Escape and equilibrium trace fossils in association with Conichnus conicus as indicators of variable sedimentation rates in Tortonian littoral environments of SW Spain

Numerous and large vertically extensive specimens of Conichnus conicus are described from the Tortonian deposits of the central Guadalquivir Basin (SW Spain). These burrowing structures form a low bioturbation index of the ichnofabric, which allows one to infer the original depositional setting. Both ichnological and sedimentological features suggest a coastal to shallow marine environment dominated by high sediment supply and high-energy sedimentary processes. Conichnus exhibits a characteristic internal fabric that reflects changes in sedimentation rates linked to various processes: (1) initial substrate penetration by the tracemaker; (2) gradual retrusive movement in response to rapid but continuous sediment aggradation; 3) moderate to high rates of retrusive migration as a consequence of rapid to very rapid sediment aggradation and tracemaker escape; and, finally, (4) abandonment of the burrow or burial of the tracemaker subsequently in response to extremely high sediment accumulation rates. Conichnus has been also described from other areas with similar deposits of different ages, where its important value as a paleoenvironmental indicator similarly has been indicated. (c) 2005 Elsevier SAS. All rights reserved.