PALEOGEOGRAPHIC IMPLICATIONS OF SIMILAR DETRITAL ZIRCON SIGNATURES IN MOSTLY JURASSIC AND CRETACEOUS STRATA FROM IDAHO TO SOUTHERN BAJA CALIFORNIA

The terrigenous strata of the Late Jurassic Galice Formation have long been interpreted to have been derived from erosion of older Klamath Mountain terranes following Middle Jurassic orogenesis. New detrital zircon age data are not in agreement with this model. Of the 165 analyzed grains 60% are older than 245 Ma. Of these grains about 45% are Late Paleozoic through Neoproterozoic in age with the remainder having significant populations of Grenville age grains and 1.4 to 1.8 Ga grains. These data are essentially indistinguishable from the Tithonian part of the Great Valley Group (GVG) collected from the Coast Ranges of northern California as well as from two well rounded and undeformed boulders collected from a Tithonian conglomerate of the GVG. They are also similar to Late Jurassic strata of the Blue Mountains province (Lonsome Formation of the Izee terrane). Older Klamath Mountain terranes are an unlikely source for the detrital zircon age distribution of the Galice and GVG, and local sources are also ruled out for the Lonsome formation. The detrital zircon signature of these now widely separated units are, however, remarkably similar to modern Colorado River sands, as well as the Late Triassic-Jurassic flysch belt in the Peninsula Ranges of Baja California and the Jurassic and Early Cretaceous Coloradito and Eugenia Formations. Some differences do exist, however. For example, the southernmost samples from the flysch belt and the Eugenia and Coloradito Formations of Baja California contain a significantly larger Permian detrital component. Sources of this age zircon are present in mainland Mexico. All of the units under discussion have Paleozoic and older detrital ages that are similar to the erg deposits of the Colorado plateau which are a likely source for this age component. However, published data for the ergs are distinct in terms of some ages and percentages of age components suggesting to us that there may also be a significant Gondwana derived component in our samples. These data suggest to us that the Late Jurassic Galice,Coast Range Great Valley and Izee terranes were located substantially farther south during stratal accumulation and subsequently moved northward by strike-slip faulting related to the MSNI fault system proposed by Wyld and Wright (2005).