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This chapter notes the occurrence of the pyrite dimorph, marcasite, as a minor diagenetic cement in light purple to bluish-white Miocene calcareous oozes. It occurs as the only mineral phase cementing small, elongate, gray aggregates with the morphology of worm burrows and may represent a worm's last stand. Scanning electron micrographs reveal clumps of typically fluted or curved orthorhombic dipyramids of marcasite supporting a framework of broken and corroded siliceous fossil fragments. Pyrite co-occurs in the same stratigraphic horizons. Marcasite has apparently formed in association with animal decay that creates a microenvironment where pW is near or below 6.0. It may later invert to pyrite, possibly in association with the initial cementation and compaction processes leading to a chalk marked by pyritic liesegang rings. Iron sulfide occurrences in Hole 315 are consistent with the very high sedimentation rates reported in the variegated Miocene oozes and chalks.

Marcasite in Miocene Calcareous Sediments from Hole 315A