Amanda C. Reynolds

Bio: Amanda C. Reynolds is an academic researcher from University of Arizona. The author has contributed to research in topics: Canyon & Bedrock. The author has an hindex of 8, co-authored 9 publications receiving 364 citations. Previous affiliations of Amanda C. Reynolds include ExxonMobil & ConocoPhillips.

Dissected hydrologic system at the Grand Canyon: Interaction between deeply derived fluids and plateau aquifer waters in modern springs and travertine

Abstract: Geochemical study of water and gas discharging from the deeply incised aquifer system at the Grand Canyon, Arizona, provides a paradigm for understanding complex groundwater mixing phenomena, and Quaternary travertines deposited from cool springs provide a paleohydrologic record of this mixing. Geochemical data show that springs have marked compositional variability: those associated with active travertine accumulations (deeply derived endogenic waters) are more saline, richer in CO2, and elevated in 87Sr/86Sr relative to springs derived dominantly from surface recharge of plateau aquifers (epigenic waters). Endogenic waters and associated travertine are preferentially located along basement-penetrating faults. We propose a model whereby deeply derived fluids are conveyed upward via both magmatism and seismicity. Our model is supported by: (1) gas analyses from spring waters with high He/Ar and He/N2 and 3He/4He ratios indicating the presence of mantle-derived He; (2) large volumes of travertine and CO2-rich gases in springs recording high CO2 fluxes; and (3) 87Sr/86Sr in these springs that indicate circulation of waters through Precambrian basement. Geochemical trends are explained by mixing of epigenic waters of the Colorado Plateau aquifers with different endogenic end-member waters in different tectonic subprovinces. Endogenic waters are volumetrically minor but have significant effects on water chemistry. They are an important and largely unrecognized component of the hydrogeochemistry and neotectonics of the southwestern United States.

A late Miocene–early Pliocene chain of lakes fed by the Colorado River: Evidence from Sr, C, and O isotopes of the Bouse Formation and related units between Grand Canyon and the Gulf of California

Abstract: We report strontium isotopic results for the late Miocene Hualapai Limestone of the Lake Mead area (Arizona-Nevada) and the latest Miocene to early Pliocene Bouse Formation and related units of the lower Colorado River trough (Arizona-California-Nevada), together with parallel oxygen and carbon isotopic analyses of Bouse samples, to constrain the lake-overflow model for integration of the Colorado River. Sr isotopic analyses on the basal 1–5 cm of marl, in particular along a transect over a range of altitude in the lowest-altitude basin that contains freshwater, brackish, and marine fossils, document the 87 Sr/ 86 Sr of first-arriving Bouse waters. Results reinforce the similarity between the 87 Sr/ 86 Sr of Bouse Formation carbonates and present-day Colorado River water, and the systematic distinction of these values from Neogene marine Sr. Basal Bouse samples show that 87 Sr/ 86 Sr decreased from 0.7111 to values in the range 0.7107–0.7109 during early basin filling. 87 Sr/ 86 Sr values from a recently identified marl in the Las Vegas area are within the range of Bouse Sr ratios. 87 Sr/ 86 Sr values from the Hualapai Limestone decrease upsection from 0.7195 to 0.7137, in the approach to a time soon after 6 Ma when Hualapai deposition ceased and the Colorado River became established through the Lake Mead area. Bouse Formation δ 18 O values range from −12.9‰ to +1.0‰ Vienna Pee Dee belemnite (VPDB), and δ 13 C between −6.5∐ and +3.4‰ VPDB. Negative δ 18 O values appear to require a continental origin for waters, and the trend to higher δ 18 O suggests evaporation in lake waters. Sr and stable isotopic results for sectioned barnacle shells and from bedding planes of the marine fish fossil Colpichthys regis demonstrate that these animals lived in saline freshwater, and that there is no evidence for incursions of marine water, either long-lived or brief in duration. Lack of correlation of Sr and O isotopic variations in the same samples also argue strongly against systematic replacement of Sr in Bouse carbonates after deposition. Our results reinforce the conclusion that the Bouse Formation was deposited in a descending series of basins connected by overflow of Colorado River water. The Hualapai Limestone records a separate and earlier lake that may have been progressively influenced by Colorado River water as the time of river integration approached.

