PUTATIVE ELDONIOIDS FROM THE CAMBRIAN GIBSON JACK FORMATION, IDAHO

The Gibson Jack Formation is a Cambrian shale unit which crops out in southeastern Idaho. It is a known Burgess-Shale Type (BST) fossil lagerstätte, containing trilobites preserved via kerogenization (carbonaceous compression), with pyrite accessory mineralization. In this study, several discoidal, putative fossils from the Gibson Jack Formation were analyzed via scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS), to assess their biogenicity. These discoids are highly abundant in certain horizons, likely having been transported and sorted by turbidity currents. The pyrite, carbon, and phosphate association is consistent with BST preservation of soft-bodied organisms, though they have been subjected to substantial diagenetic alteration, including some thorium enrichment. Abiotic causes are inadequate to explain the morphology and chemistry of the fossils. The morphology is reminiscent of eldonioids, a group of enigmatic discoidal organisms common in other Cambrian BST lagerstätten. Analysis of Eldonia specimens from the Marjum Formation and Spence Shale are used for comparison. The Gibson Jack fossils constitute a novel fossil or mode of fossilization, and could constitute a new occurrence of eldonioids.