SEDIMENTOLOGY OF MARINE VERTEBRATE BURIAL IN THE MIOCENE PISCO FORMATION, PERU

The Miocene Pisco Formation on the central coast of Peru is known for a remarkable abundance of well preserved, articulated, marine mammals. The goal of this project is to provide a better sedimentological model to explain how such large carcasses could be buried and preserved so well. Because of the unusual abundance of the fossils in the Pisco basin, combined with numerous, readily-accessible outcrops in canyons and hills, we were able to find several specimens in cross section. This provided us with a special opportunity for sedimentological studies of the cross sections, and comparison with the previous taphonomic research done in the Pisco Formation. Going into this study, we planned to test models that included burial in channels and submergence in the sediment by liquefaction.

We studied 11 large marine vertebrate cross sections, ranging in size from dolphins to whales. All of them were fully articulated, and only one whale and the dolphin skeleton were not encased in a concretion. We have organized the sites into three categories: 1) burial in channels, probably during storms, 2) near-shore storm deposits, and 3) storm deposits in deeper water on the shelf. Four whales and the dolphin were buried in channels. Sediments are diatomaceous, tuffaceous, and contain silt-size clastic grains. One whale was buried in a near-shore storm deposit of coarse, fossiliferous sediments associated with shallow water facies. Three whales were deposited in a shelf environment in diatom-rich, tuffaceous, and silty sediments. The burial of these whales before decomposition required substantial advection of sediment and in some cases included volcanic ash deposition. Two of the whale skeletons were on distinct bedding surfaces suggestive of hiatuses in deposition. Other whales were on indistinct contacts. We found no evidence that liquefaction was a mechanism for burial. This study suggests that storms were the primary process driving burial and were effective in at least three different depositional settings.