An erg landscape mystery: An exotic boulder in Jurassic aeolian‐fluvial deposits, Grand Staircase Escalante National Monument, Utah, USA

Abstract

An unusually large exotic boulder occurs in strata close to the interfingering transition of the fluvial deposits of the Lower Jurassic Kayenta Formation and aeolian deposits of the Lamb Point Tongue of the Jurassic Navajo Sandstone in Grand Staircase Escalante National Monument, Utah. This boulder is composed of a weakly bedded, pink, thrombolitic burrowed mudstone and siltstone in a carbonate mudstone matrix. Comparison with similar carbonate units of the region—interpreted as lacustrine in origin and associated with interfingering deposits of Kayenta‐Navajo strata—indicates that the strata from which the boulder originated was likely a product of wind‐blown silt deposited in carbonate mud of a shallow lake. The exotic boulder is clearly embedded in and is eroding out of mostly massive to weakly bedded Jurassic, fine‐ to medium‐grained host sandstone. Nearby, the sandstone is disrupted by soft‐sediment deformation. The upper half of the enclosing sandstone shows low‐angle cross‐stratification that abuts against the edge of the boulder, indicating that Jurassic sedimentation lapped up against the sides of the boulder. Surprisingly, the exotic boulder does not co‐occur with any other clasts of similar lithology, nor is any obvious source lithology present within the immediate area. Lake beds of similar lithology in the same region are not physically traceable to the site where the exotic boulder occurs. The exotic boulder is evidence of the dynamic processes of the Jurassic landscape. The collective characteristics of the massive sandstone with typical polygonal weathering patterns and clusters of clastic pipes in the lower host rock suggest that the exotic boulder was emplaced in one of three ways: (A) fluid overpressurization, likely triggered by strong ground motion, produced the massive and deformed sandstone host and transported the boulder upward in the subsurface; (B) the block was an erosional remnant produced by Jurassic landscape degradation; or (C) the block was transported by a hyperconcentrated overland flow.