Turbidite depositional influences on the diagenesis of Beecher's Trilobite Bed and the Hunsruck Slate; sites of soft tissue pyritization

Abstract

Chemical signatures of enrichment of highly reactive iron, and framboid size distributions, are reported in turbidite sediments that host soft tissue pyritization (Beecher's Trilobite Bed, Upper Ordovician, and the Hunsrück Slate, Lower Devonian). These signatures demonstrate that the sediment of Beecher's Trilobite Bed was enriched in highly reactive iron prior to turbidite transport but that no enrichment was present in the Hunsrück Slate. Turbidite transport and re-sedimentation altered framboid size distributions. Small diameter framboids (< 5 μm) that formed in the sediment at the pre-transport site were lost during transport due to oxidation and/or size sorting. Larger diameter framboids (∼5 –15 μm) that formed at the pre-transport site were transported without alteration. The oxidation of the original small framboid population formed highly reactive iron (oxyhydr)oxides that were reduced during post-transport suboxic diagenesis to produce porewaters rich in dissolved iron. In some turbidites a bimodal framboid population resulted where a minor population of small diameter framboids, produced by limited sulfate reduction at the post-transport site, was added to the transported large diameter population. Soft tissue pyritization in this setting was facilitated by the presence of suboxic, iron rich porewaters where dissolved sulfide formed during soft tissue decay, confining iron sulfide precipitation to the decay site.