Pebble in the Wind. Micromorphological Features at the Contact between Gravelly Clasts and Silty Groundmass in Loess-Debris Sediments, Central Asiatic Loess Zone

Abstract

Loess-debris sediments are featured by two modes of accumulation, that are aeolian and colluvial. In Central Asia, such sediments are common at archaeological sites, where the ‘debris’ component is represented by geogenic pebble-sized clasts of colluvial origin and archaeological artefacts. Therefore, the understanding of microscale processes affecting geogenic coarse clasts is crucial to the evaluation of post-sedimentary processes that could have impacted the archaeological assemblages, such as re-orientation, in situ damage or translocation. The objective of this work is to describe the relationship between the coarse mineral material and the fine material at their contacts. We studied four Upper Pleistocene-Holocene archeological sites located at the toe or along a slope in Kyrgyzstan and Uzbekistan, that are: Obishir-1, Obishir-5, Katta Sai 1 and Katta Sai 2. The main methodology used here is micromorphology supported by grain size analyses. Micro-features occurring at the surfaces of pebbles were described and classified, recognizing mainly four typical features, that are: loamy cappings, loamy coatings, secondary calcium carbonate accumulations, and cracks. Also the groundmass was analyzed. We found that the main processes related to these features are alteration of the mineral grains, frost action, redeposition, pedogenetic and anthropogenic processes. And that in most cases, the primary contact between the two partial fabrics has been disrupted. Analogies with gravel-sized artefacts can be drawn especially for what concern secondary calcium carbonate accumulation. Our study may well be of support when analyzing site formation and post-depositional processes at archaeological sites.

Keywords: micromorphology, Colluvial Deposits, Quaternary, Geoarchaeology, Central Asia