CRevolution 2: Origin and Evolution of the Colorado River System II themed issue Crossey et al. Importance of groundwater in propagating downward integration of the 6–5 Ma Colorado River system: Geochemistry of springs, travertines, and lacustrine carbonates of the Grand Canyon region over the past 12 Ma L.C. Crossey1, K.E. Karlstrom1, R. Dorsey2, J. Pearce3, E. Wan4, L.S. Beard5, Y. Asmerom1, V. Polyak1, R.S. Crow1, A. Cohen6, J. Bright6, and M.E. Pecha6 1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, 87131, USA 2Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403-1272, USA 3U.S. Forest Service, Grand Mesa, Uncompahgre and Gunnison National Forests, Paonia Ranger District, 403 North Rio Grande Avenue, P.O. Box 1030, Paonia, Colorado 81428, USA 4U.S. Geological Survey, 345 Middlefield Road, MS977, Menlo Park, California 94025, USA 5U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, Arizona 86001, USA 6Department of Geosciences, The University of Arizona, 1040 East 4th Street, Tucson, Arizona 85721, USA ABSTRACT travertines, suggesting a long-lived spring- basins. Bouse carbonates display a southward fed lake/marsh system sourced from western trend toward less radiogenic 87Sr/86Sr values, We applied multiple geochemical tracers Colorado Plateau groundwater. Progressive higher [Sr], and heavier d18O that we attribute (87Sr/86Sr, [Sr], d13C, and d18O) to waters and up-section decrease in 87Sr/86Sr and d13C and to an increased proportion of Colorado River carbonates of the lower Colorado River sys- increase in d18O in the uppermost 50 m of the water through time plus increased evapora- tem to evaluate its paleohydrology over the Hualapai Limestone indicate an increase in tion from north to south. The d13C and d18O past 12 Ma. Modern springs in Grand Canyon meteoric water relative to endogenic inputs, trends suggest alternating closed and open reflect mixing of deeply derived (endogenic) which we interpret to record progressively systems in progressively lower (southern) fluids with meteoric (epigenic) recharge. increased input of high-elevation Colorado basins. We interpret existing data to permit Travertine (<1 Ma) and speleothems (2–4 Plateau groundwater from ca. 8 to 6 Ma. the interpretation that the southernmost Ma) yield 87Sr/86Sr and d13C and d18O values Grand Wash, Hualapai, Gregg, and Temple Blythe basin may have had intermittent mix- that overlap with associated water values, basins, although potentially connected by ing with marine water based on d13C and providing justification for use of carbonates groundwater, were hydrochemically distinct d18O covariation trends, sedimentology, and as a proxy for the waters from which they basins before ca. 6 Ma. The 87Sr/86Sr, d13C, paleontology. [Sr] versus 87Sr/86Sr modeling were deposited. The Hualapai Limestone and d18O chemostratigraphic trends are com- suggests that southern Blythe basin 87Sr/86Sr (12–6 Ma) and Bouse Formation (5.6–4.8 Ma) patible with a model for downward integra- values of ~0.710–0.711 could be produced by record paleohydrology immediately prior to tion of Hualapai basins by groundwater sap- 25%–75% seawater mixed with river water and during integration of the Colorado River. ping and lake spillover. (depending on [Sr] assumptions) in a delta– The Hualapai Limestone was deposited The Bouse Limestone (5.6–4.8 Ma) was marine estuary system. from 12 Ma (new ash age) to 6 Ma; carbon- also deposited in several hydrochemically We suggest several refinements to the ates thicken eastward to ~210 m toward the distinct basins separated by bedrock divides. “lake fill-and-spill” downward integration Grand Wash fault, suggesting that deposition Northern Bouse basins (Cottonwood, Mojave, model for the Colorado River: (1) Lake was synchronous with fault slip. A fanning- Havasu) have carbonate chemistry that is Hualapai was fed by western Colorado Pla- dip geometry is suggested by correlation of nonmarine. The 87Sr/86Sr data suggest that teau groundwater from 12 to 8 Ma; (2) high- ashes between subbasins using tephrochro- water in these basins was derived from mix- elevation Colorado Plateau groundwater was nology. New detrital-zircon ages are consis- ing of high-87Sr/86Sr Lake Hualapai waters progressively introduced to Lake Hualapai tent with the “Muddy Creek constraint,” with lower-87Sr/86Sr, first-arriving “Colorado from ca. 8 to 6 Ma; (3) Colorado River water which posits that Grand Wash Trough was River” waters. Covariation trends of d13C arrived at ca. 5–6 Ma; and (4) the combined internally drained prior to 6 Ma, with limited and d18O suggest that newly integrated Grand inputs led to downward integration by a or no Colorado Plateau detritus, and that Wash, Gregg, and Temple basin waters were combination of groundwater sapping and Grand Wash basin was sedimentologically integrated downward to the