Extremely intersting picture. Along the North American west coast there are Miocene siliceous "shales" that are sulphur-enriched. Off of California, this unit is known as the Monterey Formation. The are other names for similar units in other areas.
Research into the Monterey Formation over the past 20 years, particularly by my colleague at the USGS, Caroline Isaacs, suggest that it is in part a bioclastic unit (highly diatomaceous, associated with upwelling events.
Thanks for the ping.