Tracing δ18O and δ2H in Source Waters and Recharge Pathways of a Fractured-Basalt and Interbedded-Sediment Aquifer, Columbia River Flood Basalt Province

The heterogeneity and anisotropy of fractured-rock aquifers, such as those in the Columbia River Basalt Province, present challenges for determining groundwater recharge. The entrance of recharge to the fractured-basalt and interbedded-sediment aquifer in the Palouse region of north-central Idaho is not well understood because of successive basalt flows that act as restrictive barriers. It was hypothesized that a primary recharge zone exists along the basin’s eastern margin at a mountain-front interface where eroded sediments form a more conductive zone for recharge. Potential source waters and groundwater were analyzed for δ18O and δ2H to discriminate recharge sources and pathways. Snowpack values ranged from −22 to −12‰ for δ18O and from −160 to −90‰ for δ2H and produced spring-time snowmelt ranging from −16.5 to −12‰ for δ18O and from −120 to −90‰ for δ2H. With the transition of snowmelt to spring-time ephemeral creeks, the isotope values compressed to −16 and −14‰ for δ18O and −110 and −105‰ for δ2H. A greater range of values was present for a perennial creek (−18 to −13.5‰ for δ18O and −125 to −98‰ for δ2H) and groundwater (−17.5 to −13‰ for δ18O and −132 to −105‰ for δ2H), which reflect a mixing of seasonal signals and the varying influence of vapor sources and sublimation/evaporation. Inverse modeling and the evaluation of matrix characteristics indicate conductive pathways associated with paleochannels and deeper pathways along the mountain-front interface. Depleted isotope signals indicate quicker infiltration and recharge pathways that were separate from, or had limited mixing with, more evaporated water that infiltrated after greater time/travel at the surface.

The spatial and temporal evolution of the Portland and Tualatin forearc basins, Oregon, USA

The Portland and Tualatin basins are part of the Salish-Puget-Willamette Lowland, a 900-km-long, forearc depression lying between the volcanic arc and the Coast Ranges of the Cascadia convergent margin. Such inland seaways are characteristic of warm, young slab subduction. We analyzed the basins to better understand their evolution and relation to Coast Range history and to provide an improved tectonic framework for the Portland metropolitan area. We model three key horizons in the basins: (1) the top of the Columbia River Basalt Group (CRBG), (2) the bottom of the CRBG, and (3) the top of Eocene basement. Isochore maps constrain basin depocenters during (1) Pleistocene to mid-Miocene time (0–15 Ma), (2) CRBG (15.5–16.5 Ma), and (3) early Miocene to late Eocene (ca. 17–35 Ma) time. Results show that the Portland and Tualatin basins have distinct mid-Miocene to Quaternary depocenters but were one continuous basin from the Eocene until mid-Miocene time. A NW-striking gravity low coincident with the NW-striking, fault-bounded Portland Hills anticline is interpreted as an older graben coincident with observed thickening of CRBG flows and underlying sedimentary rocks. Neogene transpression in the forearc structurally inverted the Sylvan-Oatfield and Portland Hills normal faults as high-angle dextral-reverse faults, separating the Portland and Tualatin basins. An eastward shift of the forearc basin depocenter and ten-fold decrease in accommodation space provide temporal constraints on the emergence of the Coast Range to the west. Clockwise rotation and northward transport of the forearc is deforming the basins and producing local earthquakes beneath the metropolitan area.

Annually resolved sediments in the classic Clarkia lacustrine deposits (Idaho, USA) during the middle Miocene Climate Optimum

The world-renowned Miocene Clarkia paleolake in northern Idaho (USA) is closely associated with Columbia River Basalt Group volcanism. The flood basalt dammed a local drainage system to form the paleolake, which preserved a plant fossil Lagerstätte in its deposits. However, the precise age and temporal duration of the lake remain unsettled. We present the first unequivocal U-Pb zircon ages from interbedded volcanic ashes at the P-33 type location, constraining the deposition to 15.78 ± 0.039 Ma. Using micro–X-ray fluorescence and petrographic and spectral analyses, we establish the annual characteristics of laminations throughout the stratigraphic profile using the distribution of elemental ratios, mineral assemblages, and grain-size structures, as well as organic and fossil contents. Consequently, the ~7.5-m-thick varved deposit at the type location P-33 represents ~840 yr of deposition, coincident with the end of the main phase of Columbia River Basalt Group eruptions during the Miocene Climate Optimum. The timing and temporal resolution of the deposit offer a unique opportunity to study climate change in unprecedented detail during global warming associated with carbon-cycle perturbations.

