U-PB DATING OF DETRITAL ZIRCON FROM TURBIDITES OF THE CHUGACH AND PRINCE WILLIAM TERRANES IN PRINCE WILLIAM SOUND, ALASKA: IMPLICATIONS FOR THE SIGNIFICANCE OF THE CONTACT FAULT SYSTEM AS A TERRANE BOUNDARY

The Contact fault system has traditionally been viewed as the terrane boundary between the Chugach and Prince William terranes in southern Alaska and is thought to divide the Campanian-Maastrichtian Valdez Group from the Paleocene-Eocene Orca Group in an arcuate trace across Prince William Sound (PWS), Alaska. The purpose of this study is to determine whether the Contact fault functions as a terrane boundary in PWS by analyzing the affinity of strata on either side of fault segments. Elements of the Contact fault system have different names across PWS. In NE PWS the low-angle Landlock fault merges with the high-angle Jack Bay fault, defining the Landlock Block. Early maps included the Landlock block in the Valdez Group, but current maps show it as part of the Orca Group and align the Contact fault with the Jack Bay fault. In N PWS the contact is the Eaglek fault, which cuts a 41 Ma pluton of the Eshamy Suite. In W PWS the Contact fault is less defined and mapped as the Culross fault, which passes though Kings Bay. To the SW near Seward the contact had originally been mapped east of the Resurrection ophiolite, however recent work has shifted it farther west due to the presence of Orca Group rocks on both sides of the ophiolite. Detrital zircons were analyzed from sandstone samples collected from either side of the Jack Bay, Landlock, and Eaglet faults between Valdez Arm and Seward. U-Pb ratios were measured at the Arizona Laserchron Center using LA-ICP-MS. Grain-age distributions and maximum depositional ages (MDA) of rocks in the Landlock block suggests that it has a similar provenance to rocks of the Orca Group farther east in PWS on Hinchinbrook Island. The MDA of rocks on either side of the Jack Bay fault in Jack Bay show that it is not the terrane boundary. In Unakwik Inlet, the Eaglek fault separates older Orca Group rocks from typical Valdez Group, but to the SW the Culross fault in Kings Bay does not because typical Valdez rocks are east of the fault. Overall, the data suggest that the Contact fault system as currently mapped in PWS may be a series of late strike slip and dip slip faults instead of a major tectonic boundary. A critical component in future analyses will be documentation of the stratigraphic relationship between the turbidites of the Valdez and Orca Groups because there may not be a terrane boundary between the two.