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Neogene avian and mammalian tracks from Death Valley National Monument, California: Their context, classification and preservation

1986; Elsevier BV; Volume: 57; Issue: 2-4 Linguagem: Inglês

10.1016/0031-0182(86)90017-9

1872-616X

Paul J. Scrivner, David J. Bottjer,

Fish Ecology and Management Studies

The Copper Canyon Formation consists of over 3000 m of basalt and alluvial fan and lacustrine strata deposited in a small Neogene basin. Lacustrine strata are of two typs: fine-grained dolomitic carbonates deposited in a playa-lake environment and calcitic bioclastic carbonates deposited in a freshwater lake environment. Bedding surfaces of the playa-lake carbonates deposited in shoreline environments commonly contain avian and mammalian footprints. Tracks of artiodactyls are most abundant; those of birds, horses, carnivores and proboscideans are less common. Using the binomial avian and mammalian footprint classification scheme of Vialov (1966), four different mammalian ichnogenera containing thirteeen ichnospecies, as well as six ichnospecies of avian tracks are recognized. Environmental factors of the ancient Copper Canyon playa-lake which contributed to good footprint preservation included: (1) periods of flooding and intense evaporation which resulted in receding shorelines that presented wet, fine-grained sediment surfaces for impression and subsequent subaerial exposure; and (2) periodically high sedimentation rates which insured quick burial of the tracks. While vertebrate footprints also occur in sandstones and conglomerates deposited in alluvial fan environments, the morphological distinctness of tracks is best preserved in fine-grained dolomitic carbonate and siltstone strata deposited in the playa-lake environment. Distinctness of preserved tracks also increases with apparent decrease, within a certain range, of sediment water content at the time of impression. An examination of preserved morphologies has allowed several sediment water content states important to footprint preservation to be deduced. Changes in sediment water content at the time of impression acted to produce a wide range of track morphologies for each individual trackmaker. An examination of track abundance and occurrence within the formation suggests increasing visitation by larger vertebrates to the playa-lake as lake water freshened. The rare ocurrence of tracks of small mammals and low amount of vertebrate bioturbation within the playa deposits also suggests that the ancient Copper Canyon playa-lake lacked a resident vertebrate population of its own, probably receiving only migrating visitors seeking food and water. The footprints in the Copper Canyon Formation reflect part of the wide diversity of vertebrates present in southern California during the Hemphillian.