Linking exhumation, paleo-relief, and rift formation to magmatic processes in the Western Snake River Plain, Idaho

<p>Southwest Idaho has experienced substantial topographic changes over the Cenozoic that are reflections of complex tectonic and mantle processes. The western Snake River Plain (WSRP) in southwest Idaho has been characterized as an intracontinental rift basin but differs markedly in topography and style from other western Cordilleran extensional structures. It also differs in orientation and structural style from the down warped lava plain of the eastern Snake River Plain that follows the path of the Yellowstone hotspot (YHS). Potential magmatic drivers for WSRP formation include the ~12-10 Ma Bruneau-Jarbidge eruptive center of the YHS or the ~17-16 Ma Columbia River Basalt (CRB) large igneous province. To better constrain the timing and style of rifting in the region we sampled granitoid bedrock from Cretaceous and Eocene-aged plutons from the flanks of the WSRP to detail their exhumation history with apatite (U-Th)/He (AHe) thermochronometry. We present new AHe dates from seventeen samples, with cooling dates ranging range from 7 Ma to 55 Ma. The majority of cooling dates for the Cretaceous plutons are Eocene, and the Eocene intrusions yield Miocene dates. The AHe dates provide thermochronological evidence of rapid cooling and exhumation of the Idaho batholith during the Eocene. This supports the presence a high relief landscape in Idaho associated with regional uplift due to Farallon slab rollback and Challis magmatism. We also find evidence for a post-Eocene decrease in relief, seen in the negative slope on date-elevation relationships in the southwest flank of the WSRP. Our AHe dates indicate limited exhumation on the flanks of the WSRP during Miocene rift formation. We interpret this to be evidence of extension dominated by magmatic intrusions and intrabasin faults rather than basin-bounding faults. Miocene AHe dates show rapid exhumation along the Middle Fork Boise River that had begun by ~17 Ma. We take this to indicate focused incision along the river due to base level fall in the WSRP and the timing suggests that CRB activity was responsible for initiation of WSRP formation</p>

Igneous Rock Associations 27. Chalcophile and Platinum Group Elements in the Columbia River Basalt Group: A Model for Flood Basalt Lavas

The Columbia River Basalt Group is the youngest and best preserved continental Large Igneous Province on Earth. The 210,000 km3 of basaltic lavas were erupted between 16.6 and 5 Ma in the Pacific Northwest, USA. The peak of the eruptions occurred over a 700,000-year period when nearly 99% of the basalts consisting of the Steens, Imnaha, Picture Gorge, Grande Ronde and Wanapum Basalts were emplaced. In this study we examined the Platinum Group Elements (PGEs) Pt and Pd, and the chalcophile elements Cu and Zn in the Columbia River Basalt Group. The presence of Pt, Pd and Cu in the compositionally primitive Lower Steens, Imnaha and Picture Gorge Basalts suggests that the Columbia River Basalt Group magma was a fertile source for these elements. The PGEs are contained mainly in sulphides in the earliest formations based on their correlation with immiscible sulphides, sulphide minerals and chalcophile elements. Grande Ronde, Wanapum and Saddle Mountains Basalts are depleted in PGEs and chalcophile elements compared to earlier formations. Sulphur was saturated in many flows and much of it probably came from assimilation of cratonic rock from a thinned lithosphere. We propose a model where the presence or absence of PGEs and chalcophile elements results primarily from the interaction between an advancing plume head and the crust/lithosphere that it encountered. The early lavas erupted from a plume that had little interaction with the crust/lithosphere and were fertile. However, as the plume head advanced northward, it assimilated crustal/lithospheric material and PGE and chalcophile elements were depleted from the magma. What little PGE and chalcophile elements remained in the compositionally evolved and depleted Grande Ronde Basalt flows mainly were controlled by substitution in basalt minerals and not available for inclusion in sulphides.

