MAXIMUM DEPOSITIONAL AGE OF THE PALEOCENE TO EOCENE ORCA FLYSCH, PRINCE WILLIAM SOUND, ALASKA

The Chugach-Prince William (CPW) composite terrane is a Mesozoic-Tertiary accretionary complex exposed for ~2200 km in southern Alaska, which is principally composed of deep-water turbidites with abundant quartzofeldspathic and volcanic-lithic sandstones and interleaved basaltic rocks and pillows of the Valdez and Orca Groups. The Paleocene to Eocene flysch of the Orca Group is interpreted to have been deposited in a trench adjacent to a trench-ridge-trench triple junction during subduction of the Kula-Farallon or Kula-Resurrection spreading center. The thickness and compositional homogeneity of the Orca flysch is remarkable, and most of this unit appears to have been deposited in the Paleocene based on sparse fossils and dates of cross-cutting plutonic rocks. However, the age of the youngest (outboard) parts of the Orca flysch are largely unconstrained.

In this contribution we report U/Pb detrital zircon ages from two transects across the Orca flysch in eastern and western Prince William Sound, Alaska. Combined preliminary results from five locations (n = 486 zircons) have an age distribution with prominent grain-age populations at 73, 106, 188, and 365 Ma. Forty-two zircons from the preliminary data set are Precambrian, with modes at 1164, 1826, 1912, and 1988 Ma. The youngest 10 zircons analyzed range from 55 to 59 Ma; thus, maximum depositional ages from these preliminary results suggest that at least some of the flysch of the Orca Group was deposited in the Eocene, and that deposition immediately preceded intrusion of the rocks by near-trench plutons of the Sanak-Baranof belt. In fact, these data almost require that flysch of the Orca group covered and essentially smothered the ridge that was subducted to form the near-trench plutons. Detrital zircon ages and field relations suggest deposition of some flysch of the Orca Group was contemporaneous with formation of the Resurrection and Knight Island ophiolites. Additional samples in process include sandstone from the farthest outboard exposures of the Orca flysch (Montague Island) and farthest inboard (Contact fault), which will constrain the full depositional age range of the flysch exposed in Prince William Sound.