NEW MAPPING, STRATIGRAPHIC AND PROVENANCE ANALYSIS OF CRETACEOUS-PALEOGENE CONGLOMERATES IN NORTHERN UTAH

Stratigraphic and structural relations of synorogenic strata deposited in foreland and wedge-top basins in northern Utah constrain the timing, location, and magnitude of thrust sheet emplacement and exhumation. Herein, we present new geologic mapping, conglomerate clast counts, sandstone grain point counts, and detrital zircon data from proximal Cretaceous to Paleogene synorogenic strata deposited along the flank of the basement-cored Wasatch anticlinorium and across parts of the Crawford and Absaroka thrust sheets. New mapping has identified four distinct conglomeratic packages in the study area that were previously mapped mostly as the Tertiary Wasatch Formation. New unit identification is primarily based on changes in conglomerate petrofacies that have also been identified in correlative strata to the east. Conglomeratic units overlie Paleoproterozoic basement of the anticlinorium and folded Paleozoic-Jurassic rocks with angular unconformity, and comprise from base to top: (i) proximal facies of the Aptian to Turonian Kelvin and Frontier Formations; (ii) coarse conglomerates with local grown relations along the East Canyon backthrust correlated with the Echo Canyon and Weber Canyon Conglomerates; (iii) boulder conglomerate above an unconformity that thickens northward and is interbedded with finer-grained strata correlative with the Maastrichtian-Paleocene Evanston Formation; and (iv) polymict conglomerate with lenses of lacustrine limestone correlative with the Eocene Wasatch Formation. Preliminary U-Pb geochronologic analysis of detrital zircon in samples of synorogenic strata show “classic” North American zircon populations originally sourced from the Yavapai-Mazatzal, Mid-continent rhyolite, Grenville, and Appalachian basement provinces, along with an apparent lack of Cordilleran arc-sourced grains. Patterns are consistent with unroofing of multiple stratigraphic levels across frontal and piggy-back thrust sheets, recycling from erosion of previously deposited synorogenic strata, and development of a topographic barrier along the Wasatch anticlinorium that separated the foreland basin in this area from sources in the hinterland to magmatic arc, whereas arc-sourced grains are common in correlative synorogenic strata farther north and south.