Paleontologic data on the Age of the Orca Group, Alaska Report 85

TitlePaleontologic data on the Age of the Orca Group, Alaska Report 85
Publication TypeReport
Year of Publication1985
AuthorsPlafker, G, Keller, G, Barron, JD, ,
Series TitleUnited States Department of the Interior, Geological Survey
Document Number85-429

The Orca Group is a widespread, very thick, and complexly deformed accretionary sequence of flysch and tholeiitic basalt in the Prince William Sound area of Alaska (Winkler, 1976; Winkler and Plafker, 1981). Despite a number of extensive field studies of the Orca Group, reliable data on the age of the unit has been exceedingly elusive. On the basis of sparse paleontologic and radiometric data, the sequence was assigned a Pal eocene and early Eocene(?) age (Addicott and Plafker, 1971; Plafker and Lanphere 1974; Winkler, 1976, Winkler and Plafker, 1981). New paleontologic data suggest that some strata assigned to the Orca Group are of middle Eocene age and possibly as young as late Eocene or Oligocene. However, data suggesting an age younger than about 50 Ma appear to be incompatible with radiometricanydetermined ages for plutons that intrude the Orca. This paper summarizes the published and unpublished paleontologic data from the Orca Group, considers their implications for the age of the unit, and points out the problems in age assignments. PDF