Distribution and Relationships of Traces, Sedimentary Facies, and Physicochemical Conditions in Lake Eyre (Australia) and Lake Tanganyika (East Africa), and Their Comparison to Marine Systems: Implications for Ichnofacies, Paleoenvironmental, and Sequence Stratigraphic Analysis

Abstract

Actualistic studies of modern lacustrine environments, with respect to the spatial and temporal distribution of traces as proxies for terrestrial and aquatic organisms, were carried out in Lake Eyre and Lake Tanganyika. These studies were done within modern environments in order to better understand how to interpret and differentiate lacustrine from marine and fluvial deposits in the geologic record. The variability of ichnofossil-lithofacies associations are significant to interpreting stratal patterns and significant surfaces in lacustrine deposits. Our study methods of these lacustrine systems used the physicochemical factors that control the occurrence, diversity, abundance, and tiering of organism behavior in continental settings. This approach parallels the physicochemical factors that control behavior of epi-, endobenthic, and nektic organisms in marine environments. The distribution of traces in Lake Tanganyika (an overfilled lake in a tropical rift basin setting) and Lake Eyre (an underfilled lake in an arid, mid-latitude ephemeral playa setting) are described, synthesized, and compared to the Mermia, Coprinisphaera, Celliforma, Termitichnus, Skolithos, and Scoyenia ichnofacies models proposed for continental deposits, as well as the established ichnofacies for marine deposits. Comparisons show that all terrestrial, lacustrine, and marine ichnofacies models are inappropriate for the fluvial-lacustrine settings of Lake Tanganyika and Lake Eyre because they do not reflect the distinctive collection of traces across these environments. As such, they do not provide accurate interpretations for paleoenvironments. For example, similar ichnofossil morphologies may result in lacustrine, floodplain, and marine settings, but the behaviors that created them and the physicochemical conditions under which they were created are vastly different. In comparison to marine basins, most lakes are typically short-lived geologic features and exhibit great spatial and temporal heterogeneity. The most important factors controlling tracemaking biota in those lake types are the degree of water body mixing, thermal stratification, nutrient input and productivity, dissolved oxygen concentration with depth, and salinity-alkalinity. The multiple ichnocoenoses for each subenvironment observed in the balanced-filled and underfilled lacustrine systems of our study more accurately record the environmental uniqueness and distinctive collection of traces found in each environment.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018