Abstract

ORTEGA GUERRERO, Beatriz; LOZANO GARCIA, Ma. Socorro; CABALLERO, Margarita  and  HERRERA HERNANDEZ, Dimitris A.. History of the depositional evolution of the Chalco lake, Mexico, since MIS 3. Bol. Soc. Geol. Mex [online]. 2015, vol.67, n.2, pp.185-201. ISSN 1405-3322.

Lacustrine sedimentary sequences are natural archives of past climatic and environmental changes. Lake level fluctuations in these sedimentary records are frequently associated with climatic changes. However, in active volcanic and tectonic settings, lacustrine sedimentation may be altered by these processes by affecting the place of deposition and controlling the volume and type of detrital material available for removal by surface processes. In consequence, it is necessary to analyze volcanic and tectonic processes in order to achieve a better understanding of climatic and environmental changes in the lacustrine sedimentary sequences in this type of environment. Additionally, volcanic material preserved in lacustrine sequences document the recurrence of volcanic activity in a region. Lake Chalco basin, located in the central part of the Transmexican Volcanic Belt, offers an ideal site to evaluate the impact of volcanic and tectonic activity on lacustrine sedimentation. In this work, we analyze the uppermost 26.7 m of the lacustrine sequence collected in seven cores drilled in different parts of Lake Chalco. In the obtained sequences, we determined the sedimentary facies succession, established their lateral distribution, and analyzed the sedimentary architecture and the impact of volcanotectonic activity on the lacustrine system. The time scale was constructed from radiocarbon determinations and well dated marker tephras from the region. The composite sequence spans the last ca.60000 yr B.P. (MIS 3-MIS 1). Thirteen facies were recognized and grouped into three main categories: 1) detritic, 2) volcaniclastic, and 3) biogenic. Volcaniclastic deposits comprise 18 individual layers 1 - 50 cm thick, two of which are from well documented eruptions from Popocatépetl and Nevado de Toluca volcanoes. From our cores and other available information, we suggest that the monogenetic volcano Teuhtli, in the southwestern part of the Chalco lacustrine plain, is the origin of the so called Gran Ceniza Basáltica tephra, dated between ca. 27000 y 41000 cal B.P. The accumulation of Teuhtli derived products altered the basin morphology and affected the lacustrine sedimentation, which is observed in the diatom record. We suggest that an ancient depocentre was located south of the modern lake, and that its location was controlled by synsedimentary normal faulting, active until ca. 26500 years ago.

Keywords : Lacustrine sedimentation; central Mexico; volcanic settings; tephrachronology; Quaternary.

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