- Copyright © 2016, SEPM (Society for Sedimentary Geology)
A well preserved plant assemblage at the Colwell Creek Pond locality of Leonardian (Kungurian) age provides an opportunity to evaluate taphonomic conditions in a dryland alluvial setting. A narrow channel body incised to 5 m depth through red paleo-Vertisols contains 2 m of varicolored laminated mudstone with graded layers and plant material. X-ray diffraction analysis of individual laminae indicates the presence of chlorite, illite, kaolinite, and mixed-layer clay, with hematite in red and gray layers and goethite in yellow-brown laminae. No carbonate was identified, and the total organic carbon content is minimal. The fine sediment accumulated in a shallow abandoned channel from suspension and gentle underflows, probably linked to seasonal inflow, and analysis of lamina thickness suggests that standing water may have persisted for up to a few millennia. The preservation of lamination is attributed to a lack of bioturbation, possibly linked to a paucity of subsurface oxygen, low productivity, elevated salinity, rapid deposition, or a combination of these factors; minimal bioturbation may also reflect the limited use of freshwater ecospace during the Early Permian. Clay-rich paleo-Vertisols complete the fill, with drab root traces that indicate growth of vegetation in a strongly seasonal setting.
Abundant plant material in the laminated beds includes branches of walchian conifers, the possible cycadophyte Taeniopteris spp., and the comioid, possible peltasperm, Auritifolia waggoneri. They were derived from an adjacent riparian zone and preserved as 3D goethite petrifactions. Much of the foliage shows evidence of arthropod herbivory. Although a humid climatic episode cannot be ruled out, the exceptional abundance and preservation of the plants probably reflects the persistence of an oxbow lake on a relatively arid alluvial plain, where riparian plants experienced periodic moisture stress but had access to groundwater nearly year round. Rapid burial in standing water, the lack of bioturbation in the laminated sediments, and early biomineralization probably explain the exceptional preservation of the plant remains.