Turning Over an Old Leaf--Taphonomy of Plant Remains in Lacustrine Sediments

Fredrick J. Rich

Lakes may never have been as abundant in the past as they are now. Consequently, their sediments constitute a minor portion of the geologic record. In spite of that, comparatively rapid rates of accumulation of undisturbed lacustrine sediments, and close proximity of depositional sites to forests and marshes have resulted in preservation of extraordinarily rich plant-fossil assemblages. Some classic deposits (Green River Formation) yield exquisite fossils, but little is known of the taphonomy of the remains, and floristic reconstructions are highly speculative. The relative abundances of taxa preserved in Holocene sediments, and their state of preservation can be related to the degree to which the plant fragments have been transported, the mode of transportation, and susc ptibility to biological and mechanical degradation. Like organs of different species, and different organs within a species may accumulate quite differently. The diversity of depositional subsystems in lake-fill sediments (fans, deltas, pelagic mud, etc) further complicate matters. Leaf size and taxonomic diversity may decrease with distance from shore, but complete leaves may represent either deep or shallow water deposition. On the other hand, wood may be more common in deep, offshore sites while inflorescences remain near shore. Any paleofloristic study must be accompanied by detailed sedimentology and careful collection of all plant remains (including pollen, and not just the pretty leaves). Equal effort should be made to collect vertebrate and invertebrate remains which may be prese ved, and which may expose the sedimentological and preservational biases that the leaves will not reveal.

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