EARLY PLEISTOCENE LACUSTRINE SEQUENCES FROM THE VALSEQUILLO BASIN, CENTRAL MEXICO

Early Pleistocene palaeoenvironmental reconstructions are presented using stable isotopes, diatoms, tephras and pollen from lake sections, containing the 1.3 Ma Xalnene Ash as a stratigraphic marker. The sediments and diatoms suggest that shallow water lakes occurred and that soils became increasingly abundant. At the sites we studied, Malde’s (1968) suggested “deep lake sediments” were derived from reworked volcanic deposits, with localised lakes filled with calcareous sediments.

From the stable isotope results (Rio Atepezingo) there is evidence for a lake pre-Xalnene ash, although probably neither large, nor deep. After the Xalnene ash there is a change in the hydrologic regime with no evidence of an extensive lake at the locations studied, although there were pools present, with much sediment reworking and volcanic ash deposition.

Trace amounts of organic material found at the base of the Xalnene Ash itself suggest that the organic material came from both terrestrial and wetland plants. Sparse pollen from the base of the ash, indicate terrestrial plants, including pine and woody herbs. Although the basin has been generally volcanically dynamic, groundwater levels were high at times which allowed spring-fed, small lakes to form in topographical lows. The Valsequillo Basin was more humid than today, immediately before and after the Xalnene Ash eruption and was able to support a large community of Quaternary fauna.

The Xalnene Ash was formed by a subaqueous, hydromagmatic maar, basaltic eruption from Cerro Toluquilla. The initial volcanic eruption was explosive and wholly subaqueous but subsequently the eruption built a subaerial volcanic cone, overlain by basaltic lava. The ash is at least 5.4 m thick close to the cone, but thins to 10 cm within 4 km northwest of the vent, although the thickness is controlled by the irregular basin floor topography over which it was deposited, both as subaerial ashfalls and subaqueous density flows. Xalnene Ash consists predominantly of graded coarse lapilli ash, interbedded with fine ash, accretionary lapilli, and cross-bedded, base surge deposits close to the volcano. This ash divides the lake sediments in the Valsequillo Basin into lower and upper units, with conformable contacts between the ash and the lake sediments.