Climate change on orbital to tectonic time scales: Facies development in the Miocene terrestrial succession of the Aktau Hills (SE Kazakhstan)

Throughout the Cenozoic, continental settings of Central Asia witnessed increased desertification. This climate shift is thought to be connected to global cooling, orogeny and the final Paratethys retreat. Despite general... [ view full abstract ]

Throughout the Cenozoic, continental settings of Central Asia witnessed increased desertification. This climate shift is thought to be connected to global cooling, orogeny and the final Paratethys retreat. Despite general aridification, a period of lacustrine sedimentation during the Miocene indicates elevated moisture transport into Asia´s continental interior. In this study, we present field results from an exceptionally well exposed Miocene succession located in the Aktau Hills (Ili Basin, SE Kazakhstan). Dated to Early Miocene (MN4-5) by mammal remains at its base, the investigated interval grades from reddish-coloured mudflat siltstones to light-grey calcareous lacustrine sediments. Along this transition, five facies associations can be differentiated, in stratigraphic order: 1. mudflat environment, 2. ephemeral playa lake, 3. intermittent saline lake, 4. distal open lake, 5. marginal lake. Occurrences of aquatic fossils (ostracods, fish bone fragments, charophytes) are positively correlated with the trend towards freshwater conditions and increasing shoreline proximity. Each facies association exhibits cyclic alternations of single facies types, that correspond to higher and lower lake or groundwater level, respectively.
A cyclostratigraphic framework, based on field measurements of magnetic susceptibility and rock colour, suggests astronomical forcing of the rhythmic facies changes. Evolutive spectral analysis shows considerable variations in sedimentation rate. Nonetheless, an influence of the 405 ka and the ~100 ka eccentricity cycles on short-term lake and groundwater level fluctuations can be deduced from spectral analysis. The entire succession comprises approximately 3.5 Ma, displaying a long-term humidification trend. Increased water influx into the Ili Basin could have been triggered by the uplift of the Tian Shan, acting as a topographic barrier for the westerly winds. Enhanced orographic precipitation possibly leads to a positive net water budget and flooding of the basin. In addition, global warming and the transgressive extension of the eastern Paratethys during the the Middle Miocene Climatic Optimum could have promoted the elevated transport of moisture to Central Asia.