DUST IN THE WIND: INFLUENCES ON THE PRESERVATION OF VOLCANICLASTICS WITHIN THE EOCENE-OLIGOCENE WHITE RIVER SEQUENCE OF THE GREAT PLAINS AND MIDDLE ROCKY MOUNTAINS

Climatic and environmental changes associated with the terrestrial Eocene-Oligocene transition (EOT) of the northern Great Plains manifest as a combination of up-section transitions in depositional environments, paleopedology, and faunal associations within a regionally diachronous lenticular package of strata that thins in multiple directions over hundreds of kilometers from a depocenter of over 300 m in western NE. The EOT is preserved within this diachronous stratigraphic succession, referred to as the White River sequence, and preserves both local and global climatic and environmental influences. At any one locality depositional environments change up-section from meandering river systems with forested landscapes to progressively open ecosystems with flashy and ephemeral fluvial systems. This up section change in fluvial style and paleopedology is associated with an increase in the proportion of eolian volcaniclastic materials introduced into the system, an increase in sedimentation rates, and is diachronous across the region, occurring earlier in WY. Traditionally this up section change has been interpreted as a response to global cooling and/or drying across the EOT, although this same transition in fluvial geomorphology and associated paleopedology can be produced by an increase in sedimentation rates, such as the dramatic influx of aeolian volcaniclastics into the White River depositional complex. The introduction of volcanic materials, as ash falls and reworked volcaniclastics, began in the late Eocene and continued into the Oligocene. Depending on the dominant environmental conditions at any one time or place, reworked volcanic materials were either completely altered to smectitic clays or preserved as pristine ash shards. Today this difference in preservation is recognized as the formal lithostratigraphic units of the Chadron and Brule members/formations in WY and NE. With progradation of reworked volcaniclastics into any given area, depositional systems were eventually overwhelmed, resulting in up-section changes in fluvial and pedogenic facies suggestive of increasingly arid conditions. Further study within regionally extensive isochronous intervals is required in order to clarify the influence of local vs. global climatic and environmental drivers on these changes.