Zebra layers and palaeoenvironment of Late Miocene Stratum in the Linxia Basin, northwestern China

Miocene strata in the Linxia Basin (Gansu, China) are usually interpreted as lacustrine sediments. However, the red-grey inter-beds known as 'Zebra layers' commonly tilt with respect to the terrain on the side slopes of the modern valley, which may be due to mantling palaeotopography (similar to aeolian loess). The anisotropy of magnetic susceptibility, which reflects the original arrangement of magnetic particles in sediments, was applied to investigate this phenomenon. The results showed that the tilting of the inter-beds in the side slope was due to mantle palaeotopography rather than soil creep, and that they were not deposited in a subaqueous environment. The grain sizes of sediments showed similar features as aeolian loess. We speculate that Miocene sediments were deposited by mantling the palaeotopography where aeolian materials accumulated. After deposition, flowing water submerged these strata, which caused the side slope to become gradually thinner from top to bottom and stirred the magnetic particles in these sediments. The grey colour of the Zebra layers may not be original, as it may be due to waterlogging and deoxidation after deposition; finally, when the iron oxides in these sediments were transformed or removed, their colours became grey. The formation of Zebra layers indicates that the Late Miocene palaeoenvironment of northwestern China was similar to that in which Quaternary aeolian loess was deposited.