Sedimentation conditions and rock-magnetic properties of Quaternary deposits from Laguna de Santa Rosa, Iturbide region, northeastern Mexico

We report initial results of an interdisciplinary study of the lacustrine sequence of Laguna de Santa Rosa, based on a 26 m long core recovered from its northern-central sector. The Laguna is an intramontane elongated basin with a small catchment area at an altitude of 1520 m asl in the central sector of the Sierra Madre Oriental in the Iturbide region, northeastern Mexico. The stratigraphic variations of rock-magnetic properties, total carbonate content and isotopic carbon, and mineralogy define three major zones separated by peaks of the calcite to quartz+phyllosilicate+feldspar ratio. The upper zone, from surface to 7-9 m, is formed by an organic clay with abundant gasteropods and two black to green clays. It is characterized by magnetic susceptibility around 10 -6 SI, frequency dependent factor of 0 %, δ 13 C ranging from -20.3 ‰ at surface to -26.4 ‰, presence of quartz and phyllosilicates with generally more than 20 % calcite, and sporadic occurrence of plagioclase (albite) with K-feldspars nearly absent. The middle zone, from 9 m to 17.5 m, is formed by five clay units with colors ranging from green gray to brown. This zone is characterized by variable susceptibility up to 17 10 -6 SI, negative frequency dependent factors, low total carbon content, quartz and phyllosilicate enrichment, less calcite and presence of K-feldspars. This middle zone shows higher detrital input and/or reduced productivity, which suggests a cold and less humid interval. The lower zone extends from 17.5 m to 26 m and shows two brown to green gray clay units. This zone is characterized by magnetic susceptibility between 8 10 -6 SI and 14 10 -6 SI, variable frequency dependent factors, and δ 13 C values around -24 ‰. Its mineralogy is similar to the upper zone, again enriched in calcite and occurrence of plagioclase. The phyllosilicate fraction is mainly composed of mica, chlorite, smectite and kaolinite. The magnetic minerals are allogenic, derived from the lutites that form the catchment basin.