Origin and geologic characteristics of the paleo-Lake of Amajac, Hidalgo
The sedimentary lithofacies and the fossils reported in this study belong to the continental clastic sequence of the Atotonilco El Grande Formation, which crops out in the Santa Maria Amajac town, in the Hidalgo state. This sequence, named in this study as the paleo-Lake of Amajac, was accumulated in a high-altitude lacustrine environment. The origin of this paleolake was caused by the closing of a canyon over 1,000 m deep, with sub-vertical walls, carved into the carbonate platform rocks from El Abra Formation. The canyon walls were particularly unstable north of the Doñana town. This instability was caused by an intense fracturing, the steeply dipping walls and the presence of water within fractures. As a consequence of these conditions, large volumes of rocky material of varying sizes detached from the canyon, forming a talus-deposit at the base of the canyon. This deposit, which was over 250 m thick, caused the temporal damming of the Amajac River and the formation of a fresh-water lake covering approximately 85 km2 at times of maximum ﬂooding. Immediately after the closing of the canyon, lacustrine sedimentation began and conglomeratic, sandy, clayey-silt and clayey lithofacies accumulated in the basin, with more than 150 m of detrital sediments and pyroclastic material of varying granulometry.
The coarse-grained sediments are found at the base of the sequence and in channels, while the ﬁne-grained sediments have a broader distribution and have a great amount of fossil ﬂora and fauna of freshwater environment. The ﬂora belongs to the families Salicaceae, Platanaceae, Fagaceae, Rosaceae and Equisetaceae; whereas the fauna consists of gastropods from the genera Planorbis, ostracods, small ﬁshes, vertebrae rests and some insects belonging to the order Coleoptera. On the basis of stratigraphic and paleontological records, it is considered that the Atotonilco El Grande Formation has a Pliocene age (Blancan). The lacustrine sedimentation ended when the river eroded the barrier that originated the lake.
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