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AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Bioturbations, Storms and Earthquakes: The Depositional Environment of the Cambrian Alum Shale in Southern Sweden

Warren Newby1; Previous HitSvenTop Egenhoff1; Joerg Maletz1; Per Ahlberg2

(1) Geosciences Department, Colorado State University, Fort Collins, CO.

(2) Department of Earth and Ecosystem Sciences, Lund University, Lund, Sweden.

The Cambrian Alum Shale Formation is considered the most important black shale unit in southern Scandinavia. With up to over a hundred meters of cumulative thickness and TOC 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 shows two distinctly different types of proximal-distal facies transects as a function of input and availability of detrital quartz silt. During times of abundant quartz silt supply much of it was brought onto the deep shelf during storms. This resulted in a shelf environment with successively less and finer-grained quartz silt towards distal parts with the most distal facies being organic-rich black shales. When the siliciclastic sediment source was shut off the proximal shelf areas turned to carbonate production reflected in the onset of carbonate mud input onto the deep shelf. On a proximal to distal transect the highest energy deposits consist of shell debris pack- to grainstones with glauconite that grade into carbonate mudstones and carbonate-shale intercalations. The distal end of this shelf transect is again a dark organic-rich shale.

All facies are characterized by abundant bioturbation, with more proximal types being horizontal and larger, and distal ones subvertical and smaller. All facies also show signs of bed load transport in the form of irregular laminae or ripple structures. Millimeter-thick slump units are present in the Alum Formation, indicating active tectonic movements in the area. The Alum Shale succession exemplarily shows that even in the very early Phanerozoic the deep shelf environment was colonized by a variety of organisms burrowing already into the soft substrate. Currents brought in silt from the shoreline 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.