CYCLIC SEDIMENTATION PATTERNS CAN RESULT FROM CYCLIC INCREASES IN VOLCANICLASTIC INPUT DRIVEN BY CHANGES IN THE ORIENTATION OF THE GRAVITATIONAL PULL ON THE EARTH (MILANKOVITCH CYCLES)

Milankovitch cyclicity in sedimentary rocks is thought to result from changes in solar insolation and associated climate or sea level variability. But cyclic changes in the orientation of the pull of gravity on the earth may temporally focus magmatism and volcanism whose volcaniclastics then become the dominant driver of cyclic sedimentation. Kutterolf et al. (2019) recognized Milankovitch frequencies in tephra records at volcanic arcs. Three "black shales", the Eagle Ford of south Texas, the Green River of western North America, and the Kimmeridge Clay of the North Sea, exhibit sedimentary cyclicity in volcaniclastic input.

Lee et al. (2018) described volcanic ash as one driver of organic carbon burial in the Cretaceous while Oceanic Anoxic Event II was a time of massive subaerial volcanism. The Eagle Ford (EF) was deposited across OAEII and contains 300 recognized layers of volcanic ash. Frebourg et al. (2016) attributed EF organic enrichment and cyclicity to ash derived minerals. Lehrmann et al. (2019) and others recognize Milankovitch cyclicity in the EF. Eighty-five percent of the silica in the formation is authigenic and not evidence of cyclic terrestrial input. The Milankovitch cyclicity of the EF is from eolian ash-derived minerals, not from changes to solar illumination, climate, or sea level. The EF reflects a cyclicity in volcanic explosions.

Voluminous volcaniclastics emanated from the Eocene North Atlantic Igneous Province, while active Eocene volcanism in North America included the Absaroka, Challis, Chilcotin, and other volcanic complexes. The Wyoming Geological Survey describes plentiful volcanogenic zeolites in the Eocene Green River Shale. The Green River has many recognized layers of ash, contains the world's largest trona deposit which is an evaporative alteration product of ash, and it is organically enriched (also a sign of ash fall). Fischer et al. (1991) described Milankovitch cyclicity in the Green River. Cyclic volcaniclastic input to the Green River was likely triggered by cyclic volcanism.

The North Sea Kimmeridge Clay has changes in lithofacies and clay mineralogy Jean (2006) attributed to a volcanic influence and not climatic control, and a cyclicity recognized by House (1985) and others to be linked to Milankovitch cycles.