LONG-TERM HYDROLOGY CONTROLS THE CONTINENTAL PLANT-FOSSIL RECORD

It is known that labile macrofloral parts (e.g., leaves) require isolation from physical and/or biological decay before preservation. These conditions are associated with geochemical conditions that inhibit ion exchange. Actualistic studies have detailed the environments of deposition (e.o.d.) where such conditions exist and the processes associated with preservation. It is assumed that each e.o.d. has an equal probability to be found as a characteristic depositional facies in the stratigraphic record, and the plant assemblages record environmental conditions across the climate spectrum (extreme wet to dry). This is not the case for continental regimes, particularly those controlled by fluvial processes. The continental plant record is controlled by landscape evolution within a sedimentary basin, which, in turn, is controlled by tectonics and climate. Burial processes (e.g., avulsion, floods, ashfall) may promote preservation potential of accumulated debris but, ultimately, the water-table level maintained in the basin over the multi-millennial scale (and beyond) controls its fate. Three conditions control basin hydrology: increased accommodation and landscape aggradation, landscape stasis, and landscape degradation. Highest preservation potential occurs when accumulation directly follows increased accommodation. As base level changes in response to subsidence, accomodation is filled rapidly and a new, long-term water table is established higher in the section. Plant-bearing deposits above each disconformity, buried through aggradational processes, provide the highest fidelity record comprised of adpressions or compressions. Landscape stasis occurs when aggradational processes reach equilibrium, resulting in pedogenesis and alteration within subjacent sediment due to water table fluctuations. Prolonged intervals of severe drought or rainfall result in rapid loss of buried debris via oxidation and pedogenesis. Plant parts are removed from the record, and only those assemblages existing below the level of deepest penetration may remain. Landscape degradation via uplift or lowering of the fluvial equilibrium profile by changes in sediment supply or discharge removes buried plants via erosion and resets regional water table. The introduction of pore waters reacts with organic debris, resulting in impressions of once labile organs.