Quaternary landscape evolution of the Helmand Basin, Afghanistan: Insights from staircase terraces, deltas, and paleoshorelines using high-resolution remote sensing analysis

The Helmand Basin in southern Afghanistan is a large (310,000 km2), structurally controlled, endorheically drained basin with a hyperarid climate. The basin hosts a high elevation (~200 m) plateau (the Dasht-i Margo), 11 fluvial staircase terraces (T11 to T1), 7 delta systems (D1 to D7), and 6 paleolake shorelines (SL1 to SL6) within the Sistan Depression on the western side of the basin. Mapping and surveying of these features by remote sensing is integrated with geological observations to reconstruct Quaternary landscape evolution of the basin. The fluvial systems, deltas, and paleolake shorelines are correlated with one another and with the younger terraces (T7 to T1). The shape of fluvial longitudinal profiles changes depending on whether they formed pre-, syn-, or post-growth of the Koh-i Khannesin volcano on the southern margin of the Helmand River. The age of the volcano (~0.6 Ma) and correlation of the terraces with the global history of glacial-interglacial cycles constrain the age of the younger terraces to the late Pleistocene and indicates that the older terraces are middle Pleistocene (dating back to 800 ka). The Helmand Basin once hosted a large lake, called here the Sistan paleolake, which at SL6 times and before had a surface area >50,000 km2. Since that time the lake elevation and area have decreased, evolving to the present-day dried out Sistan Depression with small ephemeral playa lakes. Episodic formation of terraces, deltas, and paleolake shorelines is attributed to changes in base level modulated by climate change related to Milankovitch cycles.