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Isabel Israde-Alcantara, Universidad Michoacana de San Nicolas de Hidalgo (Mexico)
Garduño Victor Hugo, Universidad Michoacana de San Nicolas de Hidalgo (Mexico)
Miller Wade, Department of Geology (United States)
Rodriguez-Pascua Miguel, Instituto Geológico y Minero de España. (Spain)
Barron John, US Geological Survey (United States)
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An initial phase of lacustrine sedimentation developed in west central Mexico during the late Miocene after 11-7 Ma. This is in response to tectonic extension associated with the initial emplacement of the late Miocene Intermediate sequence that constitutes the substrata of the Trans Mexican Volcanic Belt. Lacustrine diatomites of this period contain the genus Mesodictyon, which is also documented in upper Miocene deposits in the western United States, Peru, France, Europe and North Africa. One new species of biostratigraphic interest is Tertiariopsis (Khursevich and Kociolek) bas. Thalassiossira cuitzeonensis found in upper Miocene deposits of the Cuitzeo Basin, north Michoacán, Mexico. Climatic conditions in west central Mexico during this interval were relatively warm and humid based on the widespread distribution of lacustrine deposits; however some were shallow lakes.
Following a latest Miocene (8.0 -5.2 Ma) stage of arid conditions and greatly reduced deposition of fine-grained lacustrine sediments, a period of extensive, relatively deep, perennial lakes extending for more than 250 km in an east-west direction marked the early Pliocene period between 5.2 and 4.0 Ma. Lower Pliocene diatomites found in the Chapala region contain the same diatom species of Stephanodiscus carconensis and Tertiarius aff. baikalensis found in rocks of this age in the western United States. The relatively warm and humid conditions that characterized this interval in central Mexico coincide with a period of high-latitude warming, higher global sea level, and a reduction in size of the Antarctic Ice Sheets. Restriction of the Central American Seaway during the latest Miocene may have also acted to increase precipitation in central Mexico during the early Pliocene. Pliocene mammalian faunas from central Mexico, Guanajuato, Jalisco and southernmost Baja California sur, also support a savanna setting with moist and warm conditions.
Shallow lakes and fluvial deposits dominate after 4.0 Ma, and vertebrate remains of La Goleta Michoacan and Rancho Viejo in Guanajuato document a period of dispersal of vertebrate faunas, both from North and South America. A combination of reduced precipitation and volcanic and tectonic processes are presumed to have been the cause for this mid Pliocene reduction in lake size in central Mexico. Early Pliocene is also a period of great tectonic activity in central Mexico producing deformation of the lacustrine successions that are well represented in Acambay, Patzcuaro, Cuitzeo, Zacapu, Chapala and Ixtlahuaca paleolakes. Deformation structures ranges from Slumping, faulting and folding generated unconformities, that record different sedimentation episodes in the basins, all those are cut by normal or inverse faulting that also interest lacustrine sediments and soils with vertebrate and lithic activity remains.
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