Identifying origins of and pathways for spring waters in a semiarid basin using He, Sr, and C isotopes: Cuatrociénegas Basin, Mexico B.D. Wolaver1,*, L.J. Crossey2,*, K.E. Karlstrom2,*, J.L. Banner3,*, M.B. Cardenas3,*, C. Gutiérrez Ojeda4,*, and J.M. Sharp, Jr.3,* 1Bureau of Economic Geology, University of Texas at Austin, 10100 Burnet Road, Austin, Texas 78758, USA 2Department of Earth and Planetary Sciences, University of New Mexico, MSCO3-2040, 1 University of New Mexico, Albuquerque, New Mexico 87131, USA 3Department of Geological Sciences, University of Texas at Austin, 1 University Station, Austin, Texas 78712-0254, USA 4Instituto Mexicano de Tecnología del Agua (IMTA), Paseo Cuauhnáhuac 8532, Colonia Progreso, 62550 Jiutepec, Morelos, México ABSTRACT mantle-derived He (to 23% of the total He) species (Hendrickson et al., 2008; Fig. 2). ≤ –1 and CO2 (pCO2 10 atm). Mantle degas- Springs have varied spatial distribution and geo- He, C, and Sr isotopes are used to infer sing is compatible with the thinned North chemistry. Spring vents are often obscured by spring sources in a water-stressed area. American lithosphere, as shown in tomo- valley-fi ll alluvium, but high-discharge springs Spring-water origins and pathways in the graphic images. Sr isotopes in both Cuatro- generally issue directly from fractures in Creta- Cuatrociénegas Basin are revealed by linking ciénegas Basin springs and spring-deposited ceous carbonate rocks. Other springs are located structure and geochemistry via regionally travertine (87Sr/86Sr = 0.707428–0.707468) at the base of alluvial fans (Wolaver and Diehl, extensive fault networks. This study presents indicate that carbonate rocks of the regional 2011). CO2-rich spring water with pCO2 1.5–3 the fi rst dissolved noble gas and He isotopic Cupido aquifer (87Sr/86Sr = 0.7072–0.7076) orders of magnitude higher than atmosphere data from northeastern Mexico. Basement- are the main source of Sr. Rock-water inter- facilitates deposition of rare modern freshwater involved faults with complex reactivation actions with mafi c volcanic rocks (87Sr/86Sr = stromatolites (Dinger et al., 2006). histories are important in northeastern 0.70333–0.70359) are not inferred to be an The study area is in the Chihuahuan Des- Mexico tectonics and affect hydrogeologic important process. Groundwater-dissolved ert, where long-term (1942–2003) annual pre- systems. The importance of faults as con- inorganic C origins are modeled using major cipitation of 200 mm exceeds annual potential duits for northeastern Mexico volcanism is elements and C isotopes. C isotope data show evaporation (1964–1988) of 1960 mm (Aldama recognized, but connections between fault- that ~30% ± 22% of CO2 in spring water is et al., 2005). Irrigated agriculture since the mid- ing and the hydrogeologic system have not derived from dissolution of aquifer carbon- 1990s in adjacent valleys caused groundwater been extensively investigated. This research ates (Ccarb = 30%), 24% ± 16% is from soil declines of ~1 m/yr, and the main springs that tests the hypothesis that Cuatrociénegas gas and other organic sources (Corg = 24%), formed the initial basis for irrigated agriculture Basin springs are divided into two general and 46% ± 33% is from deep sources [Cendo in the town of Cuatrociénegas no longer fl ow classes based upon discharge properties: (endogenic crust and mantle) = 46%]. This (Wolaver et al., 2008). In addition to support- (1) regional carbonate aquifer discharge study demonstrates the presence of mantle- ing groundwater-dependent ecosystems with 3 (mesogenic) mixed with contributions from derived He and deeply sourced CO2 that unique endemic species, the Cupido aquifer that deeply sourced (endogenic) fl uids contain- ascend along basement-penetrating faults fl ows to Cuatrociénegas springs supplies water 3 ing He and CO2 from the mantle that ascend and mix with Cupido aquifer groundwater to more than four million people in northeastern along basement-involved faults; and (2) car- before discharging in Cuatrociénegas Basin Mexico and is correlative to the prolifi c Texas bonate aquifer discharge mixed with locally springs. Edwards-Trinity aquifer (Johannesson et al., recharged (epigenic) mountain precipi tation. 