Copper deposits on the Colorado Plateau are well exposed variants on the theme of sediment-hosted copper systems. Study of two contrasting areas in the Colorado Plateau, Lisbon Valley and the northern Moab fault, allows evaluation of controls on mineralization and their possible relationships to broader basinal processes, through a combination of published work with new mapping, core logging, petrography, short-wave infrared spectroscopy (SWIR), and geochemical analyses. The focus of this study was on the GTO pit in the Lisbon Valley Mine and surface exposures along the northern Moab Fault. Both areas occur in the vicinity of step-over zones along major NW-trending normal faults. Both also areas also exhibit local bleaching of red beds and rare bitumen, suggesting flow of reduced fluids (hydrocarbons?). However, in detail, both areas differ in host, recognized alteration, and styles of mineralization. At Lisbon Valley, economic copper mineralization, composed principally of chalcocite and its weathering products, is mainly restricted to pyrite-depleted, carbonate-cement-poor strata in the variably pyritic sandstones and conglomerates of the Early Cretaceous Burro Canyon formation. In this area, bleaching is found throughout the stratigraphic column from the Permian to the Jurassic and are associated with formation of secondary kaolinite and other acid-stable sheet silicates. Rare hematite preserved under quartz overgrowths suggests that some of the Burro Canyon may have been red prior to (hydrocarbon-related?) bleaching. Mineralization typically occurs within 100 meters of major strands of the Lisbon Valley fault system. By contrast, along the northern Moab Fault the bleached upper part of the Moab Member of the Entrada formation contains widespread sparse copper mineralization, in the form of Cu(±Fe) sulfides and their weathering products. Fracture-controlled concentrations of Cu-Fe sulfides (now cuprian limonites) are locally abundant along structures at along the northern Moab fault. However, unlike at Lisbon Valley, the typical strata hosting Cu are Fe poor show little evidence for widespread early pyrite. The copper occurrences (now oxidized) have high Cu:Fe (>5:1) indicating a chalcocite-like, iron-poor precursor. Relatively pure calcite cement and nodules are abundant close to the fault splays whereas variably ferroan and manganoan carbonate is extensive in the lower Moab Member. Evidence of selective bleaching of original red beds near faults and in favorable (sandy) strata is well developed in both the Curtis and Slick Rock members of the Entrada Formation. Where present, copper occurs with bleaching indicating that it may have been localized by the presence of hydrocarbons or sour gas along the northern Moab fault and Lisbon Valley.. Mineralization postdates bleaching and broadly correlates with structures (second-order) if not specifically structurally controlled (first-order). Based on published work, these relationships imply that mineralization is Laramide (Late Cretaceous) or younger. Oxidized basinal brines, focused by faults and favorable stratigraphy, are inferred to have carried Cu and other metals which were then trapped by reaction with sulfide. The sulfide came from combinations of: (1) preexisting pyrite in the host rock, (2) H2S in a second fluid, and/or (3) S2- generated by thermochemical or bacterial sulfate reduction by organic carbon. At Lisbon Valley, stratigraphic control by preexisting pyrite and a reducing, perhaps sulfide-bearing fluid appear to have been the key traps in the Cretaceous rocks; along the Moab Fault, stratigraphic localization of copper at the top of the Curtis in iron-poor rocks suggests a hydrocarbon or gas (H2S-bearing?) trap. These results parallel evidence for similarly diverse controls on faults and stratigraphy elsewhere in the Paradox Basin.

Changes in channel morphology of the ephemeral streams of the Rillito Creek system through time have constituted greater hazards for the Tucson metropolitan area than has overbank flooding. The behavior of this stream system has been investigated to document past channel instability, determine potential sites of bank erosion, and suggest floodplain management alternatives to the federal regulations currently applied to semiarid regions. Rillito Creek and its main tributaries, Pantano Wash and Tanque Verde Creek, were mapped from Houghton Road to the mouth of Rillito Creek using aerial photographs taken between 1941 and 1979. These maps, in conjunction with streamflow and channel composition data, detail channel change within this stream system. Prior to and including 1941, the Rillito Creek system exhibited braided plan-view patterns. By the early 1960's, single channel patterns had developed, accompanied by extensive decreases in channel widths. Pantano Wash continued to narrow to 1979 as its depth increased. Tanque Verde Creek and Rillito Creek widened extensively in 1965 and 1978 during prolonged winter flows, and narrowed during intervening periods dominated by low magnitude summer flows. Channel banks were eroded locally throughout this system during the periods of channel narrowing, however. The greatest amounts of bank erosion have occurred on the outer banks of channel bends and at locations where the silt-clay content of the banks and the density of riparian vegetation have been the least. In compliance with federal regulations, management of the floodplains of the Rillito Creek system has been focused mainly on those areas subject to flooding by 100-year floods. Frequent changes in channel morphology have created problems for floodplain management by altering the areas subject to flooding. In addition, bank erosion during lower magnitude flows historically has posed a greater hazard than has overbank flow. For more effective floodplain management, zones of potential bank erosion based on past erosional sites, historical channel positions, and current stream channel patterns should be considered for land-use regulation as well as potential areas of flooding. The principles established herein are applicable to other ephemeral stream systems in semiarid regions.