EXTREME ECOLOGIC PATCHINESS UNIQUELY RECORDED IN ICHNOFABRICS OF A CHALK-MARL SUCCESSION, UPPER CRETACEOUS DEMOPOLIS CHALK

The Upper Cretaceous Demopolis Chalk (eastern U.S. Gulf coastal plain) is characterized by decimeter-scale alternation of lighter chalks and darker marls reflecting Milankovitch-scale clastic dilution cycles on a passive-margin shelf. Ichnofabrics are preeminently expressed only below bed contacts—aka piped zones—wherein the fills of traces produced by deeper dwelling transition-layer organisms contrast markedly with background sediments that were earlier homogenized during passage through a surface mixed layer. These piped zones represent a special form of preservation that permits assessments of time-averaging, temporal resolution, and temporal completeness of ichnofabrics. Based on piped-zone thicknesses, which approximate thicknesses of transition layers, and accumulation rates of 3-5 cm/ka, Demopolis ichnofabrics are time-averaged composites that formed over periods exceeding 8 ka. Ichnologic fidelity is low; distinct biogenic structures (Thalassinoides, Zoophycos, Teichnichnus, Chondrites, and others) reflect only the work of elite deep-tier trace makers whose burrows or burrow systems were passively or actively filled with sediments derived at or near the sediment-water interface. Modeling that employs densities of elite burrows and burrow systems, sedimentation rates, and conservative estimates of average trace-maker lifespans and periods of burrow occupancy (<10 years) indicates that preserved ichnofossils represent less than 5-10% of the time recorded in associated host sediments. This indicates extreme ecologic patchiness; deeper substrate levels (i.e., transition layers) were only rarely and sparsely inhabited by trace-producing macrobenthic organisms. Behaviors inferred from preserved traces (sediment caching, microbial farming) may have been in response to pulsed sediment-delivery events and thus suggest temporal as well as spatial patchiness. Observations made on the Demopolis Chalk likely apply to other pelagic deposits, particularly similar shelf- and deep-sea chalks. This calls for caution in ichnofabric-based studies of paleoenvironmental conditions and paleoecologic aspects (e.g., infaunal tiering).