Episodic rainfall events characterise complex sediment deposition in a fault-bounded sag pond in Northwest China

Fault-bounded sag pond sediment records are commonly found to be excellent archives of palaeoearthquakes and more rarely, they can provide robust evidence of local environment and climate change. We examined the sedimentary and geomorphological record of the Aksay Pond within the Karachingar Valley along the southern margin of the Altai Mountain Range in northwestern China. This pond is associated to a shutter ridge resulting from cumulative deformation associated with successive earthquakes along the Fuyun Fault, including the most recent 11 August 1931 M w 7.9 earthquake. However, detailed chronology based on 14 C Accelerator Mass Spectrometry, 210 Pb and 137 Cs dating suggests that only deformation related to the 1931 earthquake have been preserved at the pond site. A small wedge of sediments comprising cobble and gravel sized clasts are separated by two palaeosoil deposits suggesting that very minor sediment could accumulate prior to 1931. In the northern part of the pond, sediment was deposited predominantly from a colluvial fan that has periodically been mobilised from the steep mountain slopes to the east. In the southern part of the pond we find that the dry climate of the region has experienced periodic storms that have resulted in the deposition of 11 fining upwards packages formed by remobilised and deposited sand to mud-sized sediments within the pond. Associated with these fining upward cycles are two colluvial wedges that emanate from the fault scarp. The upper wedge is dated at approximately 1986 CE and coincides with a high precipitation event recorded at the Fuyun Meteorological Station in 1986. The middle wedge may be related to an earlier high precipitation event, but due to challenges in obtaining accurate ages, cannot be verified. Alternatively, these two colluvial wedges could coincide with minor ca. M w 5 earthquakes in the region. Importantly, these wedges are not associated with large scale rupturing similar to the 1931 earthquake. Based on the sedimentary and geomorphological evidence of the Aksay Pond, we propose a model for the sequential formation of sag ponds highlighting the influence of both climatic and tectonics processes. • Sag pond sediments record storm deposits in the southern Altai Mountains • Preserved colluvial wedges are not related to tectonic activity • Multiple earthquakes required to displace shutter ridge prior to pond formation • A joint climatic and tectonic process model for sag pond formation is presented