AN ERG LANDSCAPE MYSTERY: AN EXOTIC BLOCK IN JURASSIC EOLIAN-FLUVIAL DEPOSITS, GRAND STAIRCASE ESCALANTE NATIONAL MONUMENT, UTAH

An unusual large exotic clast ~1.8 x 0.85 x 0.7 m occurs in fluvial-eolian deposits close to the interfingering transition of the Lower Jurassic fluvial Kayenta Formation and eolian Lamb Point Tongue of the Jurassic Navajo Sandstone in Grand Staircase Escalante National Monument, east of Johnson Canyon, Utah. The isolated exotic clast is a weakly bedded, pink, burrowed siltstone in a carbonate mudstone matrix; likely a product of wind-blown silt deposited in carbonate mud of a shallow lake.

The clast is clearly embedded in and is eroding out of mostly massive to weakly bedded Jurassic fine- to medium-grained sandstone, indicating disruption and soft-sediment deformation. Surprisingly, the exotic clast does not co-occur with any other similar lithologic clasts or lags, nor is any obvious source lithology present within the immediate area (~ 1 km²). The host sandstone exhibits polygonal weathering patterns typical of massive sandstone and shows clusters of carbonate-cemented clastic pipes with convoluted bedding, although no obvious path of liquefaction occurs directly beneath the clast. The upper half of the enclosing sandstone shows low-angle cross-stratification that butts up against the edge of the exotic clast, indicating Jurassic stratification lapped up against the sides of the clast.

The collective characteristics of the massive sandstone with clastic pipes in the lower host rock suggest that the exotic clast was related to strong ground motion that either provided movement through overpressurized waters to produce the massive and deformed sandstone host and transport of the clast, and/or was an erosional remnant produced by Jurassic landscape degradation that was incorporated into the massive sandstone through strong ground motion.