Downhole gamma ray data to reconstruct an age-depth model of the terrestrial record at Lake Chalco, Central Mexico

Understanding the evolution of lower latitude climate from the most recent glacial periods of the latest Pleistocene to post glacial warmth in the continental tropical regions has been obstructed by a lack of long and continuous time series. Here 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. We use borehole logging to conduct a cyclostratigraphic analysis of the Lake Chalco sediments, and incorporate other available dating information. More than 400 m of sediments are logged for several geophysical properties including magnetic susceptibility and spectral gamma radiation.

Gamma radiation can be used to identify elemental isotopes in the geological record, which is used for stratigraphic correlation and paleoclimatic investigations. Among the lake deposit of Chalco sub-basin, 388 total tephra layers (≥1 mm in thickness) were reported from the core description. Tephra layers with specific gamma ray signatures, presenting a challenge for extracting the primary signals caused by environmental and climatic agents. Here, we apply a tailored protocol to identify tephra layers embedded in other sediments using high-resolution gamma ray spectroscopy. This facilitates dividing the overall sediment column into representative horizons of tephra and non-tephra.

After extracting the non-volcanic primary signal, we apply a suite of evolutive cyclostratigraphic methods to the Lake Chalco logging data, with a focus on gamma ray data. The high-resolution results suggest that the Lake Chalco sediments contain several rhythmic alterations with a quasi-cyclic pattern comparable with the Pleistocene benthic stack. This allow us to calculate a ~500-kyr time span for the sediment deposition in Lake Chalco. By using cyclostratigraphic analysis on data captured by geophysical downhole logging, we demonstrate the potentially broad applicability of this method for downhole logging data and provide further insight into the sedimentation history of Lake Chalco.