Santonian-Maastrichtian Arc Related Basins: Age Constraints on Deformation in Western Mexico, Colima and Michoacán States

Western Mexico is mostly formed by Jurassic-Early to mid Cretaceous arc related strata that form the Guerrero Composite Terrane. These rocks were deformed and partially metamorphosed sometime after the Cenomanian, which is the youngest age of marine sedimentary rocks in the region. Mapping and zircon geochronology of redbeds that unconformably overlain the deformed marine successions indicate that a mayor compressional event occurred prior to Santonian to Maastrichtian time. Redbeds of the Cutzamala Basin are 3,000 km thick. They are exposed in Tierra Caliente region consist of basal conglomerate made up of clast derived from the basal Arteaga Complex and Early Cretaceous marine volcanic rocks; a thick paleosol succession and volcanic and volcaniclastic rocks that yielded a 84 Ma zircon age. This unit contains fossil dinosaur bones. Volcanic and volcaniclastic redbeds at the Coalcoman region were previously interpreted to underlie Early Cretaceous limestone. However, our new detrital zircon analyses show a cluster at 84.7 Ma (Santonian) and at 70.6 Ma (Campanian-Maastrichtian boundary). At the Colima Region a > 500 m thick redbed section of sandstones, conglomerate and siltstone, with silicic lava flows lies unconformably on Aptian-Cenomanian limestone. The lava flows have K/Ar ages of 80 to 78 Ma The presence of silicic lava flows indicates proximity to vent regions.

These units are similar to coeval continental arc rocks of Sonora State and form part of a continental margin arc that was built across all of the accreted terranes of western Mexico, constraining the age of mayor deformation. The arc-related rocks Santonian-Maastrichtian are gently folded and strike-slip faulted at the Cutzamala Basin, suggesting a post-Maastrichtian transpressional event.