CONSTRAINING THE CONTROLS OF DEPOSITIONAL ENVIRONMENT ON ABUNDANCE OF SYNDEPOSITIONAL ZIRCONS WITHIN THE LOWER CRETACEOUS CEDAR MOUNTAIN FORMATION

A primary application of detrital zircon (DZ) U-Pb geochronology is the use of the youngest dates to constrain the maximum depositional age (MDA) of a sedimentary deposit. Despite the widespread use of MDA in DZ studies, the controls of depositional facies on the number of syndepositional zircon grains being deposited, and thus MDA approximations, are not well constrained. The Lower Cretaceous Cedar Mountain Formation (CMF) consists of a variety of terrestrial depositional environments, including fluvial (single-channel, braided-channel, and over-bank) and lacustrine deposits, making it an ideal target for assessing the controls of depositional facies on MDA approximations. Here we present new paired laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and chemical abrasion-thermal ionization mass spectrometry dates from both randomly selected and picked euhedral zircon grains from the Buckhorn Conglomerate (BC), Yellowcat (YC), Ruby Ranch (RR), and Short Canyon Conglomerate (SCC) members. Preliminary LA-ICP-MS results from euhedral zircons from fine-grained paleosol and silty over-bank deposits in the YC (Berriasian-Hauterivian) indicate modest abundances (~2%-10%) of Cretaceous zircons. Fine-grained over-bank deposits in the RR (late Aptian-early Albian) yielded nearly 100% Cretaceous euhedral zircon. Coarse-grained braided channel deposits from the BC yielded at most 6% Cretaceous zircon while the coarse-grained fluvial channel sample from the SCC (late Albian-Cenomanian) yielded 72% Cretaceous zircon between 115 Ma and 93 Ma. Overall, samples from fine-grained depositional facies contained more abundant euhedral grains (3.6% vs. 1%), and euhedral grains were more likely to be Cretaceous (37.8% vs. 26.5%). These results indicate that finer-grained depositional facies yielded a greater proportion of syndepositional zircons relative to older, recycled zircons. However, the tempo of arc magmatism during the time of deposition appears to also exert a strong control on the abundance of syndepositional zircons, as young zircon is more abundant in late Albian to Cenomanian strata that were deposited during the mid-Cretaceous Cordilleran arc flare-up (60%-68.2%) versus older strata deposited during a lull in Cordilleran arc magmatism (0%-6.7%).