DIAGENESIS OF EXCEPTIONALLY PRESERVED TRILOBITES FROM THE WHEELER SHALE

Trilobites from the middle Cambrian Wheeler Shale, Utah, are exceptionally well preserved. A previous sedimentary model suggested that bacterial sulfate reduction acting on decaying trilobites led to cone-in-cone (CIC) calcite precipitation that buttressed the ventral side of the exoskeletons in early diagenesis. In addition to the sediments, it is quite possible that the exoskeletons might hold petrographic and geochemical signatures consistent with this model. We examined two Wheeler Shale trilobite species, benthic Elrathia kingii and putatively pelagic Peronopsis interstricta, to determine if they had different petrographic and geochemical signatures either reflecting their habitats prior to burial or burial diagenesis. Similarly, we examined whether non-molted trilobites with presumably more organic carbon had different petrographic and geochemical signatures than molts and exoskeletal fragments. We found that elemental Ca, Mg, Fe and S composition of both trilobite species was similar and indicative of diagenetic overprints. Both trilobites, regardless of molt status, had similar thicknesses of CIC calcite that was not solely restricted to the ventral surface; fragments also had CIC calcite in different orientations. Growth of CIC calcite displaced compacted and foliated sediment, indicating CIC calcite growth occurred after sediment compaction. Cubic pyrite inclusions in CIC calcite also suggest a low-organic carbon, late-diagenetic precipitation. Thick and thin twins on CIC calcite crystals indicate temperatures up to ~200 °C, while microfractures and microstylolites indicate contractional strain. Recrystallization and fracture-fill blocky calcite also indicate reworking of CIC calcite. While it is possible that CIC calcite might form in early diagenesis, late diagenetic conditions are more consistent with CIC formation in the Wheeler Shale, likely tied to compression and compaction from burial and rotation of the House Range Embayment fault block.