Evidence from paleosols for low to moderate elevation of the India-Asia suture zone during mid-Cenozoic time

Abstract: The post-collisional uplift history of the India-Asia suture zone in southern Tibet is important for understanding the geodynamic conditions of the India-Asia collision and how it may have modified regional and global climate. Here we use stable isotope and major element analyses of paleosols preserved in the Liuqu Conglomerate to reconstruct India-Asia suture zone paleoclimate and paleoelevation. Paleosol carbonate δ13C (Vienna Peedee belemnite) average values of −9.4‰ ± 1.1‰ indicate that Liuqu paleosols were well vegetated by C3 plants, unlike thinly vegetated and arid modern Tibet. Major element weathering indices show that these soils experienced significant collapse and loss of cations due to weathering of similar intensity to that in the wet, low elevations of the Neogene Himalayan foreland. Age estimates for the Liuqu vary, but current evidence points to an early Miocene age. Our evidence requires that the India-Asia suture zone experienced wet, well-vegetated conditions during Liuqu Conglomerate deposition. Geodynamically this implies that the suture zone was at relatively low elevation and was topographically open to monsoon moisture as late as 40 m.y. after the start of the India-Asian collision. This challenges the idea that uplift of the India-Asia suture zone was monotonic and tied directly to crustal thickening.

Mesozoic─Cenozoic geological evolution of the Himalayan-Tibetan orogen and working tectonic hypotheses

Abstract: The Himalayan-Tibetan orogen culminated during the Cenozoic India – Asia collision, but its geological framework and initial growth were fundamentally the result of multiple, previous ocean closure and intercontinental suturing events. As such, the Himalayan-Tibetan orogen provides an ideal laboratory to investigate geological signatures of the suturing process in general, and how the Earth9s highest and largest orogenic feature formed in specific. This paper synthesizes the Triassic through Cenozoic geology of the central Himalayan-Tibetan orogen and presents our tectonic interpretations in a time series of schematic lithosphere-scale cross-sections and paleogeographic maps. We suggest that north-dipping subducting slabs beneath Asian continental terranes associated with closure of the Paleo-, Meso-, and Neo-Tethys oceans experienced phases of southward trench retreat prior to intercontinental suturing. These trench retreat events created ophiolites in forearc extensional settings and/or a backarc oceanic basins between rifted segments of upper-plate continental margin arcs. This process may have occurred at least three times along the southern Asian margin during northward subduction of Neo-Tethys oceanic lithosphere: from ∼174 to 156 Ma; 132 to 120 Ma; and 90 to 70 Ma. At most other times, the Tibetan terranes underwent Cordilleran-style or collisional contractional deformation. Geological records indicate that most of northern and central Tibet (the Hoh-Xil and Qiangtang terranes, respectively) were uplifted above sea level by Jurassic time, and southern Tibet (the Lhasa terrane) north of its forearc region has been above sea level since ∼100 Ma. Stratigraphic evidence indicates that the northern Himalayan margin of India collided with an Asian-affinity subduction complex – forearc – arc system beginning at ∼60 Ma. Both the Himalaya (composed of Indian crust) and Tibet show continuous geological records of orogenesis since ∼60 Ma. As no evidence exists in the rock record for a younger suture, the simplest interpretation of the geology is that India – Asia collision initiated at ∼60 Ma. Plate circuit, paleomagnetic, and structural reconstructions, however, suggest that the southern margin of Asia was too far north of India to have collided with it at that time. Seismic tomographic images are also suggestive of a second, more southerly Neo-Tethyan oceanic slab in the lower mantle where the northernmost margin of India may have been located at ∼60 Ma. The geology of Tibet and the India – Asia suture zone permits an alternative collision scenario in which the continental margin arc along southern Asia (the Gangdese arc) was split by extension beginning at ∼90 Ma, and along with its forearc to the south (the Xigaze forearc), rifted southward and opened a backarc ocean basin. The rifted arc collided with India at ∼60 Ma whereas the hypothetical backarc ocean basin may not have been consumed until ∼45 Ma. A compilation of igneous age data from Tibet shows that the most recent phase of Gangdese arc magmatism in the southern Lhasa terrane initiated at ∼70 Ma, peaked at ∼51 Ma, and terminated at ∼38 Ma. Cenozoic potassic-adakitic magmatism initiated at ∼45 Ma within a ∼200-km-wide elliptical area within the northern Qiangtang terrane, after which it swept westward and southward with time across central Tibet until ∼26 Ma. At 26 to 23 Ma, potassic-adakitic magmatism swept southward across the Lhasa terrane, a narrow (∼20 km width), orogen-parallel basin developed at low elevation along the axis of the India – Asia suture zone (the Kailas basin), and Greater Himalayan Sequence rocks began extruding southward between the South Tibetan Detachment and Main Central Thrust. The Kailas basin was then uplifted to >4 km elevation by ∼20 Ma, after which parts of the India – Asia suture zone and Gangdese arc experienced >6 km of exhumation (between ∼20 and 16 Ma). Between ∼16 and 12 Ma, slip along the South Tibetan Detachment terminated and east-west extension initiated in the northern Himalaya and Tibet. Potassic-adakitic magmatism in the Lhasa terrane shows a northward younging trend in the age of its termination, beginning at 20 to 18 Ma until volcanism ended at 8 Ma. We interpret the post-45 Ma geological evolution in the context of the subduction dynamics of Indian continental lithosphere and its interplay with delamination of Asian mantle lithosphere.

