ONSET OF PLEISTOCENE CLIMATE GENERATED A PULSE OF COARSE SEDIMENT, PRESERVED AS THE SHARPYLDAK FORMATION IN THE TIEN SHAN RANGE OF KYRGYZSTAN

Geologic mapping, magnetostratigraphy constrained by fossils in the underlying fine-grained Chu Formation, overlying terraces, and regional structural and stratigraphic analysis can be used to constrain the age of the Sharpyldak Formation (SH) and the cause of events that created this formation. Our geologic mapping has shown that in most places the SH gradationally and conformably overlies the Chu Fm indicating that there is no time gap between these formations. Analysis of Pmag samples constrained by fossils collected in the Kochkor, Chu, Naryn, and At Bashi basins require that the transition from the Chu Fm to the SH occurred between 2.5 and 3.0 Ma. Abundant Miocene and Pliocene fossils found in the Chu Fm include Chilotherium (rhino), Hipparion (horse), Samotherium (giraffe), Pliocervus (deer), and Hyaenaictitherium (hyena). While this data from the underlying Chu Fm dates the beginning of the SH, post-SH fluvial terraces limit the end of the SH. We have used cosmogenic isotope dating methods to determine the age of many younger terraces (up to ~250,000). Using uplift, incision, and slip rates derived from the young terraces we can extrapolate to the ages of older terraces; this has revealed that the oldest terraces are about 1 Ma, which requires that the SH to be older than this. In addition, Pleistocene fossils have been found in the SH, including Equs (horse), Bison (bison), and Mamut (mammoth). Some of these fossil sites have reversed polarity, so the SH must be older than 0.8 Ma. It is important to note that the SH is deformed and uplifted in a manner similar to the Chu Fm, which makes the SH structurally closer to the Chu Fm than subsequent fluvial terraces. This suggests that the SH formed early in our 3-1 Ma window provided by the age control. Based on our age constrains for the SH we infer that the SH was formed by a pulse of coarse sediment into basins due to the onset of Quaternary climate. Once the fluvial system adjusted to the new climate re-incision occurred and progressive downcutting due to ongoing uplift resumed.