Sedimentary sequences of Early to middle Cretaceous age that are rich in organic carbon (C-org) are common in the deep seas and in shallow interior and shelf basins of the world. High concentrations of C-org and the presence of fine laminations suggest that these strata accumulated in environments that were anoxic or nearly anoxic at and possibly above the seafloor. Many organic-rich sequences display a marked cyclicity in amount of C-org and, if deposition was near or above the carbonate compensation depth, in CaCO3 as well. These cyclic C-org and CaCO3 variations result in interbedded lighter and darker shale, marlstone, and limestone that have cycle periods ranging from 20,000 to 100,000 years. Variations in degree of bioturbation parallel changes in C-org and CaCO3 because burrowing organisms were most active at times of deposition of relatively low C-org and high CaCO3 sediments.
Such cycles are best developed in sediments deposited during episodes of lower dissolved oxygen concentration (so-called “oceanic anoxic events”), but similar cycles of C-org and CaCO3 variation having periods of tens of thousands of years occur in strata of almost every geologic age. The periodicities are similar to those in pelagic sediments of Pleistocene age that are known to be climatically induced. Studies of geochemistry, stable isotopes, and mineralogy show that older organic cycles are also related to climatic changes that produced fluctuations in the amount of wind- and river-borne clastic sediment and terrigenous organic matter delivered to the oceans, and to coincident variations in surface-water salinity, productivity, and mid- to deep-water oxygen concentration. Regional paleogeography and paleodepth determined the amount and type of organic matter preserved within the sediments.