Decoding tufa and travertine (fresh water carbonates) in the sedimentary record: The state of the art

Abstract: Traditionally, fresh water carbonate research has focused on the sedimentology and palaeontology of ancient lacustrine deposits. Lithofacies in such low-energy deposits are typically fine-grained, developed uniformly in a generally concentric distribution (‘bulls-eye’ pattern) and are predictable even when preserved imperfectly. In contrast, because of their local lithofacies and palaeontological complexities, fluvial carbonates were either delegated to a status of ‘minor geomorphological features’ or barely considered prior to the 1970s. This viewpoint was based on the depositional record of fluvial and spring-fed fresh water carbonates, which were considered to be restricted generally to localized karstic areas. Such deposits are often preserved as scattered patches of ambient temperature tufa. Occasionally, however, in active tectonic areas, localized travertine deposits are also developed from deeply circulating hydrothermal waters. With a few exceptions (for example, basins with high subsidence rates or in arid climate zones), these fresh water carbonates are prone to erosion from continuing river incision and thus may not be preserved in the geological record. A partial record of fluvial and spring-deposited carbonates is often preserved in Quaternary deposits, but the record in older deposits is typically fragmentary and often diagenetically modified. Yet once their unique facies architecture (and specialized nomenclature) is understood, these carbonates provide an important record of past sedimentological cycles of great value in palaeoenvironmental landscape modelling. The emphasis of modern research is to acquire information that explains how active systems function. In this respect, tufas reveal much of how carbonate precipitation is a shared product of physico-chemical and microbiological biomediation processes. Likewise, travertines not only show an intimate interrelation with active tectonism but also hold great potential as monitors of past volcanic carbon dioxide emissions. In addition, both tufas and travertines contain palynological records that can be used as proxy indicators of climate change. Perhaps no other field of sedimentology has witnessed more developments and applications over such a brief period of study.

Degassing of mantle-derived CO2 and He from springs in the southern Colorado Plateau region—Neotectonic connections and implications for groundwater systems

