Late Cretaceous basement foundering of the Rosario embayment, Peninsular Ranges forearc basin: backstripping of the El Gallo Formation, Baja California Norte, Mexico

Excellent exposure, well-controlled palaeobathymetry, and tightly-spaced, high-precision radiometric age control in the El Gallo Fm. permit rigorous quantitative analysis. Backstripping of these proximal nonmarine, forearc basin deposits reveals that, during the Late Cretaceous, the Rosario embayment of the Peninsular Ranges forearc was undergoing an episode of rapid tectonic subsidence. This subsidence had several marked effects on the sedimentology of the Rosario embayment: formation of a broad alluvial plain consisting of coarse-grained clastics; rapid (～ 600 m Myr^-1) aggradation of sediments; and a retrogradational succession of facies, capped by a marine transgression, as deposition failed to keep pace with eustatic rise and subsidence.
Long-term sedimentation is driven by some combination of two allocyclic mechanisms: tectonic subsidence and eustatic sea-level rise. In order to evaluate which force predominated during deposition of the El Gallo Fm., the processes of sedimentation, compaction, and isostasy are evaluated through the interval in question. A sensitivity analysis is performed, in which the maximum tectonic and maximum eustatic contributions are estimated, along with the best-fit model. These results are qualitatively the same: tectonic subsidence was the major driving force of sedimentation in the Rosario embayment in late Campanian time. Regional sedimentological similarities suggest that this tectonic subsidence may have characterized the Peninsular Ranges forearc margin at this time, reflecting an episode of active down-faulting during the Late Cretaceous.