Cottonwood sequential lake spillover that first delivered distinct from Gregg and Temple basins until and Mojave basins at ca. 5–6 Ma. Southern, Colorado Plateau water and detritus to the after 6 Ma. New isotopic data from Hualapai potentially younger Bouse basins are distinct Salton Trough at ca. 5.3 Ma. We propose Limestone of Grand Wash basin show values hydrochemically from each other, which sug- that the groundwater sapping mechanism and ranges of 87Sr/86Sr, d13C, and d18O that gests incomplete mixing during continued strongly influenced lake evolution of the are similar to Grand Canyon springs and downward integration of internally drained Hualapai and Bouse Limestones and that Geosphere; June 2015; v. 11; no. 3; p. 660–682; doi:10.1130/GES01073.1; 17 figures; 5 supplemental tables. Received 16 May 2014 ♦ Revision received 17 January 2015 ♦ Accepted 19 March 2015 ♦ Published online 13 May 2015 660 For permissionGeosphere, to copy, contact June [email protected] 2015 © 2015 Geological Society of America Colorado River system springs, travertines, and lacustrine carbonates groundwater flow from the Colorado Plateau to Grand Wash Trough led to Colorado River integration. INTRODUCTION 37° N Late Miocene to modern carbonate deposits Hualapai basin SL (720 m) along the lower Colorado River (Fig. 1) provide Grand Wash F basin (912912 mm)) r a rich source of information about the paleohy- LM Rive TR W H K drology of spring, lake, and river waters in this Las Vegas basin GW ColoradoL H F (650 m) D G B region over the past 12 Ma. Recent models for 36° N T GW the Bouse Formation have inferred a series of Black Canyon Lake Bidahochi divide events that took place during a short period of G TGTG Explanation Grand Canyon springs time (bracketed between 5.6 and 4.8 Ma), when C Hu first-arriving Colorado River water caused a Grand Canyon travertine Mojave basin SA faults (Neogene) chain of lakes and internally drained basins to (560 m) Pyramid paleo divides between basins divide progressively fill and spill in a downward-prop- 35° N proposed lakes and B agating sequence (Blackwelder, 1934; Spencer M highstand elevations and Patchett, 1997; House et al., 2005, 2008; EC (Spencer et al., 2013) House and Pearthree, 2014; Roskowski et al., Topock subsurface Bouse Br divide Havasu basin Bouse outcrops 2010; Spencer et al., 2008, 2013). The Bouse H (360 m) Bristol Parker Hualapai outcrops Formation provides a record of these events, basin divide T basins of Hualapai and Bouse which some studies conclude took place in 34° N (330 m) r area of Wind Caves Mbr. less than 50–100 ka (Spencer et al., 2008). The 240 km 110m contou of Latrania Fm. inferred “event stratigraphy” of the Bouse For- BN Split Mtn. Gorge mation challenges notions of Walther’s law, the SJ Blythe S alton Se basin proposed 110 asl marine resolution of available geochronology, and time SA BS (330 m) incursion (McDougall C scales needed for establishment of continental- a MWW et al. 2014) SM Chocolate Mtns. scale river systems. Earlier models for deposi- E Lake Cahuilla 33° N Lake Cahuilla divide tion of all of the Bouse and Hualapai Lime- (12 m) Yuma basin Ridge Basin lake restored stones in a marine estuary (e.g., Metzger, 1968; 9 - 6 Ma CerroCer Prieta Subsurface Smith, 1970; Buising, 1988, 1990) have been 230 km divide Bouse / Imperial Index Map more recently refuted based on isotopic and GC Fm. stratigraphic data (Spencer and Patchett, 1997; ~ 5 Ma Colorado ID WY River delta (restored) Roskowski et al., 2010; Spencer et al., 2013; area of 32° N g. 1 Pearthree and House, 2014). However, a marine CP UT A CO influence in the southernmost Bouse basins is NV still supported by some data sets (McDougall, 2008, 2010; McDougall and Miranda Martinez, 0630 0120 CA AZ NM km SM restored 2014). Resolution of the debate about a possible TX marine connection for the southern Blythe basin -116° W -115° W -114° W -113° W -112° W is needed in order to test competing models for the magnitude of post–6 Ma uplift of the Colo- Figure 1. Map of the Grand Canyon and lower Colorado River region with inferred Bida- rado Plateau relative to the sea level (e.g., Luc- hochi, Hualapai, and Bouse paleolakes. Labeled springs (blue dots in Grand Canyon) are: chitta, 1979; Karlstrom et al., 2007, 2008). B—Blue Spring, F—Fence, TR—Thunder River, H—Havasu, L—Lava Warm Spring, This paper uses new and previously published TG—Travertine Grotto. Labeled travertines (yellow triangles in Grand Canyon) are: K— data on the geochemistry of carbonates to test Kwagunt, H—Havasu, TG—Travertine Grotto. Surface exposures of the Hualapai Lime- and refine models for initiation of the Colorado stone are yellow; Bouse Formation are red; red dots show drill holes that encountered River by downward integration, and explore Bouse Formation. Paleolake maximum elevations are from Spencer et al.