REGIONALLY EXTENSIVE MASS KILL OF LARGE ORTHOCONE NAUTILOIDS, REDWALL LIMESTONE (LOWER MISSISSIPPIAN), GRAND CANYON NATIONAL PARK, ARIZONA

Billions of large fossil orthocone nautiloids occur within a single lime packstone bed of the Redwall Limestone through the Grand Canyon region, northern Arizona and southern Nevada. The uppermost 2-m-thick packstone bed of the Whitmore Wash Member of the Redwall Limestone (Osagean Series of the Mississippian System) contains a coplanar horizon averaging 1 nautiloid fossil per m². The bed with abundant nautiloids extends westward 290 km from Marble Canyon on the Colorado River to Frenchman Mountain near Las Vegas. The platform facies of the bed with abundant nautiloids originally occupied an area of at least 1.5 x 10⁴ km². Nautiloids resemble the genus Rayonnoceras, but the siphuncle differs from any described in the literature.

Mean length of nautiloids is 0.8 m with log-normal size distribution indicating mass kill of an entire population. Implosion structures and collapse of the body cavity argue that bodies were within the shells at the time of burial. Orientations of nautiloids indicate they were swept up in a westward or southwestward sediment flow. About 15% of nautiloids are vertical within the bed. The packstone bed has inverse grading and abundant fluid-escape pipes indicating strongly fluidized condition and deposition by abrupt freezing from a hyperconcentrated sediment gravity flow. The enormous hyperconcentrated flow hydroplaned westward at a velocity of over 5 m/sec through a shallow, carbonate platform environment, sweeping up, smothering and depositing an entire seafloor population of nautiloids.

Discovery of the extent of the packstone bed, inventory of nautiloid fossils, and interpretation of depositional process were made possible within Grand Canyon National Park by special use permits allowing motorized raft operations with geologists on the Colorado River. Float boulders with nautiloids directed our attention to the source bed within the Redwall cliff. Because of the Antiquities Act, we chose to collect nautiloids for research from outside the national park. Our investigations provide an interesting example of how paleontological discoveries can be made in remote areas of national parks.