CARBONATE CONCRETIONS FROM THE WHEELER FORMATION PRESERVE EVIDENCE SUGGESTING MICROBIAL REDUCTION OF STRUCTURAL FE(III) FROM CLAY MINERALS IN CAMBRIAN SEDIMENTS

Mudstone hosted carbonate concretions have long been a vexing aspect of diagenesis, yet they can provide insights into the nature and timing of host rock biogeochemical processes. Concretions found within the Wheeler Formation preserve a diverse suite of authigenic mineral assemblages. Complex relationships among mineral assemblages, in combination with geochemical data, allow timing of multiple phases of concretion formation to be assessed within the broader context of host rock diagenesis. The Wheeler Formation is a middle Cambrian fossil lagerstätte, which contains Burgess Shale-type preservation of carbonaceous macrofossils; thus, the nature and timing of diagenetic reactions is of additional interest. In this study petrographic, isotopic (d34S, d18O, d13C), SEM, and XRD data from concretions as well as the surrounding shale were analyzed in order to elucidate mechanisms and timing of concretion growth. Morphological and geochemical data reveal the signatures of several distinct, microbially mediated mineralization processes that were active throughout early diagenesis, and persisted after brittle deformation associated with burial compaction of the concretions. 24% of concretions contain microcrystalline silica, closely associated with authigenic pyrite. The intergrowth of these minerals indicates high pore water concentrations of silica, Fe(II) and sulfide during precipitation of these phases. The paucity of biogenic and detrital silica in Wheeler sediments implicates clay minerals as the most likely chemical precursor to authigenic silica. The pore water concentrations of Fe(II) and silica necessary for the precipitation of this mineral assemblage could have been produced by dissolution of clay minerals resulting from the reduction of octahedrally coordinated structural Fe(III) by Fe-respiring bacteria. Multiple lines of evidence suggest that bacterial processes were likely catalytic in producing this mineral association; it is also unlikely that the Wheeler Formation reached burial temperatures required to drive the necessary reactions abiotically. When considered in the context of other mineral phase relations within the concretions, these mineral associations suggest that iron reduction happened later in the diagenetic sequence than predicted by conventional models.