JOINT CONTROL ORIGIN AND AGE OF PHANTOM SHIP, CRATER LAKE NATIONAL PARK, OREGON

Phantom Ship, a part of the Phantom Cone which is the oldest exposed above-water feature within the Crater Lake caldera, forms a striking WNW-trending “sail-like” shape. This commonly-visited tourist stop projects above water level within the caldera’s rim. An early hypothesis to explain the shape, as suggested by Howel Williams, is that the sail-like profile is due to one or more WNW-trending dikes. Recent work by Charles Bacon mentions another possibility: fracturing and differential erosion of interlayered flows and pyroclastic rocks. The latter hypothesis has been validated by field observations made possible through access granted by the Crater Lake Park Service to collect samples and measure joint sets on the island. Three dominant steeply-dipping joint sets (N.27W., N.54W., and N.80E.) are responsible for the above-water shape of Phantom Ship as well as contributing to the fractured nature of the feature. Rare slickenlines (~75, S.60W.) suggest minor dip-slip movement along the joint sets. The presence of a dike to the WNW below water level has been documented by Bacon but no apparent dikes occur above the surface on Phantom Ship or on the adjacent caldera wall. Although the flows are altered, petrologic and XRF analyses show they are silicic augite andesites. Plagioclase phenocrysts exhibit a variety of textures (such as oscillatory zoning and dissolution) suggesting rapid growth and a complex cooling history. Interlayered pyroclastic rocks are comprised of angular heterolithic silicic andesite clasts in a highly altered fragmental matrix. Besides broken crystals and rock fragments, the matrix contains post-depositional celadonite, calcite, quartz, and zeolites. Although a 403 +/- 12 ka age from a lava flow near water level has been published by Bacon, a recent 930 +/- 120 ka ⁴⁰Ar/³⁹Ar date on 100-180 μm plagioclase separates from an upper flow of Phantom Ship suggests an older age for this part of the Phantom Cone.