The Rifle alluvial aquifer along the Colorado River in west central Colorado contains finegrained, diffusionlimited sediment lenses that are substantially enriched in organic carbon and sulfides, as well as uranium, from previous milling operations. These naturally reduced zones (NRZs) coincide spatially with a persistent uranium groundwater plume. There is concern that uranium release from NRZs is contributing to plume persistence or will do so in the future. To better define the physical extent, heterogeneity and biogeochemistry of these NRZs, we investigated sediment cores from five neighboring wells. The main NRZ body exhibited uranium concentrations up to 100 mg/kg U as U(IV) and contains ca. 286 g of U in total. Uranium accumulated only in areas where organic carbon and reduced sulfur (as iron sulfides) were present, emphasizing the importance of sulfatereducing conditions to uranium retention and the essential role of organic matter. NRZs further exhibited centimeterscale variations in both redox status and particle size. Mackinawite, greigite, pyrite and sulfate coexist in the sediments, indicating that dynamic redox cycling occurs within NRZs and that their internal portions can be seasonally oxidized. We show that oxidative U(VI) release to the aquifer has the potential to sustain a groundwater contaminant plume for centuries. NRZs, known to exist in other uraniumcontaminated aquifers, may be regionally important to uranium persistence.

This paper synthesizes the framework and geological evolution of the Arctic Alaska–Chukotka microplate (AACM), from its origin as part of the continental platform fringing Baltica and Laurentia to its southward motion during the formation of the Amerasia Basin (Arctic Ocean) and its progressive modification as part of the dynamic northern palaeo-Pacific margin. A synthesis of the available data refines the crustal identity, limits and history of the AACM and, together with regional geological constraints, provides a tectonic framework to aid in its pre-Cretaceous restoration. Recently published seismic reflection data and interpretations, integrated with regional geological constraints, provide the basis for a new crustal transect (the Circum-Arctic Lithosphere Evolution (‘CALE’) Transect C) linking the Amerasia Basin and the Pacific margin along two paths that span 5100 km from the Lomonosov Ridge (near the North Pole), across the Amerasia Basin, Chukchi Sea and Bering Sea, and ending at the subducting Pacific plate margin in the Aleutian Islands. We propose a new plate tectonic model in which the AACM originated as part of a re-entrant in the palaeo-Pacific margin and moved to its present position during slab-related magmatism and the southward retreat of palaeo-Pacific subduction, largely coeval with the rifting and formation of the Amerasia Basin in its wake.

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21The Yukon Koyukuk Basin is a large depression that covers ~118,000 sq. km in 22 western interior Alaska that is divided into two sub-basins by a volcanic arc assemblage. Sedimentary basins that flank orogenic belts serve as archives that record the 58 long-term deformational and uplift history of orogenic systems. When associated with a 59 topographic effect, the kinematic evolution of an orogen affects the geometry, filling 60 pattern, and provenance signature of the adjacent synorogenic sedimentary basins 61 (Bayona, et al., 2008). Processes such as tectonic uplift and surficial denudation, rainfall, 62 sediment supply, sediment transport and dispersal have direct implications on the 63 evolution of synorogenic sediment routing systems (Whitchurch et al., 2011). As 64 observed in several Cenozoic orogenic belts (e.g. Himalayas and Andes), differences in 65 sedimentary filling patterns are recognizable by variations in large-scale axial and 66 transverse river systems and smaller transverse rivers observed draining into and within 67 the foreland and hinterland basins separated by an orogenic high (Burbank et al., 1996; 68 Clark et al., 2004; DeCelles, 2012; Dietze et al., 2014; Hessler and Fildani, 2015). This 69 4 variation in drainage systems leads to differences in provenance signatures recorded 70 within the basin sediments (Lease et al., 2007; Bayona, et al., 2008; Cina, et al., 2009; 71 DeCelles et al., 2014). Therefore, the provenance signature of the sediments that occupy 72 these basins should help in understanding the evolution of the synorogenic sediment 73 routing systems on either side of the orogen. 74In northern Alaska, the latest Jurassic -Early Cretaceous Brooks Range orogen is 75 flanked by two synorogenic sedimentary basins, the northern Colville foreland basin and 76 southern Yukon Koyukuk Basin (Fig. 1). Renewed deposition of thick successions of 77 clastic rocks in the Colville Basin occurred coincidently with the formation of Yukon 78Koyukuk Basin, which led authors to conclude that these events were the result of 79 orogenic uplift of the Brooks Range (Molenaar et al., 1984(Molenaar et al., , 1986 Miller and Hudson, 80 1991;Till, 1992). Seismic reflection data collected within the Colville Basin delineate a 81 series of turbidite-dominated sedimentary packages that longitudinally filled the basin 82 from west to northeast from the late Early Cretaceous to the Cenozoic (Houseknecht et 83 al., 2009; Houseknecht and Wartes, 2013). Despite all the work performed in 84 characterizing the clastic rocks of the Colville Basin, little emphasis has been paid to 85 understanding the formation and sedimentary evolution of the southern Yukon Koyukuk 86 Basin. 87The Yukon Koyukuk Basin is a large wedge-shaped physiographic depression, 88 underlain by volcanic and marine sedimentary rock units, that covers ~118,000 sq. km 89 (Fig. 1) formed in a syn-collisional forearc setting (Till et al., 1993) to its formation during post-95 accretion lithospheric extension of the hinterlan...