Preliminary observations on phytotaphonomic assemblages in a subtropical/temperate Holocene bayhead Delta: Mobile delta, Gulf Coastal Plain, Alabama

Abstract

Studies conducted in the Holocene bayhead Mobile Delta, southeastern Gulf Coastal Plain, have begun to aid in elucidating the relationships between vegetational communities and macrodetrital litter recovered in various sedimentary environments. The bayhead delta is the result of the Mobile-Tensaw River progradation through a drowned Pleistocene river valley. Geomorphic sites sampled in the upper delta plain include channel forebank, levee, swamp, and abandoned channel (bayou). Channel forebanks are littered with parallel to subparallel oriented logs and branches, but subsurface accumulations of plant organs have not been identified, to date. Although levees are sites in which forest floor litter accumulates, bioturbation by rooting and fluctuating water conditions degrade plant parts. Alluvial and Deep Alluvial Swamps are sites in which prostrate logs and erect tree bases are more prone to being preserved by suspension load sedimentation. Forest floor litter beds of these swamps rarely may be preserved by event deposition (i.e. anomalous flooding). Recovered subsurface plant parts include fragmented woody detritus, bark, and seeds. The site most prone to preserving bedded leaf litter is the abandoned channel. Vegetation surrounding the bayou contributes canopy parts resulting in organic-rich muds. The lower delta plain is the area where passive and active sediment accumulation occurs in open water distributary bays and flanking subaerial levee/marshes. Marshes are inhabited by a herbaceous community in which rooting mats predominate the subsurface samples. Most aerial plant litters degrade or are consumed on the surface of the marsh. Root: shoot ratios have not been assessed, to date. Interdistributary bays infilled by passive sedimentation (flooding) preserve an extensively degraded assemblage of plant parts in a high-organic mud. Interdistributary bays infilled by active sedimentation (crevasse splay) preserve (1) bedded leaf litter horizons in distributary mouth bar sands and adjacent estuarine silty-muds and (2) transported detrital peats in shallows near crevasse channel distributary mouth bars. These litter horizons are composed of plant parts transported from distal Levee and Swamp communities of the transitional and upper delta. Proposed hypotheses to reconstruct plants (organic connection) through the repeated associations of parts found in clastic sedimentary environments must be reevaluated with respect to the taphonomic processes occurring in specific depositional environments.