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Trace Fossils in the Green Zone: Ichnology and Sedimentology of Cambrian Glauconitic Greensands

Harding, Sherie C.1; Ekdale, A. A.2
1 Department of Geology and Geophysics, University of Utah, Salt Lake City, UT.
2 Department of Geology and Geophysics, University of Utah, Salt Lake City, UT.

Glauconite-rich sandstones (greensands) are common throughout the geologic record, but their depositional interpretation is often unclear. Trace fossil associations, ichnofacies and ichnofabrics offer some promising clues. Two Cambrian greensand sequences were examined and compared on the basis of trace fossils, ichnofacies, ichnofabric, glauconite, sedimentology and primary sedimentary structures. The Lion Mountain Member of the Upper Riley Formation in central Texas exhibits a low diversity of trace fossils and an upward increase in burrowing within beds to a maximum ichnofabric (ii5). Greensands with up to 95% glauconite exist in both the laminated and bioturbated zones. The occurrence of cross bedding and scour surfaces indicates a dynamic sedimentary marine environment within wave base. The Reno Member of the Lone Rock Formation in southern Wisconsin, also exhibits a low diversity of trace fossils and a gradual increase in burrowing within each bed to a maximum ichnofabric (ii5). Greensands with up to 90% glauconite are concentrated within flat pebble conglomerates at the base of each bed. A typical bed includes flat pebble conglomerate at the base grading to flat and hummocky laminations with increasing bioturbation upward and topped by an erosional contact. The Cambrian sites in Texas and Wisconsin exhibit similar ichnofacies and “lamscram” ichnofabrics associated with glauconite, and they probably represent comparable paleoenvironments characterized by “r-selected” ichnocoenoses in the Skolithos (and possibly Cruziana) ichnofacies.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009