NEOGENE DIATOMS OF CUITZEO LAKE, CENTRAL SECTOR OF THE TRANS-MEXICAN VOLCANIC BELT AND THEIR RELATIONSHIP WITH THE VOLCANO-TECTONIC EVOLUTION

Lake Cuitzeo, within the central sector of the Trans-Mexican Volcanic Belt, forms part of a series of palaeolakes of tectonic origin, limite d by E-W or NE-SW trending major faults. The lake basin itself is subd ivided into half-grabens, where the blocks are tilted southward; this tectonic disposition results in a series of sedimentary environments s uch as terrigenous facies (near step faults) that have an influence on the lake hydrology and, therefore, on sedimentary and biogenic facies associations. In outcrops of palaeolake Cuitzeo, the deposits are for med by three members that correspond to three well marked cycles of se dimentation. The basal, predominantly formed by clays, a diatom-rich i ntermediate one and an upper one, formed by silts and pyroclastic depo sits that close the limnic cycle. Two interruptions are observed withi n the lacustrine development, the first associated with an oxidized ho rizon and the second with an erosional conglomerate of tectonic origin . A marker diatom species for the Miocene-Pliocene boundary (Thalassio sira cuitzeonensis nov. sp.) was found that enables differentiation of the most ancient sequences from the most recent ones. (C) 1997 INQUA/ Elsevier Science Ltd.