A PRELIMINARY GRAVITY AND MAGNETIC ANALYSIS OF THE CHIWAUKUM BASIN, CENTRAL WASHINGTON

The Chiwaukum Basin (CB), in the central Cascades of Washington state, encompasses a 64×21km area and 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. More recent studies suggest a transpressional setting due to episodic crustal shortening and propose the feature be considered a structural low. A gravity and magnetic analysis of the CB will serve to further understand the complex tectonic and sedimentary evolution of the CB and the central Cascades. The central objective of the analysis is to place constraint on the sediment thickness of the Chumstick Formation (CFM) and to locate faults in order to obtain 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 and are predominantly based on stratigraphic mapping. Gravity and magnetic data collection within the CB will better serve quantification of sediment thickness and sub sequential interpretations for basin geometry. A prior gravity analysis of the eastern and western sub-basins of the CB provided an uncomplete analysis of the basin. The present gravity and magnetic analysis will use existing data to produce a series of residual anomaly maps created using wavelength filtering and derivative analysis that will be interpreted to determine the subsurface extent of the basin, the basin’s geometry and the location of structural features. This analysis will form the basis of a detailed gravity survey planned for the summer of 2010.