INVESTIGATION OF THE MT TEBO STRUCTURE AT DRY CREEK, MASON COUNTY, WA

Recent aeromagnetic surveys of the Puget Lowlands reveal an extensive thrust fault, the Olympia Structure, extending farther northwest into the Olympic Peninsula than previously identified. These surveys conducted by Blakely et al., (1999; 2009) have also uncovered several smaller thrust faults such as the Frigid Creek Fault and Canyon River Fault as well as anomalies in the magnetic field and in magnetic susceptibility measurements in the Lake Cushman area, a region of the Crescent Formation that was previously thought to be relatively uniform.

This study presents the results of ground-based, proton precession magnetometer surveys performed along forest roads concentrated around Dry Creek, Mt Tebo, and the northern and western shores of Lake Cushman. The field area was selected based on mapping by Clark suggesting that faulting may have been responsible for shaping the region's present-day lithology and topography and a magnetic anomaly appearing in the aforementioned aeromagnetic survey. Magnetic intensity data show local variations on the order of 1,000 to 2,000 nT in transects along forest roads. The geometry and nature of the magnetic variations serve to further support the presence of subsurface geologic structures that may be responsible for shaping topography of the Lake Cushman and Mt. Tebo area. This study also shows the utility of magnetic surveying in mapping structures in this structurally complex region.