U-PB AGES OF DETRITAL ZIRCONS IN FLUVIAL LOWER CRETACEOUS AND DELTAIC UPPER CRETACEOUS STRATA OF THE FOUR COURNERS REGION

Detrital zircon grains were collected from the fluvial Lower Cretaceous Burro Canyon Formation (two samples, one near Bluff UT and one near Ghost Ranch NM) and three Upper Cretaceous deltaic sandstone units: Turonian Toreva Formation of the Black Mesa basin and Gallup Sandstone of the San Juan basin; Campanian Menefee Formation of the Mesaverde Group in the San Juan basin. Paleocurrent indicators and facies relations for all units sampled document stream paleoflow from southwest to northeast. U-Pb ages of 100 detrital zircon grains per sample were analyzed by LA-ICP-MS using a beam diameter of 50 microns. Analytical data were filtered to exclude detrital zircon grains with >20% age discordance. All three deltaic Upper Cretaceous samples contain a prominent spike in detrital zircon ages at 1600-1800 Ma and a more subdued spike at 1400-1500 Ma, both probably derived from Precambrian basement of the Yavapai-Mazatzal belt as exposed along the Mogollon highlands of the Bisbee rift shoulder, with Grenvillean grains (1040-1280 Ma) distinctly subordinate. Upper Cretaceous samples also include variable proportions of Mesozoic grains (<250 Ma) derived from the Cordilleran arc, with the largest proportion present in the youngest (Menefee) sample. The fluvial Lower Cretaceous (Burro Canyon) samples also contain subordinate grains (<250 Ma) derived from the Cordilleran arc, and Mesoproterozoic grains (1400-1900 Ma) probably derived mainly from Yavapai-Mazatzal exposures in the Mogollon highlands, but the most prominent age spikes are Grenvillean (900-1300 Ma), and both Paleozoic-Panafrican (300-850 Ma) and Archean (2500-2800 Ma) age clusters are present as well. The more varied detrital zircon population of the Lower Cretaceous Burro Canyon Formation, as compared to the Upper Cretaceous units, is interpreted to reflect recycling of detrital zircons from Paleozoic-Mesozoic sedimentary cover eroded from the basement of the Mogollon highlands prior to mid-Cretaceous Dakota onlap of the Bisbee rift shoulder. The shift in detrital zircon content from Lower to Upper Cretaceous strata of the Four Corners region is thus viewed as a record of unroofing (inverted stratigraphy) in the provenance lying southwest of the Black Mesa and San Juan basins.