GEOLOGY AND PALEOMAGNETISM OF THE HARO FORMATION, SAN JUAN ISLAND, WA

The San Juan Islands, WA, exhibit complex structures which were active during the mid to late Cretaceous. The structural sequence of the San Juan Islands can be divided into two main components, the complexly deformed San Juan thrust system (“the internal units”), and the less deformed external fault bounded Mesozoic units. The external units have not been subjected to the high-pressure metamorphism found in the internal units of the San Juan thrust system. The external units consist of the Upper Triassic Haro Formation, the Jurassic-Cretaceous Spieden Group, and a younger Upper Cretaceous syn-orogenic clastic sequence, the Naniamo group. The Haro Fm. is exposed on the N. side of San Juan Island, adjacent to the primary structure (the Haro Thrust) that separates the internal/external units of the SJTS. The Haro Fm. is Triassic in age (dated by embedded fossils) consists of well-bedded alternating sandstone, mudstone and massive conglomerate units, all of which have undergone tectonic deformation. This study presents petrographic, structural, paleomagnetic, and rock-magnetic data that will better outline the history of the Haro Fm.

A petrographic point count was conducted to relate the sandstone composition to a parental source and tectonic setting. Composition data plotted on QFL, QmFLt, and QpLvmLsm diagrams display trends that indicate the parental source as a transitional arc orogen, deposited in a fore-arc or back-arc basin. Preliminary structural analysis of the unit shows that most of the Haro Fm. folds have axial planes that strike NW-SE and tilt to the NE. These fold orientations agree with overall shortening directions associated with the SJTS, however are not highly constrained. Paleomagnetic data shows evidence for remagnetization of the Haro Fm., having failed a fold-test, indicating post-fold magnetization. In order to better understand the origin of this remagnetization rock-magnetic experiments have been conducted. Hysteresis parameters from whole-rock samples are somewhat puzzling, as samples with well-defined paleomagnetic vectors have only very weak saturation magnetizations, making interpretations of the rock-magnetic data inconclusive. Production of magnetic concentrates, and possible SEM study, will be needed to further identify the carriers of the paleomagnetic signal.