HIGH-LIFE ONA DEEP SHELF: SEDIMENTATION AND BIOTURBATION GRADIENTS IN THE CAMBRIAN ALUM SHALE IN SOUTHERN SWEDEN

The Cambrian Alum Shale is considered the most important black shale unit in southern Scandinavia. With more than a hundred meters of cumulative thickness and TOC (total organic carbon) values of in places more than 10%, it is thought to have widely sourced overlying Ordovician and Silurian carbonates. The sediments of this unit have been described as being anoxic to dysoxic and deposited in a tranquil setting. However, the results of this study indicate the contrary and will place the Alum Shale alongside many other high-TOC mudstones that document abundance of benthic life and tectonic movements during sedimentation.

The Alum Shale represents a ramp environment with carbonate grainstones and packstones in the proximal portion, and organic-rich siliciclastic mudstones at its distal end. The carbonates can be subdivided into inner, middle, and outer ramp facies, and similarly the siliciclastic mudstones reflect three distinct depozones. Distinctive features of the most proximal mudstone facies belt are mud ripples and phosphate lithoclasts; the central mudstone facies belt is characterized by abundant mud rip-up clasts and some fossil fragments, whereas the most distal facies belt contains mostly massive mudstones with minor siltstone laminae.

All facies are characterized by abundant bioturbation. Sub-millimeter, sub-vertical traces occur throughout the unit, whereas up to centimeter-scale horizontal bioturbations are confined to more proximal parts and often siltstone intercalations. All facies also show signs of bed load transport either as irregular laminae, as ripple structures, or erosional features such as mud rip-up clasts. Millimeter-thick slump units are present in the Alum Shale, indicating active tectonic movements in the area. The Alum Shale succession exemplarily shows that even in the very Early Paleozoic the deep shelf environment was colonized by a variety of organisms burrowing millimeter-deep into the soft substrate. Currents brought in silt from the shoreline, in places eroded the seafloor and reached even the most distal parts of this shelf. Therefore, the Cambrian deep shelves must have already presented an environment similar to later Paleozoic examples with probably dysoxic to oxic, not anoxic conditions controlling the accumulation of high-TOC sediments.