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ABSTRACT: Doomed Pioneers: Event Deposition and Bioturbation in Anaerobic Marine Environments

Kurt A. Grimm, Karl B. Follmi

Isolated horizons of Thalassinoides and Gyrolithes burrows, in exclusive association with gravity flow deposits within otherwise unbioturbated sediments, indicate that event deposition in oxygen-depleted sedimentary environments may be accompanied by the appearance of allochthonous infauna. In the Miocene Monterey Formation of Alta California and in the Oligocene-Miocene San Gregorio Formation of Baja California Sur, we observe compelling evidence that some turbulent sedimentation events entrain living decapod crustaceans. Upon deposition in anaerobic environments, these imported burrowers rework substantial quantities of laminated, commonly organic-rich sediment, in an environment from which they were previously excluded. The persistence of oxygen-depleted environmental onditions limits the survival time of these transported infaunal dwellers and renders them doomed pioneers. The occurrence of Thalassinoides and Gyrolithes, ichnogenera that are generally observed in normal-marine, neritic environments, in anoxic hemipelagic host sediments have been problematic for paleoenvironmental interpretation; in some cases, these features have been incorrectly interpreted to indicate bottom water ventilation and reoxygenation on a basin-wide scale. Accurate recognition of doomed pioneer trace fossil assemblages will permit more precise understanding of paleo-oxygen levels and basin history.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990