Mexico City is geographically located in central-southern Mexico, in what is known as the Valley of Mexico, alternately known as the Valley of the Dead. Although it resides in a valley, the valley is situated in the high plateaus and has an average elevation of 2240 meters. Mexico City sits on the North American Plate (Ferrari). The location of Mexico City is on a subduction zone, where the Cocos tectonic plate is forced through plate movement beneath the denser North American plate. This results in shallow and deep earthquakes including the earthquake of 1912. Although the stress and subsequent rebound of locked plates can also lead to tsunami’s, Mexico City is too far inland to be affected by these large waves. However, plate tectonics do result in nearby volcanic eruptions (Johnson and Harrison).
The dispositional environment was constructed over the various eras resulting in the deposits of numerous different sediment types. The Jurassic Salt Layer was formed by a shallow-manne evaporate were shallow water evaporated then refilled and over time formed a thick salt layer. The presence of tidal flats in the Cretaceous period with sediment being deposited through the tidal pattern resulted in the interbedded limestone and shale. Coral reefs that existed during the Cretaceous period left behind formations that created limestone with honeycomb shaped holes. Deep marine carbonate from this time formed chalky deposits. The return of tidal flats in the Tertiary era resulted in additional deposits of interbedded shale as well as interbedded sandstone. Deep marine clastic systems in the Tertiary era resulted in additional shale deposits and this layer was followed by a fold then a layer from the Tertiary period of interbedded shale and sandstone formed from more tidal flats. Subsurface deposits from lakes, ancestral streams and rivers resulted in layers of quarternary rhyolite, sandstone and conglomerate. Similar deposits resulted in a top layer of granite.
The drawing on the next page shows the various layers making up the depositional environment of Mexico City (Figure 1).
Sources Cited
Ferrari, Luca, et al. "The dynamic history of the Trans-Mexican Volcanic Belt and the Mexico subduction zone." Tectonophysics 522 (2012): 122-149.
Johnson, C. A., and C. G. A. Harrison. "Neotectonics in central Mexico." Physics of the Earth and Planetary Interiors 64.2 (1990): 187-210.
