000 meters of uplift. Another model suggests that the Laramide orogeny was the culprit for a majority of the uplift. A third model suggests there were three instances of mountain building, and a combination of the three led to the present >2 km height of the Guadalupe Mountains. Our study of cave spar, a deep phreatic speleothem, may provide clues to changing landscapes and add important constraints on the timing of those changes using U-Pb dating of the spar.

The cave spar used in this study is a druse of large calcite crystals formed in small geode-like caves in the Guadalupe Mountains. The stage of speleogenesis responsible for the formation of these small caves predates the sulfuric-acid speleogenesis stage. These vugs, referred to as spar caves, are generally less than 100 meters in diameter. Two models of speleogenesis are possible. These vugs could have formed early during the Permian and the calcite spar lining these caves could have formed any time after that. Or, these caves formed in response to tectonic/volcanic activity, and the calcite spar was a late stage product of speleogenesis. The latter model is based on the dissolution-precipitation curves of Dublyansky (2000). They are lined with scalenohedral or rhombohedral calcite crystals up to 0.5 meters long. The calcite spar, and possibly the spar caves, formed in response to tectonic/volcanic activity. For these crystals to be used as indicators of landscape change, they would need to be linked to this activity.

Here we report the ages, location and elevation for 12 spar samples collected from caves in the Guadalupe Mountains of southeastern New Mexico and west Texas. The U-Pb ages are bi-modal and cluster in the period of the Laramide and the period of the Basin and Range. 5 samples have ages between 90 and 35 Ma, and 5 samples have ages between 35 and 15 Ma. Two samples are still in the process of being dated. These are two important tectonic/volcanic stages of geologic history. This relationship shows that growth of the calcite spar is linked to this activity. These ages also constrain the minimum age of the formation of the spar caves. More detailed studies using Sr, C, and O isotopes and fluid inclusions are needed.