FOREARC BASIN EVOLUTION OF THE LATE JURASSIC-EARLY CRETACEOUS EUGENIA FORMATION; NEW MAPPING IN THE VIZCAINO PENINSULA, BAJA CALIFORNIA SUR

The Tithonian-Valanginian Eugenia Formation represents an enigmatic part of Vizcaino forearc stratigraphy. Previous workers have interpreted the Eugenia Formation as proximal deposits of the Sierra San Andres volcanic arc complex on the Vizcaino Peninsula. This interpretation fails to explain the presence of alkaline pillow lava interbedded in the Eugenia Formation indicating syndepositional rifting of the basin. Further, previously undocumented continentally derived quartzite clasts in conglomerate beds, which in the Eugenia range up to boulder size, have no obvious source on the Vizcaino Peninsula. U/Pb data from conglomerate igneous clasts show contamination of continentally derived zircons. The presence of these continentally derived materials conflicts with models that interpret the Vizcaino as part of an island arc terrane accreted to the North American margin in the Early Cretaceous. New mapping within the Eugenia block has refined basic structural and stratigraphic details. The basic structure is a highly faulted anticlinorium cored by Eugenia strata, flanked unconformably by the overlying Aptian-Albian Peforada Fm, and is cut by dense arrays of normal faults that extend into upper Valle Group strata. Coarse-grained Eugenia facies contain outsized volcanic blocks ranging up to 10m; new U-Pb dating and field relations suggest an ~200 x 150m volcanic block, mapped by earlier workers as an intrusive stock, is actually a volcanic olistolith block. Igneous clasts exhibit similar chemical trends with no sample segregation with respect to time progression of deposition. Additionally, U/Pb data of igneous clasts from the Eugenia and Valle formations have mean ages of 149.4 and 131.5Ma respectively. This suggests that the igneous clasts feeding this basin, from Eugenia through Valle time, were of a common source and were fed by this source for a period no less than 20Ma. Conglomerate clast counts show a progressive shift in the dominate conglomerate clast type from volcanic during Eugenia deposition to volcanic/plutonic during Valle deposition, perhaps reflecting erosional unroofing of the source arc. Finally, U-Pb ages of 156.4 ± 2.6 and 153.2 ± 3.8 Ma for Eugenia tuff horizons extend the age of the formation into the Kimmeridgian, and provide a maximum age constraint for conglomerate deposition.