CARBONACEOUS PRESERVATION OF BURGESS SHALE-TYPE FOSSILS FROM THE MIDDLE CAMBRIAN WHEELER AND MARJUM FORMATIONS, UTAH

Cambrian Burgess Shale-type deposits are characterized by the conservation of non-biomineralized tissues, which are not preserved under normal conditions. These deposits contain diverse assemblages of soft-bodied organisms that are otherwise unknown in the fossil record and provide a unique window on the Cambrian Explosion. The causes of exceptional preservation in Burgess Shale-type deposits are poorly constrained, and, recently, important differences in taphonomic pathway among some deposits have been documented. Understanding the range of taphonomic pathways in Burgess Shale-type deposits is vital to understanding how and why exceptional fossilization occurred and to the interpretation of fossil assemblages. Nonmineralized metazoans and algae from the Wheeler and Marjum Formations were analyzed using SEM-EDS. Specimens were examined under SEM using SE and BSE modes and fossil composition was determined through elemental mapping and spot analyses. Based on preliminary results, we interpret that nonmineralized fossils from both deposits are preserved as carbonaceous films. The gut of a single arthropod specimen is preserved in phosphate, however aspects of no other soft-bodied fossils are associated with mineral coatings. Some fossils, including phyllocarid carapaces and large algae, are preserved as carbonaceous “flakes” several microns in thickness. Other soft-bodied fossils, which are visually readily distinguished from the matrix by dark color, show either elevated carbon concentrations vs. matrix, or show no significant compositional difference vs. matrix. These fossils are interpreted as thin organic films. The lack of authigenic clays as a fossilizing agent, as reported from the Burgess Shale, may offer a partial explanation of why soft-bodied metazoans are rarer and typically more poorly preserved than in the Burgess Shale. In the absence of a mechanism to replicate the finer anatomical detail of the most labile tissues, only more refractory tissues would survive early diagenesis to be preserved as carbonaceous films. The predominance of “gut traces” as the most common soft-bodied metazoan fossils supports this hypothesis, and suggests that early mineralization may be important in conserving some types of paleontological information typically lost from Wheeler and Marjum fossils.