Geochronology and geochemical analysis of Quaternary travertine deposits at the Belen quarries of Mesa Aparejo, NM: Evaluation of travertine facies for paleohydrology and paleoenvironment Studies

Abstract

The depositional facies and stable isotopic composition of a large-volume Quaternary travertine deposit located in the Rio Grande rift in New Mexico provide a record of the paleohydrology of springs along the western Rio Grande rift flank. Our results show that lateral and vertical facies variations within a travertine deposit developed due to changing depositional environments, which are primarily related to the formation of travertine mounds sourced by springs along the rift-bounding Comanche and Santa Fe faults. Important travertine facies are 1) a terraced mound and sloping fan facies that represents surface flow, and 2) a vein facies that reflects artesian waters within the developing mounds. Stable oxygen and carbon isotope analyses of the two facies show similar δ18O and δ13C values, ranging between -6.6‰ and -9.3‰, and +1.3‰ and +6.1‰, respectively. The range of δ18O values is similar to nearby springs along the west side of the rift and is interpreted to reflect variable mixing of endogenic (deeply derived) spring discharge with meteoric water. The δ13C values are significantly higher than values of nearby springs (δ13C = -1 to -8) and are interpreted to be controlled by variable CO2 degassing rates. The age and volume of the travertine deposit suggests that the mounds archive an estimated minimum of 54 Gigatons CO2 that degassed over a period of 456 ka along the fault system. Significant additional amounts of CO2 likely escaped to the atmosphere during travertine formation and δ13C fractionation. Travertine mound growth was facilitated by times of high CO2 flux and times of high groundwater head in confined aquifer systems. 405