Abstract
Billions of large orthocone nautiloids occur within an extremely persistent lime packstone bed of the Redwall Limestone through the Grand Canyon region, Arizona and Nevada. The platform facies of the packstone bed is 2 m thick at the top of the Whitmore Wash Member (Osagean Series of Mississippian System). Abundant nautiloids occur within the platform facies of the bed from Marble Canyon, Arizona westward 290 kilometers to Las Vegas, Nevada. The slope facies of the bed, where nautiloids are rare, is 3-to-14-m-thick, light-gray-weathering grainstone occurring northwest of the platform facies in southern Nevada, extreme northwestern Arizona, and southwestern Utah. Both platform and slope facies of the bed are named formally Whitmore Nautiloid Bed (WNB). Bed area exceeds 3 x 10⁴ km² and bed volume exceeds 100 km³. Orthocone nautiloids assignable to the genus Rayonnoceras show evidence that bodies occupied the shells during mass kill and burial of an entire population. Shell orientation, inverse grading, outsized coral heads, and water-escape pipes indicate rapid deposition of the platform facies from a high-velocity, laminar, low-cohesion, fluidized dispersion of carbonate sand and silt. This moving dispersion can be called a hyperconcentrated flow (volume concentration sediment ~35%, flow density ~1.6 g/cm³). Nautiloids and large coral heads were separated within the dispersion being supported as a raft above the highest-density, strongly fluidized, laminar flow. The flow event resembles the process that suspends lithic fragments and pumice clasts within a pyroclastic density current. WNB has internal structure resembling ignimbrite. The hyperconcentrated flow was initiated as a liquefied flow slide, probably in southwest Colorado, and hydroplaned toward southern Nevada at velocity ~5 m/s through the carbonate platform in northern Arizona. When the flow encountered the slope in southern Nevada and southwest Utah, it was transformed into a turbulent, concentrated flow (volume concentration sediment ~25%) depositing most of its sediment mass. Further westward in Nevada the flow lost its hydroplane and became a tractive current. This is the first documentation in the geological literature of a regionally extensive hyperconcentrated sediment gravity flow upon a marine carbonate platform.