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In the framework of the International Continental Scientific Drilling Program (ICDP), a drilling campaign was performed at Lake Junin in Peru during the summer of 2015. The main objective of the Lake Junín drilling project was to obtain high-resolution paleoclimate records from lacustrine sediments in order to reconstruct the history of the continental records covering the glacial-interglacial cycles. Lake Junín is located at 4000 m above sea level in the tropical Andes of Peru and is characterized by a thick sediment package deposited at a high rate (0.2 to 1.0 mm/year). It predates the maximum extent of glaciation and is in a geomorphic position to record the waxing and waning of glaciers in the nearby Cordillera, hence making the lake a key site for investigating the Quaternary climate evolution in the inner-tropics of the Southern Hemisphere. Drilling was performed at three drill sites, with 11 boreholes in total. Continuous coring reached a maximum depth of 110.08 m at site 1 (1D borehole). Based on the element distribution of potassium (K) and thorium (Th) from spectrum gamma ray downhole logging data in two boreholes (1D and 3B), we have selected 45 samples from core run 1D and 23 samples from core run 3B at the LacCore repository in Minneapolis (USA), to compare and characterize the clay content in the lake sediments. The samples were selected based on massive, medium bedded, mottled, and thinly bedded silt. Qualitative and semi-quantitative mineralogical analyses on randomly oriented bulk rock powder samples were performed by X-ray diffraction (XRD), in order to identify the different bulk mineral phases that are present in the samples. Quartz, calcite, feldspar and clay minerals could be identified. The clay-size fraction (< 2 microns) was separated using a combination of wet-disaggregation, sedimentation and centrifugation techniques. Oriented samples were prepared and analyzed by XRD in air-dried and ethylene glycol state, to identify the individual clay mineralogy. Especially changes in interlayer size and intensity or half-width broadening in the individual clay peaks give information about changes in mineralogical properties. The main clay mineral phases showed illite, smectite, and kaolinite in different amounts. Interestingly, some specific samples showed no clay, but only calcite and quartz. Linking the abundance and the lack of the specific clay minerals in core samples with the downhole logging data (spectrum gamma ray and magnetic susceptibility), a relationship between the geological history of the lake and climate change processes can be recognized. Consequently, the different mineralogical composition of the sediments, especially the presence or absence of smectite in the clay bulk, reflects a glacial/interglacial climate cyclicity. [ABSTRACT FROM AUTHOR]

The key objective of most lake drilling projects is to retrieve drill core for reconstruction of paleoenvironmental conditions based on the sedimentation history. Such studies help to define past climate changes in space and time in order to understand current environmental processes and to provide input parameter for modelling future climate scenarios. The type of sedimentation and geophysical properties of lacustrine lithological units can be investigated with different measurement techniques on core samples and by downhole logging data. However, downhole logging data are rarely used to contribute to specific scientific research tasks in lacustrine projects, although they allow the determination of lithological, mineralogical and stratigraphic properties of the rocks and sediments especially in sections with larger lithological gaps or poor core recovery. Additionally, a well-log cluster analysis allows to construct a lithological profile, called the electrofacies log, which commonly has a higher vertical resolution than an initial visual core description and is valid even in long and frequent core gap sections. The purpose of this cluster analysis is to look for similarities/dissimilarities between data points in the multivariate space of logs and to identify zones with uniform values in each log, in order to group these features into electrofacies classes. In the framework of the International Continental Scientific Drilling Program, ICDP, a scientific drilling campaign in Peru has been supported during summer 2015. The ICDP Lake Junín Drilling Project aims to obtain high-resolution paleoclimate records from lacustrine sediments, to reconstruct the history of the continental records covering the glacial-interglacial cycles. Lake Junín (known also as Chinchaycocha) is located at 4000 m a.s.l. in the tropical Andes of Peru and is characterized by a thick sediment package (> 125 m) deposited at a high rate (0.2 to 1.0 mm/yr). Drill cores were recovered from three drill sites. After the completion of each coring operation, a full suite of downhole geophysical logging was performed in five of the 11 boreholes (1A, 1C, 1D, 2A and 3B) by the Operational Support Group of ICDP. Characterization of the physical properties of the lithologies by the use of cluster analysis allows to reconstruct a continuous lithological profile of the sedimentary record of Lake Junín. An electrofacies represents a unique set of log responses, which characterizes the physical and chemical properties of the rocks and fluids contained in the volume investigated by the logging. The downhole logs have been divided into cluster units determined by a dendrogram. Three major groups (carbonate-silt, peat and silt) have been identified by spectrum gamma ray, magnetic susceptibility, and p-wave velocity logs. With this method we are able also to attribute the lithology in correspondence of core gaps. By the end the properties of the clusters are analyzed and converted into lithological units according to the lithological information from the visual core description. [ABSTRACT FROM AUTHOR]