Deep and Intense Bioturbation in Continental Environments: Evidence from Miocene Fluvial Deposits of Bolivia

Continental ichnofaunas display a progressive increase in bioturbation depth and intensity through the Phanerozoic. Ichnologic data from Cenozoic fluvial deposits of the Chaco Basin, Subandean zone of Bolivia, indicate widespread colonization of deep infaunal ecospace by the Miocene. Trace fossils are described from the Tariquia Formation, which records deposition in anastomosed fluvial systems. Although the Tariquia ichnofauna is of low diversity and does not display significant compositional variations throughout the succession, ichnofabric analysis reveals some degree of variability linked to different taphonomic pathways that helps to understand depositional dynamics and environmental conditions during accumulation of this fluvial unit. Intense and deep bioturbation occurs in medium- to very fine-grained crevasse sandstone and overbank mudstone. Less pervasive bioturbation is recorded in deposits of abandoned main channels. The Tariquia ichnofauna is dominated by Taenidium barretti, representing an example of the Scoyenia ichnofacies. Overbank deposits are totally bioturbated (BI = 6), showing complete destruction of the primary sedimentary fabric. Main-channel and crevasse-splay sandstones display an upward increase in degree of bioturbation. The top of the channel and crevasse-splay sandstone represents colonization surfaces that allow direct measurements of maximum burrowing depth. Taenidium barretti extends up to 2.2 m into the crevasse sand sheets. Depth and intensity of bioturbation of the main-channel and crevasse sands seem to be a function of time between depositional events. Main-channel and crevasse sandstones underlying thick packages of bioturbated overbank mudstones are intensely bioturbated, recording prolonged periods of low-energy sediment fallout between crevassing events. Conversely, the lowest degree of bioturbation is found in amalgamated channel sandstone units underlying thin intervals of overbank mudstones, reflecting high-frequency depositional episodes.