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Washington and Lee University

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Rose Pettiette '13 reconstructs Alaskan terranes

The southern margin of Alaska is defined by a late Mesozoic to Cenozoic accretionary complex that comprises the Upper Cretaceous to Eocene Chugach-Prince William (CPW) terrane. The Contact Fault serves as the current boundary between the Upper Cretaceous Valdez Group of the Chugach terrane and the Paleocene to Eocene Orca Group of the Prince William terrane. However, sandstones of the Campaniam-Maastrichtian Valdez Group and the Paleocene Orca Group consist of compositionally indistinguishable feldspathic and volcanic-lithic sandstones thereby hindering the identification and fault bounding the two terranes.

During the Paleocene to early Eocene, ridge subduction led to the formation and emplacement of the Resurrection and Knight Island ophiolites and subsequent intrusion of the Sanak-Baranof belt (SBB) plutons along the 2200 km length of the CPW accretionary complex. Controversy surrounds the age of the ophiolites and the age and stratigraphic relationship of adjacent clastic strata. On the northeastern side of Resurrection Peninsula, a thrust fault is mapped between the ophiolite and Upper Cretaceous Valdez Group. However, on the western side, there is controversy about the stratigraphic affinity of clastic strata . The crux of the issue being whether the strata are Cretaceous Valdez Group and fault bounded bearing no direct relationship to the ophiolite, or whether they are Paleocene Orca Group and essentially in stratigraphic continuity with the ophiolite.

To distinguish between the Chugach and Prince William terrane and determine the age, provenance and stratigraphic affinity of the clastic rocks interbedded with (and stratigraphically above) the Resurrection Peninsula Ophiolite, U/Pb detrital zircon dates were collected from four samples (n=404) within the Resurrection Bay area. The grain age distributions from Resurrection Bay were compared with previous U/Pb detrital zircon ages obtained from strata of the Chugach terrane and Prince William terrane. All samples from Resurrection Bay were correlative to the Orca Group in the Prince William terrane that indicates the ophiolite is in depositional continuity and part of the CPW accretionary complex. These results validate previously collected paleomagnetic data obtained from the Resurrection Peninsula ophiolite that indicate a paleolatitude 13 ± 9° south of the present location to near present day northern Washington. Together, these results support large coast parallel transport of the CPW terrane since the Paleocene and the search for the original source of the clastic rocks may include terrains to the south.