2004). Understanding the Cuatrociénegas Basin Carbonate and/or evaporite dissolution is INTRODUCTION spring-water origins is important for both devel- indicated by Ca-SO4 hydrochemical facies. oping groundwater resources for human needs He isotopes range from 0.89 to 1.85 RA (RA is He, Sr, and C isotopes are used to identify and preserving spring fl ow for groundwater- the 3He/4He of air, 1.4 × 10–6) and have mini- the origins of and groundwater pathways for dependent ecosystems in similar settings. mal 3H, from which it is inferred that base- Cuatrociénegas Basin springs; the basin is an This study tests the hypothesis that Cuatro- ment-involved faults permit degassing of oasis in the Chihuahuan Desert region of north- ciénegas Basin springs are divided into two eastern Mexico (Fig. 1). Dozens of springs fl ank classes based upon discharge and geochemi- *Emails: Wolaver: [email protected]; the 2300-m-high Sierra San Marcos anticline, cal properties, which are inferred to be con- Crossey: [email protected]; Karlstrom: kek1@ unm.edu; Banner: [email protected]; Cardenas: which bisects the basin and sources critical trolled by relationships between springs and [email protected]; Gutiérrez Ojeda: cgutierr fl ows to groundwater-dependent pools, streams, fault conduits. The fi rst class has characteristics @tlaloc.imta.mx; Sharp: [email protected] and marshes that are refugia for >70 endemic that imply discharge primarily from a regional Geosphere; February 2013; v. 9; no. 1; p. 113–125; doi:10.1130/GES00849.1; 6 fi gures; 3 tables. Received 30 July 2012 ♦ Revision received 23 October 2012 ♦ Accepted 24 October 2012 ♦ Published online 13 December 2012 For permission to copy, contact [email protected] 113 © 2012 Geological Society of America Wolaver et al. La CTC STRATIGRAPHIC AND S volcanic field Coahuila Fold Belt B abia STRUCTURAL FRAMEWORK 28° LMI paleogeography F a OF NORTHEASTERN MEXICO reverse fault ^ ^ ult ^ earthquake S Cuatrociénegas Basin consists of Jurassic LMI and Cretaceous marine and nonmarine silici- San O M clastics overlying Precambrian to Paleozoic arcos Fa ult crystalline basement (Goldhammer, 1999; Fig.2 Lehmann et al., 1999). Cretaceous carbon- 27° CC ate rocks, as thick as 800 m, form the Cupido MI and Aurora Formations that are the primary regional aquifer (Wolaver and Diehl, 2011; Wolaver et al., 2008). The region has a com- C a CB lif plex tectonic history with several stages of or nia faulting (Fig. 1). During the Permian–Triassic, -T C am ^ au left-lateral motion along the Coahuila-Tamau- 26° lip as lipas transform, associated with the opening Tr ans ^ form of the Gulf of Mexico, faulted Precambrian USA and Paleozoic basement blocks of northeast- ern Mexico (Aranda-Gómez et al., 2005, 2007; Mexico ^ Dickinson and Lawton, 2001; Goldhammer, 1999). The imprinted zones of structural weak- ness infl uenced Laramide and subsequent tec- 103° 102° 101° tonic activity. First, extension along Neocomian Figure 1. Study area and regional tectonic map of Cuatrociénegas normal faults (ca. 140–136 Ma) formed fault- Basin (CC). Relief map with topography and regional tectonic fea- bounded structural highs and lows, such as the tures is after Aranda-Gómez et al. (2007, 2005), Chávez-Cabello Coahuila block and adjacent Sabinas Basin. et al. (2005), Dickinson and Lawton (2001), Eguiluz de Antuñano Then, compression folded and faulted strata in (2001), and Goldhammer (1999). Paleogeographic elements (diago- the Paleogene (ca. 45–23 Ma; Chávez-Cabello nal lines): Coahuila block (CB), La Mula Island (LMI), Monclova et al., 2005). Two less intense periods of normal Island (MI), Coahuila-Texas craton (CTC). Paleotopographic fault reactivation occurred in the Late Miocene highs (e.g., CB, CTC) are bounded by regional San Marcos and to Quaternary (Aranda-Gómez et al., 2005; La Babia faults (Eguiluz de Antuñano, 2001) with intervening Chávez-Cabello et al., 2005). downthrown Sabinas Basin (light gray shaded region; Eguiluz The tectonic activity created faults that are de Antuñano, 2001). Permian–Triassic California-Tamaulipas critical to the area hydrogeology. Mesozoic nor- transform is postulated structural feature (dashed line; Dickin- mal faults associated with the opening of the Gulf son and Lawton, 2001). Pliocene–Pleistocene volcanic fi elds: Las of Mexico created conditions that permitted the Coloradas (C), Ocampo (O), Las Esperanzas (S) (Aranda-Gómez deposition of Cretaceous carbonate rocks that et al., 2007; Chávez-Cabello, 2005); volcanic fi elds are associated now form a regional carbonate aquifer (Lehmann with Laramide-age reverse faults. Earthquakes are since 1973 et al., 1999; Wolaver and Diehl, 2011; Wolaver (U.S. Geological Survey, 2009). et al., 2008). Paleogene reactivation of reverse faults generated the ≤3000 m anticlinal uplifts that serve as recharge areas and fractured the carbonate aquifer with some deeply sourced culate mixing of three source waters that results sedimentary rocks to create secondary permea- fl uids along high-permeability fault-associated in the two spring classes. The three sources bility. Late Miocene–Quaternary normal fault- pathways. These springs have elevated discharge are: (1) epigenic waters (dominantly younger, ing fractured the carbonate aquifer, reactivated (≤550 L/s), elevated