Uranium enrichment shorewards in black shales: A case study from the Scandinavian Alum Shale

Abstract

The Scandinavian Alum Shale Formation (Middle Cambrian to Lower Ordovician) contains high levels of organic carbon (up to 25 wt.%) and syngenetic enriched trace elements. A regional survey of the uranium and organic carbon content reveals that uranium is preferentially enriched in sections located palaeo-shorewards compared with sections located farther offshore. This type of enrichment mode is evident in the Upper Cambrian (above the A. pisiformis Zone) and Tremadoc interval. In the Middle Cambrian no significant regional variation in uranium level is present. In the most enriched Upper Cambrian biozone (Peltura scarabaeoides Zone) the average concentrations of uranium (100 to 300 ppm) are inversely correlated to zone thickness. The variable bed thicknesses are believed to represent primary differences in the rate of deposition and the relationship thus indicates a strong time-dependency in the enrichment processes governing uranium incorporation in sediments. The high uranium levels generally found shorewards are interpreted to reflect a more vigorous bottom water circulation that promoted higher rates of mass-transfer across the sediment/water interface relatively to the mud deposited farther offshore. Very high levels of uranium (1000 to 8000 ppm) concentrated in discrete beds (known as kolm) are interpreted to reflect resuspension of sediment in an anoxic water column that enhanced diffusive exchange between suspended particles and sea-water. A Late Silurian to Early Devonian thermal overprint towards the Caledonian Front affected the total organic carbon (TOC) content but appears to have no effect on the uranium distribution in the thermally mature sections.