EVALUATING THE CONTROLS ON MACROINVERTEBRATE SKELETAL CONCENTRATIONS OF THE MID-LATE DEVONIAN CEDAR VALLEY GROUP OF IOWA

Invertebrate fossil skeletal concentrations (accumulations of ≥10% by volume of macroscopic invertebrate bioclasts) reflect the interplay between environmental, sedimentologic, and biologic processes. By comparing skeletal concentrations across a range of depositional environments, we can tease apart the relative influence of each of these processes. This study examines the stratigraphic, preservational, and paleoecologic context of invertebrate skeletal concentrations in the carbonate-dominated, Mid-Late Devonian Cedar Valley Group (CVG) of Iowa.

The CVG reflects carbonate shelf deposition that spans peritidal to deep subtidal settings and preserves a range of macroinvertebrate taxa, most notably brachiopods, crinoids, bryozoans, stromatoporoids, and corals. Coeval deposits are examined across an onshore-offshore transect (north-central to southeastern Iowa), along which long-term (10⁶-year time scale) sediment accumulation rates, depositional energy, accommodation space, and taxonomic abundances also vary.

In the context of stratigraphic sections, the lateral and vertical extent, preservational quality, taxonomic abundances, and packing density of skeletal concentrations are documented across a range of depositional environments. In general, skeletal concentrations are cm’s to dm’s in thickness and laterally continuous on an outcrop scale. Preliminary results suggest that the preservational quality of bioclasts does not vary systematically along an onshore-offshore transect. In fact, relatively few skeletal concentrations are poorly preserved, indicating skeletal burial rates were sufficiently high to avoid destruction and/or destruction rates were so high that preserved concentrations are dominated by the bioclasts that accumulated closest to the time of burial. Stromatoporoids and corals tend to dominate shallow subtidal settings while brachiopods, bryozoans, and crinoids are more abundant in intermediate and deep subtidal settings. Skeletal concentrations range from matrix-supported to bioclast-supported. The highest occurrence of bioclast-supported concentrations is in shallow subtidal settings where the combined effects of winnowing of fine material and high sediment supply presumably outweigh the destructive effects of these high-energy settings.