George Gehrels and William Thomas University of Arizona
This project has a specific goal of testing the hypothesis that some of the sand that make up the sandstones of the Grand Canyon were supplied primarily from the Appalachian mountains, and a more general goal of reconstructing dispersal pathways for sediment across the US during late Paleozoic (350-245 million years ago) time. Conducting source area studies for this time period is challenging because of the complex interplay of mountian building processes and climate. During this time the Ouachita-Appalachian-Caledonian mountain range formed as result of collisional interactions during assembly of the supercontinent Pangea, the Cordilleran mountain range experienced arc-type magmatism and several phases of convergent tectonism, and the Ancestral Rocky Mountains were uplifted. There were also dramatic changes in regional and global paleoclimate as North America moved northward across the paleo-equator and as southern hemisphere glaciation created large changes in sea level. North America accordingly serves as a fabulous laboratory in which to examine the interplay between these disparate processes, potentially serving as an ancient analogue of our modern world. The hypothesized Grand Canyon-Appalachian connection is based on detrital zircon grains of ~720-280 Ma that appear in increasing abundance in Mississippian through Permian strata of the Grand Canyon, as reported in the June 2011 issue of Lithosphere by Gehrels and colleagues. These young grains were interpreted to have been shed from the Appalachian orogen primarily on the basis of similarities with crystallization ages of Appalachian plutons and detrital zircon ages reported from upper Paleozoic strata of the Appalachian foreland basin. This interpretation was challenged in the August 2011 issue of Lithosphere by Thomas because there is little evidence that clastic detritus was transported westward out of the Appalachian foreland basins, and because the young sediment could have been shed from other orogens (e.g. the Fanklinian system to the north) and in part from local igneous rocks. Thomas (2011) also proposed that testing these hypotheses by combining additional DZ provenance data with facies relations, thickness patterns, and petrographic characteristics of upper Paleozoic strata would enable reconstruction of the dispersal pathways of sediment transport across North America during late Paleozoic time. We propose to combine U-Pb age and Hf isotope determinations of ~60 samples with traditional stratigraphic information to specifically test the Grand Canyon-Appalachian hypothesis and more generally reconstruct late Paleozoic dispersal pathways of southern North America.
Three impacts beyond the geologic contributions noted above include: (1) Providing opportunities for faculty members and undergraduate students at academic institutions near study areas to become involved in sample collection, analysis, and interpretation. (2) Advancing the field of provenance analysis by integrating U-Pb geochronology, Hf isotope analysis, and traditional stratigraphic/sedimentologic analysis of sandstones in an effort to reconstruct dispersal pathways and to examine linkages between tectonics and climate. (3) Sharing information with the public by providing operators of State and National Parks near our study areas with the results of our work, following the model of the 'Trail of Time' exhibits at the Grand Canyon.
