TITLE:
Timing of Onset of Volcanic Centers in the Campanian of Western North America as Determined by Distal Ashfalls
AUTHORS:
Janet L. Bertog
KEYWORDS:
Pierre Shale; Niobrara Formation; Volcanism; Forearc; Campanian
JOURNAL NAME:
Open Journal of Geology,
Vol.3 No.2,
April
30,
2013
ABSTRACT: Strata of the Late Cretaceous Niobrara
Formation and Pierre Shale Group include bentonites that provide a distal
record of volcanic activity taking place to the west. Detailed stratigraphic analysis
combined with mineralogy and geochemistry of the bentonites indicates the
following timing of events: 1) Eustatic sea level fall as a result of the end
of the Niobrara Cycle; 2) Tectonic deformation of the Western Interior Seaway
coincident with tectonism on the Absoroka Thrust in Wyoming and Late Canyon
Range Thrust in Utah; 3) Backarc volcanism in Montana associated with the
Little Elkhorn Mountain volcanic complex; 4) Forearc volcanism in the Cascades
area indicates subduction of a hot oceanic crust where plagioclase in the
oceanic crust is being incorporated into the melt; 5) Cessation of tectonic
activity results in a return of sedimentation patterns to north-south trending
belts with the Boyer Bay and Burning Brule members of the Sharon Springs
Formation deposited to the east and the Mitten Black Shale Formation deposited
in the basin.