SEDIMENTOLOGIC AND PALEOECOLOGIC IMPLICATIONS OF TRACE FOSSILS IN GLAUCONY-RICH TERRIGENOUS MARINE SEDIMENTS

Passive-margin settings in condensed sedimentary sections commonly are characterized by glaucony-rich sediments and biogenic reworking. Such sediment-starved, terrigenous, marine paleoenvironments are common from Precambrian to Recent. Two case studies of Cambrian sites in Texas and Wisconsin address the depositional interpretation of glaucony-rich beds with applications to key bounding surfaces through the study of trace fossil associations, ichnofacies and ichnofabrics. Biogenic reworking is locally abundant in glaucony-rich sediments at the two field locations selected for examination. Along with identification of trace fossils and their ichnologic attributes, associated glaucony and primary sedimentary structures are recorded for comparison. In central Texas, the Lion Mountain Member of the Upper Riley Formation exhibits a low diversity of trace fossils and an upward increase in the amount of burrowing. Greensands (up to 95% glaucony) occur in both the laminated and bioturbated zones. Cross bedding and scour surfaces indicate a dynamic sedimentary environment within wave base. In southwest Wisconsin, the Reno Member of the Lone Rock Formation also exhibits a low diversity of trace fossils and a gradual increase in the amount of burrowing within each bed. Greensands (up to 90% glaucony) are concentrated within flat pebble conglomerates at the base of each bed. A flat pebble conglomerate typically grades upward into sandstone with planar or hummocky lamination accompanied by increasing bioturbation upward and topped by an erosional contact. The Cambrian sites in Texas and Wisconsin exhibit similar trace fossil associations and ichnofabrics in the glaucony-rich facies. The paleoenvironments are shallow marine, and the trace fossil assemblages represent the Skolithos (and possibly Cruziana) ichnofacies.