Sunday, 5 October 2008
George R. Brown Convention Center, Exhibit Hall E
The Deep Spring Formation of western Nevada and eastern California records the Ediacaran-Cambrian transition from a shallow shelf environment along the Cordilleran margin of Laurentia. The formation is composed of fine siliciclastics interbedded with shelf carbonates. Siliciclastic deposits of the Middle Member of the Deep Spring Formation (Ediacaran) of Esmeralda County, Nevada, yield a low diversity assemblage of body and trace fossils together with sedimentary structures showing the interaction of microorganisms and sediment. Body fossils of an elongate, bilateral organism with an anchor-shaped “head” resembling Parvancorinaare present. In the upper part of the Ediacaran System, deposits of the Deep Spring Formation show little sediment disturbance by bioturbators; surficial scratches and traces reflecting shallow anchoring are the principal evidence of animal activity. Trace fossils include Bergaueria and a repichnial trace consisting of paired elongate impressions flanked by tiny crater-shaped impressions. Bergaueria is a sac-shaped burrow possibly produced by sea anemones or other infaunally anchored animals. The repichnial trace resembles crawling traces attributable to myriapods or euthycarcinoids of the Paleozoic. If the trace were constructed by a leg-bearing animal, this would extend the record of leg-bearers downward from the Cambrian into the Ediacaran. Wrinkled (“elephant skin”) textures on sediment surfaces are inferred to reflect microbial mat-stabilized sediment surfaces. Mudcracked surfaces imply the bonding of sediment grains by microbial consortia. Additionally, various elongate scratches on sediment surfaces record the movement of current-entrained objects across microbial mat-stabilized surfaces.