Actualistic studies of the spatial and temporal distribution of terrestrial and aquatic organism traces in continental environments to differentiate lacustrine from fluvial, eolian, and marine deposits in the geologic record

Abstract

Actualistic studies of modern continental environments and the spatial and temporal distribution of terrestrial and aquatic organisms are summarized and synthesized to understand how to better interpret the significance of trace fossils to differentiate lacustrine from fluvial, eolian, and marine deposits in the geologic record. The purpose of this approach is to develop an understanding of the physicochemical factors that control the occurrence, diversity, abundance, and tiering of organism behavior and parallels what is known for benthic and other trace-making organisms in marine environments. The distribution of traces observed in Lake Tanganyika and Lake Eyre, an overfilled lake in a tropical rift basin setting and an underfilled lake in an arid midlatitude ephemeral playa setting, respectively, are described, synthesized, and compared with the Mermia, Coprinisphaera, Termitichnus, Skolithos, and Scoyenia ichnofacies models proposed for continental environments. The comparisons show that all the models are inappropriate for the fluvial-lacustrine settings of Lake Tanganyika and Lake Eyre because the models do not support the environmental uniqueness or distinctive collection of traces across these environments, nor do they provide sufficient interpretive power. The multiple ichnocoenoses for each subenvironment observed in the balanced-filled and underfilled lacustrine systems more accurately record the environmental uniqueness and distinctive collection of traces found in each environment. Ichnocoenoses are better suited for continental depositional systems and their environments because they reflect the nature of processes and distribution of life in continental settings, which are inherently heterogeneous spatially and temporally. Ichnocoenoses also provide sufficient interpretive power for trace-fossil associations formed under different physicochemical conditions for each type of environment. General trends in trace-fossil diversity, abundance, distribution, and tiering are predicted for alluvial (fluvial), lacustrine, and eolian environments so that new models based on the distribution of ichnocoenoses and their sedimentary and pedogenic characteristics from outcrop and core can be constructed.