PALEOENVIRONMENTAL AND DEPOSITIONAL SETTING OF THE EMU BAY SHALE, A UNIQUE EARLY CAMBRIAN LAGERSTÄTTE

The early Cambrian Emu Bay Shale (EBS) is an outlier among Cambrian lagerstätten. Overwhelmingly, soft-bodied biotas of the Cambrian were preserved in distal shelf and slope environments via the Burgess Shale-type pathway. The EBS biota is noteworthy not only for its unique style of preservation, but also for its more proximal depositional setting. Previous interpretations of water depth and environmental setting of the EBS biota have varied significantly, due to the cryptic nature of the fine-grained sediments that host the lagerstätte. In order to better constrain the paleoenvironmental setting of the EBS biota, we conducted an intensive study of the EBS and adjoining strata. The EBS may be divided into three units, a basal mudstone member (8 m), a heterolithic middle member (45.5 m) comprising regularly interbedded mudstone, fine sandstone and minor polymict conglomerate, and a top member (22 m) of thin-bedded fine sand with diminutive bedforms. The soft-bodied biota occurs in mudstones of the lower member and the lowermost middle member. Several features exclude a shelfal setting for the EBS: 1. excellent sorting with only rare grading, and strong separation of sand and mud into discrete depositional beds; 2. pervasive wet-sediment loading and slump structures; 3. paraconglomerates occur at multiple horizons, bearing cobbles of diverse lithologies. These attributes suggest that the EBS was deposited in a tectonically active nearshore basin in a fan delta setting. The succession of lithologies and the transition into the overlying tidally-influenced Boxing Bay Formation indicate that the EBS represents accumulation across a range of depth and energy conditions in a prograding fan delta complex. Mudstones that contain the lagerstätte are characterized by mm-lamination and the presence of abundant hanging silt grains dispersed throughout the clay-dominated fabric. These attributes imply deposition from turbid mudflows, which possessed the strength to carry coarser grains suspended above the bed, and were critical to the entrainment and burial of the EBS biota. While these biostratinomic conditions were similar to those of typical Cambrian lagerstätten, the inferred prodelta setting of the EBS biota is unique, and may have been influenced by freshwater and nutrient delivery from terrigenous runoff.