Boletín de la Sociedad Geológica Mexicana
On-line version ISSN 1405-3322
VAZQUEZ, Gabriel; ORTEGA, Beatriz; DAVIES, Sarah J. and ASTON, Benjamin J. Sedimentary record for the last ca. 17000 years in the Zirahuen lake, Michoacán, Mexico. Bol. Soc. Geol. Mex [online]. 2010, vol.62, n.3, pp. 325-343. ISSN 1405-3322.
Lake Zirahuen (101° 44' W, 19° 26' N), located in a volcanically active region at the boundary of the two major modern climatic systems (the intertropical convergence zone and the subtropical high pressure zone) and in a region with a history of human occupation of several thousand years, provides the opportunity to investigate climatic variations, the history of vegetation and the volcanic and anthropogenic impact in central Mexico. In this paper, we present a preliminary model of lacustrine evolution of Zirahuen Lake for the last 17000 cal yr BP, based on the analysis of vertical and lateral variations of this sedimentary deposit, using two sediment cores collected in the center and northern part of the lake (5.40 and 6.61 m depth). The described sedimentary facies have been grouped into four facies associations, which describe the sedimentary evolution of the central-north part of the lake. Core log correlation is supported by matching the magnetic susceptibility peaks. The chronological framework is provided by twenty 14C dates and the recognition of two historical tephras from Jorullo and Paricutin volcanoes. According to the age models, the northern sedimentary sequence spans ca. 17000 cal yr BP, while the central one covers 11540 cal yr BP. The facies are composed of diatomaceous ooze (massive or laminated), clastic facies and volcaniclastic facies. In the northern sequence, stratigraphical and chronological evidence point to a sedimentary hiatus of nearly 1 m, equivalent to 5000 yr. In consequence, the two sedimentary sequences are only correlated for the last 7200 cal yr BP. The evolution of Zirahuen Lake for the last 17000 yr as inferred from the variations in the sedimentary components is summarized in four stages: 1) 17000-14000 cal yr BP-low lake levels and dominance of fine-grained clastic sedimentation; 2) 14000-8180 cal yr BP-increase in lake level and littoral expansion under relatively stable conditions, and between 8180 and 7200 cal yr BP, an erosive event that gave origin to the hiatus in the northern sedimentary sequence; 3) 8180-3900 cal yr BP-lake level increases; and 4) the last 3900 yr are characterized by intense erosion and clastic sedimentation. Sporadic heavy precipitation over land with scarce vegetation probably induced the high erosion. For this period, it is difficult to separate the climatic and anthropogenic signals, as agriculture and other human land uses have been documented for the region. A low erosion period is recognized between 1400 and 800 yr ago, suggesting the extension of dry conditions inferred from other sites in Mesoamerica at the end of the archaeological Classic period.
Keywords : Late Pleistocene; Holocene; paleolimnology; lacustrine sediments; tephras; central Mexico.