EXPERIMENTAL AND NATURAL ABRASION OF CONODONTS IN MARINE AND EOLIAN ENVIRONMENTS

Experimental studies of conodont abrasion and petrographic examination of conodont-bearing marine, marginal-marine, and eolian facies of the Morgan Formation (M. Pennsylvanian, Utah and Colorado) suggest that a brasion of conodonts is insignificant in aqueous environments, and lik ely to occur to a major degree only under eolian transport. High-energ y marine transport conditions simulated in a tumbler containing quartz sand produced abrasion of conodonts at a scale detectable only with S EM. Eolian transport conditions simulated in a glass ''wind tube'' usi ng a similar range of quartz sand sizes produced subtle, yet visually detectable abrasion. Conodonts sampled from high-energy subtidal carbo nate facies of the Morgan Formation (Middle Pennsylvanian, Utah and Co lorado) are unabraded, irrespective of the presence of quartz sand, an d both platform and delicate ramiform elements are represented. Conodo nts sampled from mixed siliciclastic/carbonate eolian facies are highl y abraded. Only the most robust posterior parts of Pa elements are usu ally preserved; fine surface details and denticles are worn, and anter ior blades are missing. Laboratory simulations confirm the interpretat ion that marine processes generally are insufficient to physically abr ade small phosphatic skeletal grains in the presence of sand-size mine ral grains. Variability in preservation of surface details, both in la boratory and field samples, reflects the varied histories of individua l grains with respect to transportation, repeated burial and exhumatio n, and length of exposure to bombardment and abrasion by quartz sand. Such a variable taphonomic history precludes the possibility of quanti fying the extent of residence times in eolian environments for individ ual grains.