The Relationship Between Continental Landscape Evolution and the Plant-Fossil Record: Long Term Hydrologic Controls on Preservation

Abstract

Continental depositional environments preserve the majority of the macrofloral record since the advent of land-plant colonization in the mid-Paleozoic, and wetland representatives are encountered more commonly than those that grew under more seasonal conditions. It has been assumed that preservation potential and future recovery of plant debris are high once detritus is introduced into any appro- priate environment of deposition (e.g., fluvial-lacustrine or paludal setting), regard- less of prevailing associated climate, sediment load, or geochemistry at the time of emplacement or interval thereafter. If a plant fossil is identified in any part of a stratigraphic interval, even if it occurs solely as an impression, it has been presumed that favorable conditions persisted over time to facilitate this record. Conversely, the absence of fossil plants in a stratigraphic sequence commonly has been interpreted as the result of catastrophic perturbation across the landscape, rather than the ascribing their absence to taphonomic filters that may have operated millennia after burial. Terrestrial landscapes are affected by aggradational, equilibrium, and degradational processes that control not only the local or regional water table, but also the long-term fossilization potential of organic debris entombed within these sediments. Fossil plants have the highest preservation potential when high water tables are maintained long-term within soils (e.g., histosols, entisols, gleyed soils), or in settings that are maintained below the maximum draw down of the regional water table (e.g., channel barforms, abandoned channels, lakes) of aggradational landscapes. When landscapes reach equilibrium, extensive pedogenesis ensues and the development of deep mature soils (e.g., calcisols) results in the bacterial deg- radation of any previously buried plant debris due to extreme penetration of atmo- spheric gases. When sediment is removed during landscape degradation, the local and/or regional water table is reset lower in the unconsolidated stratigraphy, once again promoting rapid decay of previously buried detritus at depth. These processes, operating under time frames of centuries to millennia and longer (lakh), control the ultimate preservational mode of plants recovered from the fossil record. This chapter reviews the factors influencing the preservation of terrestrial plants in both subaqueous and subaerial environments based on actualistic studies, and develops a conceptual framework for landscape evolution in continental regimes. A model is presented in which preservational mode is related to the taphonomic and sedimentary history of the landscape in which plant detritus is buried. Case studies of the plant-fossil record, ranging from the Triassic to the Eocene, in exclusively aggradational and in aggradational/degradational landscapes are presented.