LATE CRETACEOUS NON-MARINE SEDIMENTATION IN THE CUTZAMALA BASIN, CENTRAL GUERRERO COMPOSITE TERRANE OF WESTERN MEXICO: TECTONIC IMPLICATIONS

In Guerrero composite terrane, in the limits of Michoacán and Guerrero State, there is a large redbed basin (the Cutzamala Basin), which minimum extension is 150 x 120 km and its basin-fill is 3,500 m in thickness. The Cutzamala basin is important because it constraints the age of accretion and records the subsequent evolution of this portion of the Guerrero terrane. The Guerrero composite terrane is defined by large volumes of Upper Jurassic-Lower Cretaceous arc volcanic-sedimentary rocks. Its basement is very heterogeneous, formed by previously accreted Paleozoic deep-marine rocks and Triassic metamorphosed oceanic complex, and Middle Jurassic granitoids. The Cutzamala Basin developed on the previously deformed Upper Jurassic-Lower Cretaceous, and corresponds to an overlapping assemblage. The stratigraphy of the Cutzamala Basin is made up of a basal conglomerate, and a thick succession of thin-bedded to massive sandstone and siltstone with some conglomerate. It changes upward to volcanic and volcaniclastic rocks that contain dinosaur bones. U/Pb isotopic dating of the volcanic rocks yielded a Santonian age (84 Ma). The volcanic-volcaniclastic succession is covered by a second succession of sandstone and siltstone and conglomerate. Detailed facies analysis indicates a transition from alluvial/fluvial sedimentary environments to flood plain deposits and paleosols. They are preliminarily interpreted as filling of intermontane basins. The Santonian volcanic flows are interbedded with fluvial volcaniclastics, and mark a change from intermontane to arc-related basin. The arc probably migrated and the basin continued with subsidence that originated a second event of deposition in fluvial-flood plain conditions. It was previously interpreted that the arc was placed against nuclear Mexico during the Laramide orogeny in Late Cretaceous to Paleocene. However the discovery of the Cutzamala basin indicates that the collision of the Guerrero Composite terrane occurred earlier. The basin was deformed by a major strike slip fault and was exhumed after the Late Eocene as suggested by the age of the dikes that cut the redbeds. The redbeds of the Cutzamala Basin were already eroded and cut by a fluvial system previous the development of the Mexican Volcanic Belt, which products are filling up some of the incise rivers.