Gravity and Magnetic Analysis of the Chiwaukum Basin, Central Washington

The Chiwaukum Basin (CB), located on the eastern slope of the central Cascades in Washington, is bounded on the northeast and southwest by the Entiat and Leavenworth faults, respectively. Until recently, the CB was widely considered to be a graben structure resulting from Eocene extension or transtension and subsequent transpression with dextral strike-slip faulting. However, recent geological mapping and structural analyses propose that the feature be considered a structural low, not previously influenced by extension. In order to aid in determining which of the above hypotheses is most plausible (or if another model is more applicable), a gravity and magnetic study was performed to provide a supplemental geophysical dataset to existing geologic investigations. The central objective of the analysis is to constrain the thickness of the Chumstick Formation (CFM) and to estimate the geometry of the CB. The CFM is a middle-late Eocene, non-marine sedimentary deposit within the CB, the thickness of which is not clear-cut. Bouguer gravity anomaly maps indicate that the basin is associated with a gravity minimum while the magnetic map is more complicated with a large wavelength maximum with superimposed minima. The resulting series of 21/2D basin models suggest an average CFM thickness of 2.5 km. Ultimately, this study postulates a model that is most consistent with the conventional paradigm which accounts for an extensional component within the tectonic and depositional history of the CB