Understanding the evolution of lower latitude climate from the most recent glacial period of the latest Pleistocene to post glacial warmth in the continental tropical regions has been obstructed by a lack of continuous time series. Here we present results from a lacustrine record from tropical North America. Specifically, we examine sediments from Lake Chalco, located in the Valley of Mexico, central Mexico (19°30’N, 99°W). The basin represents a hydrological closed system surrounded by the Trans-Mexican Volcanic Belt aging from the Oligocene to the present. In order to elucidate changes in sedimentation rate across the Quaternary, we used borehole logging to conduct a cyclostratigraphic analysis of the Lake Chalco sediments. More than 400 m were logged for several geophysical properties including magnetic susceptibility and spectral gamma radiation. In order to obtain a fast understanding of such borehole logging datasets, including data from complex settings as lakes, a good understanding of the potential and specifics of relevant (time/depth) evolutive methods in cyclostratigraphy are an essential prerequisite. Therefore, we apply a suite of evolutive cyclostratigraphic methods to the Lake Chalco logging data, with a focus on gamma ray data. The high-resolution gamma ray results suggest that the Lake Chalco sediments contain several rhythmic cycles with a quasi-cyclic pattern, allowing us to calculate the time span of Lake Chalco sediment deposition. Here we present initial results from the full succession in this sedimentary archive. Tephra layers are manifest, but do not prevent a successful cyclostratigraphic analysis. The alterations between carbonates and diatomites are especially evident in the logging data and can be interpreted in a quasi-cyclic way. By using cyclostratigraphic analysis on data captured by geophysical downhole logging, we demonstrate the potentially broad applicability of this method for well logging data and provide further insight into the sedimentation history of Lake Chalco.
Mehrdad Sardar Abadi, Christian Zeeden, Arne Ulfers, Katja Hesse & Thomas Wonik
LIAG, Leibniz Institute for Applied Geophysics, Germany
Mexico, Lake Chalco