Abstract
Very large cross-beds with a measured height of 7.5 m occur in the middle of the Anchor Limestone (Lower Mississippian, Las Vegas Range, southern Nevada) and are composed of pebble-granule-sand, crinoid grainstone. The bedform is understood to be the preserved remnant of an enormous, subaqueous, 3-dimensional dune with original height estimated conservatively at 15 m with length of about 800 m. Dunes produced in laboratory flumes and dunes observed in natural shallow-water straits allow scaling of the hydrodynamics of the tractive current that produced the large-scale cross-beds within the gigantic subaqueous dune. The depositional environment of the dune is interpreted to be a marine slope with a water depth of 140 m. Flow thickness exceeded 15 m and flow velocity is estimated conservatively at 2.5 m/s toward the west. The large scale of this bedform attests to the rapid, catastrophic emplacement of coarse lime sediments in this section of the Anchor Limestone. This enormous dune is interpreted to be the distal tractive current deposit formed by the fluid turbulence generated by breakdown of a hydroplaning, fast-moving, hyperconcentrated sediment gravity flow after it had traversed northern Arizona.