Biological Sensing of Benthic Environments in Dark Shales and Related Oxygen-Restricted Facies

The stagnant basin model that is widely applied to organic-rich dark shales and laminated marls and chalks has led to the view that sea-bottom conditions during accumulation were persistently anaerobic to dysaerobic. There are however, clear indications from the presence of trace and body fossils, that the conditions were much more variable. First, there are individuals and communities adapted to lower-than-normal oxygen levels: microbial mats and “flat clams” (Inoceramidae,Posidoniidae,Monotidae, and small Pectinidae). Second, there are event populations, bedding-plane concentrations of many individuals of few species that flourished during short periods (months to years?) of higher or lower oxygenation. Third, there are often transported assemblages of taxa from adjacent shelf or slope environments or from the mass mortality of pelagic organisms.