THE FIRST QUATERNARY ALLOFORMATION FROM MEXICO

In spite of the potential usefulness of allostratigraphic classification to work out the geologic evolution in sedimentary areas of complex sedimentary architecture [including deposits of diverse lithology standing side by side, and/or superimposed, or discontinuous deposits of similar lithology], it is on the whole little used, particularly so in Mexico, where to the best of our knowledge, it has only been twice applied [in the Jurassic and Tertiary of eastern and northwestern Mexico respectively]. Its application in the Quaternary would aid to understand the influence of climate change on the sedimentary processes and stratigraphic record, and through careful characterization and correlation of discontinuity surfaces and allostratigraphic units across the country, could help to portray the Pleistocene-Holocene climate change impact in Mexico, whose academic and economic importance need not be stressed.

As a start in this direction, we describe a Quaternary alloformation from central San Luis Potosí, within the Sierra Madre Oriental Morphotectonic Province, between 22^o10’-22^o22’ N Lat. and 100^o28’-100^o40’ W Long. The Cenozoic sequence is preserved in the Peotillos Graben [a major N-S structure in western SMOr bound by horsts of Cretaceous carbonate units]. The pre-Quaternary includes Oligocene volcanics and a Late Miocene fluviolacustrine sequence. The Quaternary includes seven informal sedimentary lithostratigraphic units [namely: Puerta del Refugio, Camposanto, Los Saldaña, El Jaguey, La Concordia, and one unnamed], and three volcanic ones [Los Palau, Las Joyas (dated at 0.6 Ma), and Pozo del Carmen]. The sedimentary units were defined on the basis of gross compositional/textural features, and are laterally contiguous through gradational to arbitrary contacts, thus being of hard field recognition; however, all but the last two rest on the Tertiary and/or Cretaceous units through an angular and erosional unconformity. We feel that such units could constitute a single alloformation [tentatively designated Armadillo de los Infante Alloformation, because it largely lies within the namesake municipaliy], where the lithostratigraphic units would chiefly correspond to particular facies associations, seemingly controlled by source-area sedimentary input, hydraulic factors and climate.