A CA. 220 KYR STRATIGRAPHIC SEQUENCE FROM LAKE CHALCO, CENTRAL MEXICO

Drill cores obtained from Chalco Lake in central Mexico (19° 15' N, 98° 58' W, 2200 m asl) to a depth of 122 m, potentially contain a ~ 220 kyr record of terrestrial climate history in the northern American tropics. The active tectonic and volcanic setting of Chalco Lake in the Trans-Mexican Volcanic Belt, provides an opportunity to evaluate environmental (volcanic + tectonic vs. climatic) controls on lacustrine sedimentation. The deep cores sequence is currently under analysis of diatoms, pollen, ostracods, magnetic properties and organic and inorganic geochemistry. The stratigraphical and sedimentological analyses presented in this work provided the initial recognition of depositional environments. According to compositional criteria, 12 sedimentary facies were identified and groped into three main categories: 1) detrital and volcaniclastic, 2) biogenic and 3) carbonate-rich facies. 1) The clastic facies includes massive to laminated, silty and clayey sediments composed of clay minerals, feldspars, amphiboles, quartz, and opaques. Low organic content includes diatoms, amorphous organic matter, charcoal, ostracods and phytolites. Diatoms are the most common biological remains in all the clastic facies. Nearly 100 individual volcaniclastic layers of 1-50 cm thickness are present, which account for < 3% of the total sequence thickness, some of them of well documented eruptions of the nearby large stratovolcanoes Popocatepetl and Nevado de Toluca. 2) The biogenic facies are massive to finely laminated diatom ooze and ostracod ooze layers. 3) Carbonate-rich facies occurs as finely laminated to massive micritic mud composed of calcite, with minor amounts of dolomite and siderite. The Chalco Lake sequence has been divided into six main sedimentary units, according to their sedimentary facies. Unit 6 (122-108 m depth): finely laminated diatom ooze and micritic mud facies. Unit 5 (108-90 m): olive brown massive silt with diatoms facies. Unit 4 (90-68 m): reddish brown silt is the dominant facies. Unit 3 (68-40 m): grayish brown silt and gray silty clay facies. Unit 2 (40-7 m): yellowish brown silt facies and ostracod ooze facies. Unit 1 (7-0 m): mainly of organic-rich silty clay, diatom ooze facies.