DETRITAL ZIRCON AGES FOR THE BASAL CEDAR MOUNTAIN FORMATION (EARLY CRETACEOUS) NEAR MOAB, AND DINOSAUR NATIONAL MONUMENT, UTAH

The Cedar Mountain Formation is host to several significant vertebrate faunas, but until recently, temporal correlation with extrabasinal faunas has been hampered by a dearth of radiometric ages. The difficulty in obtaining ages is primarily a function of the rarity of well-preserved volcanic ash beds. To address this problem, silty mudstones and sandstones are utilized as sources of zircon crystals for U-Pb age analysis using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-ICP-MS). The utilization of detrital zircon ages is based on the hypotheses that (1) in terms of geologic time, there was a nearly continuous input of volcanic ash into the Cedar Mountain depositional system and, (2) that youngest U-Pb age peaks provide a maximum age of deposition.

To assess the age of the basal of the Cedar Mountain Formation, seven samples were collected from the top several meters of the underlying Morrison Formation and up into the Yellow Cat Member near Moab and in Dinosaur National Monument, Utah. Hand-picking of zircons emphasized euhedral grains—the best candidates for recent volcanic input. U-Pb ages at these sites confirm previous reports of a maximum depositional age of 124 +2/-2.8 Ma (Barremian-Albian boundary) for the base of the Yellow Cat Member and its fauna (characterized by three macronarian sauropods, plus Utahraptor, and Gastonia). The next older peaks, circa 148-154 Ma, most likely represent zircons from ash in the Morrison Formation, and confirm that much of the basal Cedar Mountain Formation consists of reworked Morrison strata.

Analysis of zircon ages from fine-grained continental clastic rocks in back-arc regions may prove to be a fruitful technique to acquire radiometric ages of deposits that might otherwise be difficult to date.