Ichnologic Signature of Oxygen-Depleted Deposits

A. A. Ekdale, T. R. Mason

The sedimentologic record of oxygen-poor depositional environments commonly includes trace fossils, especially those produced by deposit-feeding organisms that must have had broad oxygen tolerances. Endostratral fodinichnial and pascichnial traces indicate lack of oxygen within the substrate. Complex fodinichnia, such as Chondrites and Zoophycos, may form in anoxic sediment some distance below the water-sediment interface. The deposit-feeding animals can circulate oxygenated bottom water from the sea floor down through semipermanent shafts to permit respiration while they feed on unoxidized organic matter in the subsurface. Endostratal pascichnia, such as Helminthoida and Spirophycus, typically lack a continuous connection with the water-sediment interface, so interstitia water cannot be totally devoid of oxygen or else the animals cannot respire. However, endostratal pascichnia normally do not occur in oxidized sediment where digestible organic detritus has decomposed completely. In totally oxidized substrates, which typify higher energy depositional environments, permanent dwellings (domichnia) of filter-feeding organisms predominate.

The ichnologic signature of oxygen-depleted deposits is a very high-density, very low-diversity association of deposit-feeding trace fossils. We suggest an oxygen-controlled trace fossil model in which increasing oxygen concentration of the interstitial water parallels a transition from fodinichnia-dominated through pascichnia-dominated to domichnia-dominated trace fossil associations. This model provides an alternative to the more traditional depth-controlled trace fossil distribution model in certain situations.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.