35 The Mesozoic Convergent Margin of Alta and Baja California (Posters)

This session will cover aspects including the structure, petrology, geochronology, and stratigraphy of the Mesozoic convergent margin of North America from Alta California to Baja California Sur.

Friday, 24 April 2026
8:00 AM - 5:30 PM
LMH, 5th Floor Chapel

Detrital Zircon Provenance of the Vizcaino Peninsula Jura-Cretaceous Forearc Basin, Baja California, Mexico

Paper number: 35-6
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Eugenia and Valle Groups on the Vizcaino Peninsula and Cedros Island in west-central Baja California comprise a >12 km-thick Middle Jurassic through Eocene forearc volcanic-sedimentary succession deposited atop Triassic-Jurassic suprasubduction zone ophiolitic-volcanic arc basement. The forearc basin strata occur in mostly continuous, fossiliferous sections that represent the best subaerial sedimentary record preserved along the Baja and southern California margin. Eugenia-Valle strata have been broadly correlated to the Great Valley forearc sequences of northern California but there are significant differences. Here we report detrital zircon U-Pb ages from sandstone beds (n=25) throughout the Eugenia-Valle forearc succession together with zircon ages from intercalated tuff beds (n=25). The Eugenia Group as informally defined here comprises a fining upwards early Late Jurassic to late Albian age sequence encompassing the Eugenia Fm (proximal volcanic arc), Perforada Fm (deep marine) and Los Chapunes Fm (basin plain shale) succession. The age of the Eugenia Fm ranges down to at least 157 Ma based on tuff ages and the age of a cross-cutting volcanic plug. Detrital zircon age distributions define a zircon petrofacies through this sequence characterized by an abundance of Jurassic zircon with significant pre-Cordilleran (>285 Ma) grains. This provenance signature is like that exhibited by the basal Great Valley and other Jurassic Cordilleran strata distributed from NE Oregon to the Vizcaino region. The >285 Ma zircon age distribution is well-modeled by Permo-Triassic Colorado Plateau strata that contain abundant Appalachian/Gondwana-derived Paleozoic (300-480 Ma), Pan African (531-641 Ma) and Grenville (950-1335 Ma) zircon. Jurassic zircon was contributed by Early to Middle Jurassic San Andrés-Cedros arc rocks in the Vizcaino region and the western zone of the southernmost Peninsular Ranges. Dispersal of this Appalachian/Gondwana-derived sediment to the Jurassic-Early Cretaceous Cordilleran forearc is consistent with a common, topographically subdued, west-facing continental arc system. The abrupt appearance of Valle Group sandstone and conglomerate represent arc-derived sediment that flooded the forearc during the Cenomanian. This major pulse of coarse-grained sediment coincided with emplacement and syn-batholithic exhumation of voluminous LaPosta-type tonalite-granodiorite(100-88 Ma) intrusions that dominate the eastern Peninsular Ranges batholith. In contrast, Early-Late Cretaceous strata of the California Great Valley forearc continued to be dominated by Jurassic zircon, consistent with slower denudation of late Cretaceous (100-80 Ma) plutons of the eastern Sierra Nevada batholith.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3

doi: 10.1130/abs/2026CD-411059

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