U-PB AGES OF DETRITAL ZIRCONS IN THE UPPER JURASSIC MORRISON FORMATION (SALT WASH AND WESTWATER CANYON MEMBERS) OF THE FOUR CORNERS REGION, SOUTHWEST USA

Detrital zircons were collected from the Kimmeridgian Salt Wash and the Kimmeridgian/Tithonian Westwater Canyon Members of the Upper Jurassic Morrison Formation at sites in Utah, Colorado, and New Mexico near Four Corners (Salt Wash from Montezuma Canyon UT and Slick Rock CO on the Dolores River; Westwater Canyon near Recapture Creek UT and Todilto Park NM). A total of 400 detrital zircon grains (100 from each sample) were individually dated using LA-ICP-MS with a beam diameter of 50 microns. Due to >20% discordance or poor precision, 54 grains were removed from consideration, leaving 346 reliable analyses (86.5% of the total). Grains from the Salt Wash Member fall primarily into three dominant age populations at 155-200 Ma, 350-650 Ma, and 950-1250 Ma, with subordinate populations at 1380-1500 Ma, 1640-1850 Ma, and 2580-2800 Ma. Grains from the Westwater Canyon Member are mainly 150-200 Ma and 1020-1740 Ma with broad peaks in the ranges 1020-1200 Ma, 1420-1500 Ma, and 1580-1740 Ma. Prominent age spikes in the 150-200 Ma range strongly suggest derivation of some detritus from the Cordilleran magmatic arc in both members. Other age populations in the Salt Wash Member suggest reworking of sand into the Salt Wash fluvial system from older Jurassic eolianites. Reworking of older sediment is supported by the high quartz content (~90%) and low feldspar content (~3%) of the Salt Wash Member samples, with half the lithic fragments (~7%) resistant chert grains. Other age populations in the Westwater Canyon Member imply more direct derivation of detritus from basement rocks underlying Cordilleran arc assemblages. The lower quartz content (~70%) and higher feldspar content (~20%) of the Westwater Canyon samples, coupled with mainly non-chert lithic fragments (~10%), are supportive of that interpretation. Paleocurrent indicators from the Morrison Formation imply stream flow fanning across the Colorado Plateau from the west and southwest. The differences in the populations of detrital zircon grains and in petrographic modal composition between our samples of Salt Wash and Westwater Canyon Members suggest that the two members were deposited as partly overlapping but separate fluvial megafans with apices that tapped different provenances, with Salt Wash sources lying farther west than Westwater Canyon sources.