--> Soyatal Formation and Related Strata: Onset of Sedimentation in the Cretaceous Foreland-Basin System, Central Mexico

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Soyatal Formation and Related Strata: Onset of Sedimentation in the Cretaceous Foreland-Basin System, Central Mexico

Abstract

Turbidites of the Soyatal Formation and broadly equivalent units represent the beginning of sedimentation in the Cretaceous-Paleogene foreland basin system in central Mexico. The Soyatal is well exposed in the Zimapan Basin, where we identify a transitional contact with the underlying Upper Tamaulipas Formation, which represents basinal deposits equivalent to carbonate platforms (pre-foreland basin deposits) developed during the Early Cretaceous in eastern Mexico. Maximum depositional ages (MDAs) from U-Pb detrital zircon analyses obtained near the base of the Soyatal Formation are correlated with MDAs obtained from exposures of foreland-basin units exposed to the south and to the north. Geographic variation is evident in MDAs of the oldest turbidites. Western deposits of the foreland basin, corresponding to the informal sandstone at Mineral de Pozos exposed 280 km to the NW of Mexico City, consists of sediment-gravity deposits with sandstone olistoliths and angular carbonate and volcanic lithic grains, suggesting a nearby source. This sandstone yielded a weighted mean MDA of 96 ± 1 Ma (MSWD = 1.09, n = 8). Eastward in the basin, a broadly equivalent unit exposed 85 km to the SE, near of Vizarrón, named the Soyatal Formation, consists of sandstone-poor synorogenic turbidites. The stratigraphically lowest sandstone of the Soyatal, collected ca. 15 m above the transitional contact with the Upper Tamaulipas, yielded a weighted mean MDA of 92 ± 3 Ma (MSWD = 1.5, n = 4). A second sample somewhat higher in the section yielded a weighted mean MDA of 93 ± 1 Ma (MSWD = 0.92, n = 3). Eighty km SE of this site, near Tolantongo, an analyzed sandstone contains a dominant population (∼90% of total) of Early Cretaceous grains (100-81 Ma) with an MDA of 82 ± 1 Ma (MSWD = 1.04, n = 6). We interpret the observed MDAs to indicate diachronous onset of sedimentation and eastward migration of the foreland basin from Cenomanian to early Campanian time. In addition, the presence of 50% or more zircons with ages of 100-80 Ma suggests that much of the sediment was derived from a contemporary magmatic arc represented by the “La Posta-type” plutons (98-92 Ma) present along the western margin of Mexico. The active arc indicates that retroarc shortening and basin subsidence slightly postdated initial subduction of the Farallon plate, which was already established in the Cenomanian, slightly before the first deposits recorded in the Mexican foreland basin.