Palaeomagnetic and palaeoenvironmental studies in the southern basin of Mexico - I. Volcanosedimentary sequence and basin structure of Chalco lake

Abstract

Results of stratigraphic and radiometric dating of the first twelve meters of the lacustrine sedimentary and volcanic sequence from Lake Chalco, southern Basin of Mexico are reported. The study is based on four cores recovered from the central part of Chalco. Bouguer gravity models indicate that Chalco basin is divided by a N-S structural high associated with the Xico volcano into two sub-basins with different sedimentary thickness. Maximum thickness is some 400 min the east basin and 300m in the west basin. The long-term project includes pollen, diatomes, palaeomagnetic and limnological studies and aims at a palaeoenvironmental reconstruction for the late Pleistocene and Holocene. The 12m lacustrine sequence can be divided into five major units. Ten tephra layers are present in the sequence, indicating an intense volcanic activity in the region. Age control is provided by eleven radiocarbon determinations, with about 19,000 yr BP at 9 m depth. A change in the sequence is present at about 13,000 yr BP, indicated by the radiocarbon dates and the occurrence of a tephra layer followed by a thick diatomite horizon. Average bulk sedimentation rates vary from about 0.28 mm/yr in the upper 3.5 m to about 0.80 mm/yr in the subsequent deeper section. Organic carbon variations suggest that between 3 and 8 m, the sequence corresponds mainly to a shallow lake environment with high accumulation and preservation of organic matter.