GEOLOGIC STUDIES OF THE TURTLEBACK COMPLEX AND EAST SOUND GROUP, ORCAS ISLAND, NORTHWEST WASHINGTON

The main focus of study for this poster is the Turtleback Complex (TBC) and the East Sound Group (ESG) located on Orcas Island in northwestern Washington. The Turtleback Complex consists of a variety of plutonic rocks that exhibit evidence of greenschist facies metamorphism. The East Sound Group consists mainly of sedimentary and volcanic rocks with low grade metamorphism. Volcanic rocks of the East Sound Group are mainly mafic to intermediate composition, and are interlayered with, volcaniclastic rocks and limestone. Previous studies suggest that the Turtleback Complex formed the basement, and the East Sound Group formed the cover of a once coherent Chilliwack composite terrane.

We spent eight days examining the geology of the TBC and ESG during the summer of 2014. The areas with the best exposures were along the shoreline and required boat access. The TBC consists of a complex mixture of cross-cutting intrusive rocks that form an injection complex. Compositions range from gabbro to tonalite, with dominant compositions of diorite and tonalite. Most exposures are compositionally variable on a scale of meters, commonly with more mafic rocks intruded by dikes of more felsic composition. The ESG consists mainly of volcanic and volcaniclastic rocks that commonly are intruded by dikes and small bodies of diorite and tonalite. Metamorphic grade is low and primary volcanic and sedimentary features (e.g., pillows, volcanic breccia, graded beds) are well preserved. Most volcanic rocks are of andesitic to basaltic composition. The nature of the contact between the TBC and ESG is difficult to determine with certainty because of the widespread brittle faulting and the presence of similar intrusive units in both assemblages.

In an effort to reconstruct the history of the TBC and ESG, fifteen samples were collected for U-Pb geochronologic and Hf isotopic analysis. Several samples were collected from each of the compositional members of the TBC from western, central, and eastern Orcas Island. Four samples were collected from sandstones of the ESG for detrital zircon analysis. We anticipate that analyses of these samples will provide constraints on the magmatic history of the TBC and the depositional age and provenance of the ESG, and thereby help to unravel the tectonic history and significance of the Chilliwack terrane.