The Cascadian Foothills

The Lewis County Cascadian Foothills of Southwestern Washington provides a diverse geologic history of marine transgression-regression depositional cycles, shifting geosynclines, and a chaotic assemblage of faults and folds exposed to convergent margin plate subduction and plate rotations throughout the Tertiary Period. Both field mapping techniques and satellite imagery were used to reconcile confusing attitudinal observations as well as primary and secondary folds obscured by superimposition.Chapter 1 presents regional plate tectonics setting, stratigraphy, Early Eocene tectonic history, a brief summary of the North Cascade sub-continental collision, and an overview of the western geosyncline basin shifting during the Middle Eocene through Miocene Epochs.Chapter 2 presents study area stratigraphy observed in central Lewis County for the McIntosh, Northcraft, Skookumchuck, and Lincoln Creek formations. Depositional models for each formation are presented. The Northcraft formation was subdivided into two sections: the West Central Volcanics and East Central Volcanics based on andesite flows and pyroclastic ash deposits each considered mappable units.The Skookumchuck Formation exhibited cyclothemic properties based on measured sections observed in the study area. Two models are presented based on ideal cyclothems documented from the Pennsylvanian Period for comparison with Middle Eocene cycles.Chapter 3 provides an overview of geologic structure and tectonic rotation with impacts to regional deformational patterns related to folding and faulting. Plate rotation theories are presented, southwest Washington regional rotation studies and study area rotation models are presented. Plate rotation geochronologic estimates are provided for the Middle Eocene, Oligocene, and Miocene timelines. Geosyncline influences and study area tectonic sedimentation are discussed.

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