Geology, Sediments and Soils

Abstract

Lake Chapala, the major Mexican fresh water reservoir, is located in western Mexico, is a good example of a neotectonic lake with a history linked eastwards to the development of the active Transmexican Volcanic Belt geological province. Lake Chapala crosses the E-W trending Citala rift, which forms with the Tepic-Zacoalco and Colima rifts the so-called Jalisco triple junction. The Lake Chapala basin is tectonically controlled and belongs to a regional intra-arc system of half-grabens. It is assumed that the current Lake Chapala basin has a depth of about 600 m. Shallow (1.40 m) sediments in Lake Chapala are dark and silty with granulometric median ranging from 10 to 50 µm. Sediment samples were studied by isotopic, geophysical, geochemical and micro-paleontological methods. Major oxides in sediments are SiO2 (54.05%), Al2O3 (18.94%), Fe2O3 (6.75) and CaO (2.57%). The major mineralogical composition of sediments is as follows: plagioclases (25.86%), quartz (20.92%), clays (38.5%), carbonates (2.99%) and TiO2 (1.05%). Sedimentation rates (SR) were evaluated from 210Pb profiles assuming constant initial concentrations; and we can conclude that SR is at least two or three times higher on the East side of the lake than in the western part. As water depths on both sides of the lake are similar, we assume there must be a tectonic compensation on the eastern part. This explains why Lake Chapala has a uniform depth in spite of the differences detected in SR. We have estimated an approximate 1 mm yr−1 subsidence at the eastern part of the lake that compensates the higher SR. This tectonic compensation also explains the fact that the lake’s fluvial input and output are so abnormally close to each other. Scarcity of diatoms in shallow sediments of Lake Chapala is due to pH variations in both sediments and water, hence inhibiting their preservation. The major soil types around Lake Chapala are: Vertisol (50%), Pheozem (30%), Lithosol (15%) and Luvisol (5%).