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

Abstract

We applied multiple geochemical tracers ( 87 Sr/ 86 Sr, [Sr], d 13 C, and d 18 O) to waters and carbonates of the lower Colorado River system to evaluate its paleohydrology over the past 12 Ma. Modern springs in Grand Canyon reflect mixing of deeply derived (endogenic) fluids with meteoric (epigenic) recharge. Travertine (<1 Ma) and speleothems (2-4 Ma) yield 87 Sr/ 86 Sr and d 13 C and d 18 O values that overlap with associated water values, providing justification for use of carbonates as a proxy for the waters from which they were deposited. The Hualapai Limestone (12-6 Ma) and Bouse Formation (5.6-4.8 Ma) record paleohydrology immediately prior to and during integration of the Colorado River. The Hualapai Limestone was deposited from 12 Ma (new ash age) to 6 Ma; carbonates thicken eastward to ~210 m toward the Grand Wash fault, suggesting that deposition was synchronous with fault slip. A fanningdip geometry is suggested by correlation of ashes between subbasins using tephrochronology. New detrital-zircon ages are consistent with the "Muddy Creek constraint," which posits that Grand Wash Trough was internally drained prior to 6 Ma, with limited or no Colorado Plateau detritus, and that Grand Wash basin was sedimentologically distinct from Gregg and Temple basins until after 6 Ma. New isotopic data from Hualapai Limestone of Grand Wash basin show values and ranges of 87 Sr/ 86 Sr, d 13 C, and d 18 O that are similar to Grand Canyon springs and travertines, suggesting a long-lived springfed lake/marsh system sourced from western Colorado Plateau groundwater. Progressive up-section decrease in 87 Sr/ 86 Sr and d 13 C and increase in d 18 O in the uppermost 50 m of the Hualapai Limestone indicate an increase in meteoric water relative to endogenic inputs, which we interpret to record progressively increased input of high-elevation Colorado Plateau groundwater from ca. 8 to 6 Ma. Grand Wash, Hualapai, Gregg, and Temple basins, although potentially connected by groundwater, were hydrochemically distinct basins before ca. 6 Ma. The 87 Sr/ 86 Sr, d 13 C, and d 18 O chemostratigraphic trends are compatible with a model for downward integration of Hualapai basins by groundwater sapping and lake spillover.The Bouse Limestone (5.6-4.8 Ma) was also deposited in several hydrochemically distinct basins separated by bedrock divides. Northern Bouse basins (Cottonwood, Mojave, Havasu) have carbonate chemistry that is nonmarine. The 87 Sr/ 86 Sr data suggest that water in these basins was derived from mixing of high-87 Sr/ 86 Sr Lake Hualapai waters with lower-87 Sr/ 86 Sr, first-arriving "Colorado River" waters. Covariation trends of d 13 C and d 18 O suggest that newly integrated Grand Wash, Gregg, and Temple basin waters were integrated downward to the Cottonwood and Mojave basins at ca. 5-6 Ma. Southern, potentially younger Bouse basins are distinct hydrochemically from each other, which suggests incomplete mixing during continued downward integration of internally drained basins. Bouse carbonates display a southward trend ...

Constraints on the evolution of vertical deformation and Colorado River incision near eastern Lake Mead, Arizona, provided by quantitative structural mapping of the Hualapai Limestone

“…in a closed basin at the time of deposition and received no exogenous sediment (Longwell, 1936;Lucchitta, 1979), a result more recently extended by geochemical evidence to include the Hualapai Limestone in the Grand Wash Trough and Gregg Basin (Crossey et al, 2015). The Hualapai Limestone consists of several facies types, including limited evaporites and reddish siltstones, sandy limestones, the predominant wavy limestone, and travertine (for a thorough discussion of the sedimentology, stratigraphy, and geochemistry, see Crossey et al, 2015).…”

“…The commencement of Hualapai Limestone deposition was found to be 11 Ma (Faulds et al, 2001b). More recently, radiometric dating of an ash-fall tuff indicates initial deposition of the Hualapai Limestone began as early as 12 Ma (Crossey et al, 2015), while the roughly 6 Ma end of limestone deposition in the western (Temple) basin is still thought to be robust. The limestone is the uppermost member of a series of sedimentary rocks that fi ll the Grand Wash Trough.…”

“…Interfi ngering of the basal conglomerate and lower Hualapai Limestone and evaporite facies and increasingly robust limestone deposition up section indicate an environment in which ephemeral lakes periodically received clastic input from surrounding highlands, were desiccated regularly, and eventually coalesced to form paleo-Lake Hualapai (Crossey et al, 2015). In fact, the upper horizons within both the Gregg Basin and Grand Wash Trough sections are thought to represent a time when separate basins were connected by a lake highstand in late Miocene time (Crossey et al, 2015). In Gregg Basin, the Hualapai Limestone is greater than 120 m thick, and it achieves thicknesses of 210 m in the Grand Wash Trough to the east (Lopez-Pearce, 2010; this paper, section titled Syndepositional Deformation [ca.…”

“…The Hualapai Limestone consists of several facies types, including limited evaporites and reddish siltstones, sandy limestones, the predominant wavy limestone, and travertine (for a thorough discussion of the sedimentology, stratigraphy, and geochemistry, see Crossey et al, 2015). Interfi ngering of the basal conglomerate and lower Hualapai Limestone and evaporite facies and increasingly robust limestone deposition up section indicate an environment in which ephemeral lakes periodically received clastic input from surrounding highlands, were desiccated regularly, and eventually coalesced to form paleo-Lake Hualapai (Crossey et al, 2015). In fact, the upper horizons within both the Gregg Basin and Grand Wash Trough sections are thought to represent a time when separate basins were connected by a lake highstand in late Miocene time (Crossey et al, 2015).…”

“…Therefore, quantifying the reach-scale structural deformation over which the Colorado River fl owed may provide spatial context that complements singlepoint measurements of incision. At the mouth of Grand Canyon in eastern Lake Mead, the 12-6 Ma Hualapai Limestone (Longwell, 1936;Bohannon, 1984;Crossey et al, 2015) records the sedimentological and structural setting immediately prior to the arrival of the modern Colorado River and may be used to understand both the nature of the pre-Colorado River drainage in this region and the long-term differential incision rates of the Colorado River as it carved the modern canyon through eastern Lake Mead. Furthermore, the tectonic setting of the Hualapai Limestone within the transition between the Basin and Range and the Colorado Plateau, clear exposures, and temporal constraints make this unit an excellent candidate to study normal fault processes and basin development.…”