HERE’S THE ‘BEEF’—TIMING OF OVERPRESSURING IN THE CAMBRIAN ALUM SHALE, SOUTHERN SWEDEN

Elongate, bedding parallel/subparallel mineralized features observed in core from the Late Cambrian Alum Shale, Sweden, are herein interpreted as ‘beef’. The timing of ‘beef’ formation is important as it pertains to understanding the processes responsible for its formation. Overpressuring during burial likely caused the beef, but questions remain regarding the mechanism responsible for overpressuring—hydrocarbon generation or tectonic compression. Given that the Alum here is thermally mature—very low hydrogen indices (<10 mg hydrocarbon/gram TOC) based on RockEval pyrolysis—hydrocarbon generation is considered the probable mechanism that led to its formation. Nevertheless, direct observations in a detailed petrologic study were undertaken in an attempt to more fully and unequivocally address its origin.

Beef occurs as continuous elongate features across individual thin sections (>2 cm), or as shorter, en echelon segments. Beef mineralization includes calcite, Fe-dolomite, barite, K-mica, chalcedony, pyrite, and likely bitumen. The minerals and bitumen are intimately mixed; in some cases the bitumen ‘cements’ the minerals whereas in other cases the bitumen appears to be admixed insofar as it is present in alternating bands with the minerals. The close spatial and temporal relationship between the bitumen and minerals in the beef indicates that infilling of beef was at a time when bitumen was migrating in the system.

The presence and admixture of bitumen with mineralization in beef in the Alum Shale corroborates that beef infilling occurred during hydrocarbon generation. Thus, petrologic evidence constrains the mechanism of beef formation in the Alum to that associated with hydrocarbon generation.