6-6 A Review of Ichnofossil Presence and Preservation in Cenozoic Volcanic Ash Deposits on the North American Great Plains

Session: Advances in ichnology: walking in Memphis and other records of behavior

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In paleontology, invoking Pompeii or labeling a locality as Pompeiian implies extraordinary fossil preservation, often due to rapid deposition and burial by volcanic ash or other fine-grained sediments. Volcanic ash beds, however, are also exceptional for their ability to preserve post-airfall surface and subsurface ichnofossils in fine detail. Perhaps the most famous are the Laetoli hominid footprints in Tanzania, but there are other ash-preserved tracks of ancient humans in Italy, Nicaragua, and Mexico. Receiving less attention are the more numerous non-hominid trackways, burrows, coprolites, and rhizoliths that are frequently observed in ash deposits. Presented here is a review of ichnofossil assemblages and paleoenvironmental conditions in volcanic ash and the factors promoting preservation thereof, with examples from numerous volcanic ash localities in Cenozoic deposits of the Great Plains. Volcanic ash beds are stratigraphically common in the Great Plains, though typically preserved as paleodepression-filling deposits with a limited lateral extent. Ichnofossil assemblages frequently consist of abundant fossil ant nests, backfilled burrows, and simple horizontal to subvertical invertebrate burrows; small vertebrate burrow networks and large vertebrate tracks and dens; and horizontal and branching carbonate rhizoliths. Bioturbated strata are commonly separated by thin intervals of rippled, cross-bedded, or finely laminated ash, suggesting alternating periods of flooding and subaerial exposure and colonization as opposed to one airfall event. Factors contributing to ichnofossil preservation include the very fine-grained nature of airfall ash, but also it being a natural pozzolan material that is prone to form cements. Oxides of calcium and silica in the glass particles react with water to form hydroxide cements that often preferentially precipitate in void spaces left by tracemakers, along bed contacts, and during subaerial exposure. The tracemakers themselves may partially or completely backfill with clastic or organic materials from overlying or underlying strata. Ash-bed preserved ichnofauna are in situ evidence of hidden biodiversity and organismal behavior not typically decipherable from the body fossil record alone.

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