Abstract

CARRASCO-VELAZQUEZ, Baldomero E.; MARTINEZ-HERNANDEZ, Enrique  and  RAMIREZ-ARRIAGA, Elia. Stratigraphy of the Paleocene-Eocene El Morro Formation at Zimapán, Hidalgo. Bol. Soc. Geol. Mex [online]. 2009, vol.61, n.3, pp. 403-417. ISSN 1405-3322.

The Cretaceous Zimapan basin was affected by compressive Laramide deformation, which gave place to tectonic uplift in the area. After regional uplift of the rocks, erosion began to fll several continental intermontane basins with an orientation parallel to the alignment of the major folded structures. The "El Morro Formation" represents the alluvial and fluvial flling of the intermontane basins. El Morro Formation type locality has a stratigraphic thickness of 64 meters and is composed of four lithofacies with different source rocks. The earliest stratigraphic work in Zimapan area described a stratigraphic thickness that varies from several meters to 400 meters with drastic lateral changes. Sedimentation of the El Morro Formation was interrupted by the accumulation of a succession of andesitic breccias of Las Espinas Formation. Both formations are locally interfingered by a few meters and the top of El Morro Formation includes andesitic clasts, which gave a K/Ar age of 38.1±1.4 Ma. An age of 62 Ma was obtained in diagenetic illite from the Jiliapan-El Volantin thrust fault, located 25 km north of Zimapan, which constrains the end of active compressive deformation to the earliest Paleocene. The age of the El Morro Formation ranges from at least the middle Paleocene to middle Eocene by the K/Ar isotopic age of the overlying volcanic rocks and the diagenetic illite formed during the movement of the Jiliapan-El Volantin thrust faults. The palynological assemblages indicate a tropical climate with abundant precipitation and confrm the fluvial regimen that prevailed during the deposition of El Morro Formation.

Keywords : Sedimentary rocks; middle Paleocene-middle Eocene; palynology; Zimapan.