Geologic Modeling of Magnetic Data for Cypress Island, Washington

Cypress Island, Washington is composed of three distinct rock units separated by two major faults with east-west trending surface traces. The rock units are from south to north; an ultramafic unit with varyingly serpentinized harzburgite, a volcanic/ sedimentary unit composed of basalt, numerous pelagic sediments and serpentine, and a greywacke unit. The orientations of the fault contacts at depth are not interpretable by surface data. This study compiled magnetic data from several sources to model subsurface geologic aspects of the island. Magnetic anomaly profiles were created from the data, then analyzed and modeled using the computer program GM-SYS. The primary goal of this study was to model the orientation of the fault separating the ultramafic unit from the volcanic/sedimentary unit. The results indicate that the fault has a high-angle south dipping orientation. Data collected also indicate unexpected large quantities of subsurface magnetic material in the volcanic/sedimentary unit. Due to the presence of small amounts of serpentine found as outcrop in this unit, these subsurface magnetic bodies are interpreted to be serpentine as well. The distribution and quantity of this serpentine suggests that the sedimentary/volcanic unit is a serpentine melange.