Supplemental Material: The Chief Joseph dike swarm of the Columbia River flood basalts, and the legacy data set of William H. Taubeneck

Supplemental Plates. Plate S1: Large-scale map of entire extent of Chief Joseph dike swarm. Also incorporates dikes of Ice Harbor, Steens, and Monument swarms. Plate S1 represents the most complete record of dikes related to Columbia River Basalt Group (CRBG) event known. Plate S2: Simplified map of CRBG-related dikes across the inland Pacific Northwest. Dikes are colored by their orientation and dike line density is also shown. Plate S3: Simplified map of CRBG-related dikes across the inland Pacific Northwest.<br>

Supplemental Material: The Chief Joseph dike swarm of the Columbia River flood basalts, and the legacy data set of William H. Taubeneck

Supplemental Plates. Plate S1: Large-scale map of entire extent of Chief Joseph dike swarm. Also incorporates dikes of Ice Harbor, Steens, and Monument swarms. Plate S1 represents the most complete record of dikes related to Columbia River Basalt Group (CRBG) event known. Plate S2: Simplified map of CRBG-related dikes across the inland Pacific Northwest. Dikes are colored by their orientation and dike line density is also shown. Plate S3: Simplified map of CRBG-related dikes across the inland Pacific Northwest.<br>

Supplemental Material: The Chief Joseph dike swarm of the Columbia River flood basalts, and the legacy data set of William H. Taubeneck

Supplemental Plates. Plate S1: Large-scale map of entire extent of Chief Joseph dike swarm. Also incorporates dikes of Ice Harbor, Steens, and Monument swarms. Plate S1 represents the most complete record of dikes related to Columbia River Basalt Group (CRBG) event known. Plate S2: Simplified map of CRBG-related dikes across the inland Pacific Northwest. Dikes are colored by their orientation and dike line density is also shown. Plate S3: Simplified map of CRBG-related dikes across the inland Pacific Northwest.<br>

A basin-scale groundwater flow model in the Columbia Plateau (Pacific Northwest, USA); insights for management of fractured aquifer-types

&lt;p&gt;Mechanical discontinuities control groundwater flow in fractured aquifers. Bedding plane and sub-vertical discontinuities create fracture networks geometrically organized both horizontally and vertically in areas un-affected by compressional tectonic forces. In this structural setting, we use the Columbia River Basalt aquifer in the Palouse to show how the combination of previous acquired stable isotope data and geological, groundwater, and particle tracking modeling better describes groundwater flow in three dimensions. We present a steady-state flow model simulating backward particle traces from abstraction wells to the recharge boundaries. Backwards particle analysis coupled with the &lt;sup&gt;14&lt;/sup&gt;C isotope vertical concentration distribution shows how the aquifer system is characterized by two separate zones. A shallow (&lt;120 mBGL) zone of freshwater circulation is characterized by higher &lt;sup&gt;14&lt;/sup&gt;C concentrations and low particle travel times with respect to the deeper (&gt;120 mBGL) aquifer zone. Here, penetration of particles is partially impeded by the low vertical hydraulic conductivity of the volcano-sedimentary layers and recharge preferentially occurs in correspondence of discontinuities related to a geological unconformity. Hence, the outputs of a particle tracking analysis fits stable isotope data either validating a 3D groundwater flow model or aiding detail to conceptualization of a fractured aquifer.&lt;/p&gt;&lt;p&gt;The Columbia River Basalt aquifer is also horizontally anisotropic due to sub-vertical tectonic fractures which are related to gentle folding and faulting. This horizontal anisotropy significantly influences particle tracking analysis in the basin up to 120 mBGL. Well-head protection areas are defined globally by backward particle tracking analyses at shallow depths. Thus, as a consequence of this research we envisage introduction of horizontal anisotropies in groundwater flow models for definition of well capture zones.&lt;/p&gt;