The clay assemblage in Qaidam basin is mainly composed by illite, chlorite and smectite (including mixed layer minerals) with low content of kaolinite since Pleistocene. While the abonormally high content of smectite occured in core clay-rich sediments at ~12ka, and the thickness of the core sediments is about 50 cm. The lithofacies features of the core is mainly composed by the lower part (below ~50m in depth) of clay and dark peat and the upper part of halite, halite with silt and silty clay.
Check to see if this part of the core contains any bentonite/volcanic ash, that is one possibility.
Thanks for your suggestion. But we did not find any bentonite or volcanic ash in the borehole.
Additionally, the source rocks of the study area were dominated by granites, diorites and Proterozoic complex rocks since Pleistocene or earlier. While the anomalies exist only in this part of sediments.
We can confirm our results about the abnormally high content of smectite after several tests.
Mr. Towe, thanks for your suggestion, I'll try to check such minerals you said.
But what I really want to know is what kind of mechanisms or processes caused such clay mineral feature under a cold and dry climate conditions? There are no such report about the evidence of volcanic ash.
Hello Weiliang Miao,
Although the attached review article (chapter) is about clay minerals in soils per se, that's where most clay minerals are formed (which may then be recycled in geosystems), and it has a section on the formation of smectites. The article is: Churchman, G.J., Lowe, D.J. 2012. "Alteration, formation, and occurrence of minerals in soils". In: Huang, P.M.; Li, Y; Sumner, M.E. (eds) “Handbook of Soil Sciences. 2nd edition. Vol. 1: Properties and Processes”. CRC Press, Boca Raton, FL, pp.20.1-20.72. ( If it hasn't uploaded properly please go to my ResearchGate site where it is available (in final MS form) or get a copy of the Handbook 2nd ed.)
The previous comments about bentonites and weathered volcanic ash need to be considered closely - a recent review 2016 by Warren D. Huff "K-bentonites: a review" in American Mineralogist, Volume 101, pages 43–70, on these might be useful as well (in helping explain smectite formation).
During weathering producing clay minerals, there are two paths that have been identified. In the absence of calcium in the form of calcium carbonate, montmorillonite is not formed. Where calcium ions are available in pedocals, montmorillonite is one of the weathering stages. If you have calcium felspars been weathered, these will produce calcium carbonate which will accumulate in the soil in semiarid or arid climates. This could be the cause of your deposit. See: Jackson, M. L., Tyler, S. A., Willis, A. L., Bourbeau, G. A. and Pennington, R. P., 1948.
Weathering sequence of clay size minerals in soils and sediments. Journal of Physical and Colloid Chemistry 52: 1234-1260.
Thus in the Canadian Prairies, montmorillonite is every where, but on the wetter climate of Ontario, it is absent in soils. This has been used to trace the source areas of glaial tills in the region.
Stuart.
Mr. Harris, Thanks for your suggestion. This is a very helpful view. Glacial tills should be a potential source for high value of smectite in the upper part of our core. The neo-formed smectite with high value was reported in periglacial sediments of Siberia area. While suddenly glacial melting events may be suggested during the last deglaciation in Qaidam Basin, which could bring the weathering products with neo-formed smectite into the sink area in a short period. But unfortunately, we can't discuss such points because of our low resolution Age-Depth model (general high age uncertainty in salt lake area) and premilinary work.
Check the clay for euhedral zircons and volcanic quartz, which may be a clue to the origin of the clay. They might tell you something. Simply digest the clay in hydrofluoric acid and look at the microscopic residue. The virtual absence of kaolinite may be due to an alkaline environment of formation.
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