SEPM Session Explores New Frontiers in Plant Taphonomy

Abstract

New Frontiers in Old Areas was the theme for the AAPG annual meeting. The SEPM session on Plant Taphonomy-Organic Sedimentary Processes attempted to integrate the biotic and abiotic processes responsible for the development of organic-rich, hydrocarbon precursors derived from terrestrial plants. The paleophytological criteria useful in the recognition of these strata in terrestrial and nearshore sequences were also examined. The session comprised eight presentations that elucidated the relationships between sedimentary environments and the potential for preservation of plant detritus as either identifiable fossil-bearing beds or hydrocarbon sinks. Robyn Burnham (Univ. Washington) began by comparing taphonomic processes and biofacies-sediment relationships in the paratropical San Pedro fluvial system, Tabasco, Mexico, with the Eocene Puget Group of western Washington. Although fluviatile systems are easily recognized by a suite of primary sedimentary structures, subenvironments within the alluvial plain are often difficult to separate when only sedimentological criteria are applied. A transect of fluvial subenvironments (in-channel, forebank, levee, and back bank) was sampled to determine the proportion of standing vegetation potentially preservable in each environment and characteristic assemblages that may distinguish one subenvironment from another. Examination of these sample sites revealed that certain depositional settings are relatively accurate in the potential preservation of forestation, although sample size may affect the reliability of reconstructing original vegetational composition. If used to complement each other, macrofossils and sediments can be used to characterize subenvironments most likely to preserve recognizable plant detritus. This hypothesis was tested in the Tertiary record of the Puget Group in which more than 20 separate localities, each interpreted to represent a different fluvial subenvironment, were studied sedimentologically, floristically, and physiognomically (i. e., whether arborescent, vines, shrubs, etc.). Regardless of stratigraphic position within the section, similar depositional subenvironments, as interpreted from sedimentological criteria, had similar fossil floras and physiognomy. However, stratigraphically equivalent sites representing different subenvironments were not floristically identical. These differences