Geochemistry of late Quaternary tephra-sediment sequence from north-eastern Basin of Mexico (Mexico): implications to tephrochronology, chemical weathering and provenance
A ca.30 m thick tephra-sediment sequence from the north-eastern Basin of Mexico (Pachuca subbasin, central Mexico) is investigated for Stratigraphy and multi-element geochemistry to understand the tephrochronology, provenance and conditions of chemical weathering during Late Quaternary. Chemical composition of tephra layers are compared with products from surrounding volcanic structures (ApanTezontepece, Acoculco, Huichapan, Sierra de las Cruces and Tláloc) in order to identify their sources. Basal to basaltic-andesite tephra layers (Tr 1, Tr 6 and Tr 7) show similar composition with rocks from Apan-Tezontepec monogenetic volcanic field aged between 1.50 and 0.47 Ma. Felsic Tephra layers have composition comparable to Acoculco volcanic sequence. The dacitic ash (Tr 2) and rhyolitic ash and pumice fall deposits (Tr 3, Tr 4, Tr 5 and Tr 8) might be representatives of different Plinian eruptive events at Acoculco caldera and possibly occurred during <1.50 Ma and >0.24 Ma. Ternary diagrams (A-CN-K, A-C-N and A-CNK-FM) and trace element concentrations suggest that some of the sediment layers are formed from the chemical weathering of the underlying tephra and the rest of the inter-bedded sediments are derived from the erosion of rocks exposed at the Pachuca volcanic range. Indices of chemical weathering such as CIA and PIA indicate varying degrees of chemical alteration for these sediments possibly related to fluctuating humidity and duration of exposure to different weathering agents during their transportation from the hinterland to the sub-basin.
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