ADDING SOME SPICE TO THE LATE CAMBRIAN SPICE EVENT: PYRITIZATION HISTORY IN THE ALUM SHALE, ANDRARUM CORE, SWEDEN

Petrologic studies of the Late Cambrian Alum Shale were undertaken on an interval that includes the peak SPICE (Steptoean Positive Carbon Isotope Excursion) Event to evaluate pyritization across this presumed anoxic/euxinic event. Attention herein is on sulfur-bearing minerals, particularly as they may contribute to the positive δ³⁴S excursion in pyritic sulfur outlined by others. Samples were originally deposited in the distal, siliciclastic mudstone-rich end of a shelf system, and were collected from the Andrarum-3 core, drilled in Scania, Sweden; the samples are from the same intervals for which the δ³⁴S data were collected.

The Alum consists largely of siliciclastic mudstones deposited by bed-load processes. Three distinct depositional facies include, in decreasing abundance; facies (1) clay clast-rich mudstones with high organic carbon (<10 wt %), facies (2) laminated mudstones with macrofossil debris, and facies (3) laminated mudstone. In all facies abundant fecal strings and Planolites burrows are evidence of biotic colonization.

Authigenic sulfur-bearing minerals occur in all facies; pyrite is the dominant phase and the focus herein, with lesser sphalerite, barite, and melanterite(?). Pyrite occurs in texturally distinct forms including 1) microcrystalline crystals (<<1 µm) as irregular masses in matrix, 2) framboids (<5 to >20 μm), 3) concretionary masses of framboids, 4) concretions of framboids uniquely with Fe-dolomite as both a rim and a micropoikilotopic cement of the framboids, 5) concretions of euhedral crystals, 6) euhedral to anhedral crystals within discrete laminations or disseminated in matrix, and 7) massive crystals that replace other cements or disrupt and contort bedding. Importantly, our preliminary observations indicate that framboid concretions with Fe-dolomite rims/cement seem spatially restricted to the SPICE interval.

The various forms of pyrite are temporally distinct, which indicates that pyritization was an ongoing diagenetic process in the Alum. The seeming restriction of framboid concretions with Fe-dolomite rims/cement in the SPICE interval may explain the anomalous sulfur isotopic signature of this interval. Ongoing research is directed at evaluating the significance of textural variability with sulfur isotopic signature of the pyrite within the Alum.