Can “volcanic tephra fall” replace “sea level transgression” as the driver for organically enriched rock formation at the Cenomanian-Turonian Boundary (OAE2)?

Abstract

“The presence of volcaniclastic sediments within the OAE 2 interval in the subtropical North Atlantic is consistent with a major phase of explosive volcanic activity in the Caribbean and other regions.” (Huber, 1999). “Major periods of volcanism {in western North America—DMP} peaked in the Cenomanian. (Eric H. Christiansen et al.,1994). Bentonite deposits are recognized at the C/T Boundary at Black Mesa, AZ, in the Mancos Shale. The USGS study of the Utah Mancos Shale concluded, “It is enriched in uranium, copper, silver, vanadium, mercury, arsenic and, to a certain extent, gold. The metals probably came in with the volcanic tuffs that compose much of the Mancos itself.” (Marlatt, 1991). The GSSP for the C/T boundary is near Pueblo, Colorado. Glenister et al. (2012) compared the regressive facies to the transgressive facies and noted that the transgressive facies contained abundant volcanic ash, many more disconformites, and was more organically enriched The Mowry Shale (part of Greenhorn cyclothem) has been commercially mined for bentonite in Montana. The Niobrara “cyclothem” contains hundreds of ash layers (Sonnenfeld et al., 2017). Therefore, at least two Cretaceous Seaway sea level cycles were times of large-scale ash deposition. The Eagle Ford was deposited at the same time as the Greenhorn Cyclothem. James S. Eldrett et al. (2013) state, “The Eagle Ford Formation consists of a succession of calcite-rich mudstones (marls and limestones) and over 300 volcanic bentonite layers. Frebourg et al. (2016) stated about the Eagle Ford/Boquillas system, “The co-occurrence of volcanic ash beds and globigerinid-rich sediments suggests that most nutrient input was associated with ash deposition. The cyclic alternations of globigerinid argillaceous wackestones and pelagic grainstone deposits thus appear to be primarily controlled by volcanogenic nutrient input instead of other climate or sea level–driven processes.” Volcanic ash fall as a mechanism for organically enriching rock is consistent with what we observe near the Cenomanian-Turonian boundary worldwide, including the well-studied rocks of the Greenhorn Cyclothem.

AAPG Datapages/Search and Discovery Article #90357 ©2019 AAPG Rocky Mountain Section Meeting, Cheyenne, Wyoming, September 15-18, 2019