Doomed Pioneers: Allochthonous Crustacean Tracemakers in Anaerobic Basinal Strata, Oligo-Miocene San Gregorio Formation, Baja California Sur, Mexico

Laminated, organic-rich hemipelagic sediments from the Oligo-Miocene San Gregorio Formation, Baja California Sur, Mexico, contain isolated horizons (n = 41) of Thalassinoides and Gyrolithes burrows which are exclusively associated with gravity-flow event deposits (phosphatic turbidites). The geological evidence suggests that some turbulent sedimentation events entrained live infaunal Crustacea from neritic settings; upon deposition in anaerobic environments, these crustacea penetrated the substrate and reworked substantial volumes of sediment, in an environment which excluded successful larval recruitment. We suggest that the persistence of oxygen-depleted environmental conditions limited the survival time of these transported infaunal dwellers and rendered them doomed pioneers (Föllmi and Grimm, 1990). The mean size (tube width and maximum depth) of trace fossils within each burrowed horizon is strongly unimodal and differs significantly from the burrow sizes found in adjacent burrowed event horizons in the section. Since burrow diameter may be a meaningful proxy for relative size of the burrowing organism, we regard each burrowed horizon as a discrete, size-sorted sample of the infaunal population that was entrained and survived transport to create burrows within a formerly depauperate environment. In addition, the available data permit the placement of temporal constraints upon the duration of a single burrowing episode. Review of the neontological literature indicates that burrowing organisms redeposited by tidal, storm-driven and gravity-driven flows may leave a prominent ichnofossil imprint on marine sediments. In this context, we attribute each conspicuous occurrence of Thalassinoides and Gyrolithes in the San Gregorio Formation to an abrupt event spanning a very short ecological interval. In addition to their direct significance to basinal oxygen ichnofacies and ecostratigraphy, we conclude that allochthonous tracemakers are likely a geologically-common phenomenon.