Abstract: Groundwaters of the southern Colorado Plateau–Arizona Transition Zone region are a heterogeneous mixture of chemically diverse waters including meteoric (epigenic) fluids, karst-aquifer waters, and deeply sourced (endogenic) fluids. We investigate the composition of travertine-depositing CO 2 -rich springs to determine the origin, transport, and mixing of these various components. The San Francisco Mountain recharge area has little surface flow. Instead, waters discharge through major springs hundreds of kilometers away. About 70% (9340 L/s) of the total recharge (13,500 L/s) discharges 100 km to the north in the incised aquifer system at Grand Canyon. Most of this water (85%; 8070 L/s) emerges through two travertine-depositing karst spring systems: Blue Springs (6230 L/s) and Havasu Springs (1840 L/s). About 30% of recharge (4150 L/s) flows to the south and discharges along NW-striking faults in the Arizona Transition Zone, forming the base flow for the Verde River. Geochemical data define regional mixing trends between meteoric recharge and different endogenic end members that range from bicarbonate waters to sulfate waters. Water quality in the region is dictated by the percentage and character of the endogenic inputs that cause a measurable degradation of groundwater quality for water supply. Sources for the high CO 2 include dissolution of limestone and dolostone (C carb ) and “external carbon” (C external ). C external is computed as the bicarbonate alkalinity (dissolved inorganic carbon [DIC]) minus the C carb (C external = DIC - C carb ). C external is deconvolved using carbon isotopes into biogenically derived sedimentary carbon (C organic ) and deep CO 2 inputs (C endogenic ). Measured δ 13 C values are −17‰ to +3‰ versus Pee Dee Belemnite (PDB). Assuming δ 13 C carb = +2‰, δ 13 C organic = −28‰, and δ 13 C endogenic = −5‰, water chemistry mixing models indicate that an average of 42% of the total DIC comes from dissolution of carbonate rocks, 25% from organic carbon, including soil-respired CO 2 ,and 33% from deep (endogenic) sources. Helium isotope values ( 3 He/ 4 He) in gases dissolved in spring waters in the southern Colorado Plateau region range from 0.10 to 1.16 R A (relative to air) indicating that a significant component of the deeply derived fluid is from the mantle (mean of 5% asthenospheric or 10% subcontinental lithospheric mantle source). Measured CO 2 / 3 He ratios of 2 × 10 9 to 1.4 × 10 13 are adjusted by removing the proportion of CO 2 from C carb and C organic to give values 10 for all but four samples. Various mixing models using CO 2 / 3 He suggest that the mantle-derived components of the CO 2 load are highly variable from spring to spring and may make up an average of ~10% of the total CO 2 load of the regional springs. Fluid-rock interactions involving endogenic fluids are suggested by 87 Sr/ 86 Sr, δ 18 O, and other tracers. The endogenic CO 2 component, multiplied by discharge for each spring, yields an integrated annual flux of deeply derived CO 2 to the groundwater system of ~1.4 × 10 9 mol/yr. This CO 2 emission from the Colorado Plateau region reflects a complex tectonic evolution involving Laramide hydration of the lithosphere above the Farallon slab, addition of fluids from mid-Tertiary mantle tectonism during slab removal, and ongoing fluid movement induced by neotectonic small-scale asthenospheric convection.

Structural and diagenetic control of fluid migration and cementation along the Moab fault, Utah

Abstract: The Moab fault, a basin-scale normal fault that juxtaposes Jurassic eolian sandstone units against Upper Jurassic and Cretaceous shale and sandstone, is locally associated with extensive calcite and lesser quartz cement. We mapped the distribution of fault-related diagenetic alteration products relative to the fault structure to identify sealing and conductive fault segments for fluid flow and to relate fault–fluid-flow behavior to the internal architecture of the fault zone. Calcite cement occurs as vein and breccia cement along slip surfaces and as discontinuous vein cement and concretions in fault damage zones. The cement predominates along fault segments that are composed of joints, sheared joints, and breccias that overprint earlier deformation bands. Using the distribution of fault-related calcite cement as an indicator of paleofluid migration, we infer that fault-parallel fluid flow was focused along fault segments that were overprinted by joints and sheared joints. Joint density, and thus fault-parallel permeability, is highest at locations of structural complexity such as fault intersections, extensional steps, and fault-segment terminations. The association of calcite with remnant hydrocarbons suggests that calcite precipitation was mediated by the degradation and microbial oxidation of hydrocarbons. We propose that the discontinuous occurrence of microbially mediated calcite cement may impede, but not completely seal, fault-parallel fluid flow. Fault-perpendicular flow, however, is mostly impeded by the juxtaposition of the sandstone units against shale and by shale entrainment. The Moab fault thus exemplifies the complex interaction of fault architecture and diagenetic sealing processes in controlling the hydraulic properties of faults in clastic sequences.

Model for tectonically driven incision of the younger than 6 Ma Grand Canyon

Abstract: Accurate models for the incision of the Grand Canyon must include characterization of tectonic influences on incision dynamics such as active faulting and mantle to surface fluid interconnections. These young tectonic features support other geologic data that indicate that the Grand Canyon has been carved in the past 6 Ma. New U-Pb dates on speleothems are reinterpreted here in terms of improved geologic constraints and understanding of the modern aquifer. The combined data suggest that Grand Canyon incision rates have been relatively steady since 3–4 Ma. Differences in rates in the eastern (175–250 m/Ma) and western (50–80 m/Ma) Grand Canyon are explained by Neogene fault block uplift across the Toroweap-Hurricane system. Mantle tomography shows an abrupt step in mantle velocities near the Colorado Plateau edge, and geodynamic modeling suggests that upwelling asthenosphere is driving uplift of the Colorado Plateau margin relative to the Basin and Range. Our model for dynamic surface uplift in the past 6 Ma contrasts with the notion of passive incision of the Grand Canyon due solely to river integration and geomorphic response to base-level fall.