MIOCENE CATASTROPHIC GRAVITY SLIDING IN THE BEAVER DAM MOUNTAINS, SOUTHWEST UTAH

This USGS EDMAP research project aimed to better understand the deformational history and mechanisms of faulting within the Castle Cliff 7.5-minute quadrangle (CCQ) located along the western flank of the Beaver Dam Mountains (BDM) in southwestern Utah. The BDM straddles the boundary of the Colorado Plateau and Basin and Range Province and consists of an anticlinal uplift containing mainly Paleozoic sedimentary rocks flanking a central core of Proterozoic metamorphic rocks. Structural features in the CCQ illustrate several types of faults that are thought to represent a response to rapid extension-driven uplift and exhumation starting at ~17 Ma (Quigley et al., 2010). The western flank of the BDM is bounded by the high-angle Piedmont normal fault with the hanging wall down to the west forming the deep Virgin River depression filled with Miocene sediments. Within the CCQ, large gravity slide blocks of brecciated Mississippian Redwall Limestone were emplaced between alluvial-fan deposits of the Muddy Creek Fm (upper part of the basin fill of the Virgin River depression). The slide blocks appear to be klippe of a larger gravity slide sheet that was broken later by several smaller north trending normal faults located east of the main Piedmont fault. The largest gravity slide blocks (~ 16 of them) are several km long (up to 2.5 km) and up to 70 m thick. From a distance the blocks are conspicuously bedded with dark gray chert layers and limestone; however, close-up the bedding is commonly brecciated, and chert is concentrated in chaotic fractured breccia lenses.

These gravity slide blocks are interpreted to have catastrophically moved westward off the mountain front and traveled at least 6 km or more onto alluvial-fan deposits of the Muddy Creek Fm. To ascertain the age of emplacement, samples of the Muddy Creek Fm were collected for geochronological age-bracketing using U-Pb detrital zircon dating methods at the University of Arizona LaserChron laboratory. Preliminary results of three samples collected below two different slide blocks resulted in maximum depositional ages (MDA) of samples: 1) 13.58±0.29 Ma, 2) 13.29±0.21 Ma, and 3) 13.58±0.16 Ma. Samples collected above the slide blocks are currently being dated. These MDA’s indicate emplacement of the slide blocks occurred during active movement of the Piedmont normal fault.