The Lewis County Cascadian Foothills in southwestern Washington provides a diverse geologic history of marine transgressive-regressive depositional cycles, shifting geosyncline troughs, and a chaotic assemblage of faults and folds exposed to convergent margin plate subduction and plate rotation occurring throughout the Tertiary Period. Field mapping techniques and satellite imagery were used to reconcile confusing attitudinal observations as well as primary and secondary folds obscured by superposition.
Chapter 1 presents regional plate tectonic 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 occurring during 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 areas: the West Central Volcanics and the East Central Volcanics based on the presence of andesite flows and pyroclastic ash deposits each of which were considered mappable units in Lewis County.
The Skookumchuck Formation exhibited cyclothemic properties based on observed exposures. Two models are offered based on ideal cyclothemic models derived from the Pennsylvanian Period for comparison with Middle to Upper Eocene cyclothemic deposition.
Chapter 3 provides an overview of geologic structure and tectonic rotation with impacts on regional deformation 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.
