We determined U-Pb ages for detrital zircons from 26 samples of Paleozoic sandstone from the Grand Canyon. Cambrian strata yield mainly ca. 1.44 and 1.7–1.8 Ga ages that indicate derivation from nearby basement rocks of the Yavapai Province. Devonian strata contain zircons of 1.6–1.8 Ga, 1.34–1.40 Ga, and ca. 520 Ma, suggesting derivation from the Mazatzal and Yavapai Provinces, midcontinent region, and the Amarillo-Wichita uplift, respectively. Mississippian strata record a major change in provenance, with predominantly 415–475 Ma and 1030–1190 Ma grains interpreted to have been shed from the central Appalachian orogen. Pennsylvanian strata contain subequal proportions of 1.4–1.8 Ga grains derived from basement rocks exposed in the Ancestral Rocky Mountains and 409–464 and ca. 1070 Ma grains derived from the Appalachians. Permian strata contain abundant Appalachian zircons, including 270–380 Ma grains, and a lesser proportion of grains derived from the Ancestral Rocky Mountains. Transcontinental transport during Mississippian through Permian time is interpreted to have occurred in large river systems, facilitated by northeasterly trade winds during low sea level and by coastal currents. A compilation of young ages from all Upper Paleozoic strata yields age peaks of 270–365 Ma, 395–475 Ma, and 515–640 Ma, an excellent match for Alleghanian, Acadian, Taconic, and Neoproterozoic (peri-Gondwanan) episodes of magmatism along the Appalachian margin. Lag times of the youngest grains in these Upper Paleozoic strata average ∼25 m.y., suggesting relatively rapid exhumation and erosion of Appalachian source regions.
This chapter contains sections titled: Introduction Methods and Assumptions New 40AW39AR∼E Sults from Colorado and New Mexico Evidence for a Regional Thermal Episode at -1.4 GA Regional ∼ 1.4 Ga Temperatures Conclusions
CO 2 -rich springs of the western U.S. associated with Quaternary travertine and lacustrine carbonate deposits, record long-lived interactions of deeply-sourced ("endogenic") fluids with the near-surface hydrologic regime.Springs occur along faults and fracture zones associated with continental extension (e.g., Rio Grande rift, Basin and Range, Arizona transition zone).Upwelling waters may emerge as springs along basin margins or they may mix with aquifer waters in the shallow hydrologic system.They represent diffuse degassing and a generally unrecognized flux of CO 2 into regional aquifers, and also impair water quality via high solute loads and the presence of elevated trace metal concentrations (e.g., arsenic).Geochemical mixing models indicate that only a small component of saline, radiogenic, hydrothermal fluid is needed to produce observed spring chemistries.He and C isotopes are suggestive of a deep crustal or mantle origin for the gases, linking them to magmatism and extensional tectonics.Both cool (20-35ºC) and hot springs (40-80ºC) share geochemical similarities to the chemolithotrophic microbial ecosystems found in oceanic hydrothermal systems related to extensional tectonic settings (black and white smokers at mid-ocean ridges).Microbial community analysis reveals the presence of microorganisms utilizing many of the same metabolic pathways found in oceanic hydrothermal settings.Cloning and sequencing of amplified 16S rRNA genes using universal primers identifies organisms with >95% similarity to marine denitrifiers and thermophiles, as well as novel forms (<90% 16S rRNA gene similarity).Results reveal a microbial community strikingly uncharacteristic of known terrestrial springs.Bacterial communities are similar among sampled locales in CO, AZ and NM, and include many Gamma-proteobacteria sequences that exhibit strong similarity to halophilic and marine bacteria representatives from cold seeps, hydrothermal vents, saline lakes, and Arctic brine ice.Archaeal sequences are dominated by thermophilic Crenarchaeota, detected in marine and terrestrial volcanic environments.These results suggest that the springs harbor microbial communities similar to marine vent systems and seeps due to similarities in the geochemical environment.
