A GRAVITY AND MAGNETIC STUDY OF THE CHIWAUKUM BASIN, CENTRAL WASHINGTON

The Chiwaukum Basin (CB), located on the eastern slope of the central Cascades in Washington state, is bounded on the northeast and southwest by the Entiat and Leavenworth faults, respectively. Until recently, the CB was considered to be a graben structure resulting from Eocene extension or transtension. Recent geological mapping and structural analysis suggest a transpressional setting due to episodic crustal shortening and propose the feature be considered a structural low. In order to aid in determining which one of the above structural models is correct (or if another model is more applicable), a gravity and magnetic study was performed that will aid in understanding the complex tectonic evolution of the CB and the surrounding region. The central objective of the analysis is to constraint the thickness of the Chumstick Formation (CFM) and to locate faults 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 remains uncertain. Earlier hypotheses for the thickness of the CFM vary from <2km to >9km which is mainly based on stratigraphic mapping. Gravity and magnetic data collection within the CB along with density measurements on units within the basin will better serve to estimate the sediment thickness and interpretations of the basin geometry. A prior gravity analysis of the eastern and western sub-basins of the CB provided estimates of the basin's thickness but lacked density information. The present gravity and magnetic analysis uses existing data supplemented with new gravity data to produce a series of residual anomaly maps. A residual gravity anomaly maps shows that the basin is associated with a gravity minimum while the magnetic map is more complicated with a large wavelength maximum with superimposed minima. A series of two and one-half dimensional gravity and magnetic models are currently under construction that will be used to estimate the basin's thickness.