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Taphonomy of Plants in a Paratropical Fluvial System

Robyn J. Burnham

Investigation of the subenvironments of a modern paratropical fluvial system in southern Mexico indicates that certain depositional settings are relatively accurate in representing the local flora. Epiphytes, lianas, and those woody plants that are a small fraction of the standing biomass occur in sediments in approximate proportion to their representation (number of individuals) in the vegetation. A paleoenvironmental reconstruction of the flood plain as a whole can be made from the plants preserved when all subenvironments provide conditions for fossilization and when plant parts from each environment are retrieved in large quantities. However, discrepancies arise in reconstruction from the preservational qualities of distinct subenvironments and from differences in pla t species composition among the subenvironments. For example, sites at a channel margin provide ideal preservational potential, but the plants preserved are low in specific diversity relative to the flood-plain forest, reflecting only those plants that live at the edge of the water. Important sedimentologic differences also exist among subenvironments of a fluvial system. A combination of potential plant-macrofossil and sedimentologic indicators can be used to characterize the subenvironments most likely to preserve plants.

Ancient fluvial and upper deltaic sediments in Washington state have been investigated for fossil macrofloras and sedimentary structures. Subenvironments have been characterized based on the diversity and taxonomic composition of the preserved floras as well as on sedimentological criteria. The Eocene sediments support the hypothesis that within a fluvial system, dramatic floristic differences exist among subenvironments. Paleoecologic and evolutionary reconstructions can be improved with knowledge of the probable taphonomic biases of each depositional site.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.