ROCK MAGNETISM OF A 122 M LONG LACUSTRINE SEQUENCE IN CHALCO BASIN, CENTRAL MEXICO: PRELIMINARY RESULTS

The Chalco lake is part of the basin of Mexico, a high altitude tropical site that had an extensive lake system, sensitive to climatic fluctuations during the past times. A set of four cores up to 122 m depth were collected in the lacustrine sediments of Chalco sub-basin (19°15’ N, 98°59’ W, 2,250 m asl). Radiocarbon dates available from previous works on the upper 25 m of lake sediments (>20 ¹⁴C dates), indicate that -by a linear extrapolation- the 122 m record could encompass the last ca. 220-240 ka, considering a very rough mean sedimentation rate of ca. 0.54 mm/yr. The lacustrine sequence recovered would encompass at least the last glacial/interglacial cycle, providing unique information on Middle American climate. Lacustrine sediments are composed of massive brown to gray silt, banded and laminated diatom ooze, and volcaniclastic sediments.

Rock magnetism analyses will allow understanding the link between detrital input, biogenic productivity and diagenesis; and in turn these will allow us to establish the link between magnetic mineralogy and paleoclimate, and to validate the paleomagnetic variations recorded in Chalco sediments. Rock magnetism parameters characterize the magnetic components in terms of mineralogy, grain size distribution and concentration. The analyses include identification of magnetic mineral phases by Curie temperatures and FORC analyses, and measurement of magnetic susceptibility, NRM, ARM, IRM, hysteresis parameters Ms, Mr, Hc, Hcr; and the ratios S₃₀₀ and HIRM₃₀₀. In the active volcanic geological setting of Chalco sediments, most of primary magnetic minerals are Ti-magnetites/maghemites of detrital origin, with subordinate (Ti)-hematite. The preliminary results show a correspondence between rock magnetism parameters with the sedimentary facies. The main characteristics of magnetic mineralogy are:

Dark brown/organic rich facies: dissolution of Fe oxides and formation of Fe-sulphides.

Gray clayey silt facies: high coercivity (Ti)-magnetite + subordinated hematite/goethite.

Reddish brown silt: (Ti)-magnetite + hematite/goethite.

Yellowish brown calcareous mud: (Ti)-magnetite + siderite + Fe-sulfurs.

Olive brown clayey silt facies: low coercivity (Ti)-magnetite, and Fe- bearing sulfurs.