265 / 2019-07-01 10:27:14

Paleosol Identification from Logging and Application in Fluvial Strata Correlation

Paleosol identification, Logging information, Fluvial strata correlation, Sequence stratigraphic framework

Hampson’s research shows that abnormal high Th/K indicates paleosol layers in fluvial strata. According to it, a typical paleosol layer is identified at the top of the 8th Member, Lower Shihezi Formation in Sulige area, Ordos Basin, China. Besides the abnormal high Th/K, this paleosol layer is also characterized with low DEN, high CNL, low K and high Th. What’s more, some atypical paleosol layers with normal high Th/K also show low DEN and high CNL, and they seem like higher resolution indexes in identifying paleosol than Th/K.
Hampson’s conclusion is built on the base of high matured paleosol. After enough weathering, much feldspar is resolved. As a result, K is depleted in the strata, which forms an abnormal high Th/K. However, in much paleosol with a low or middle maturity after a short weathering, the K is not depleted seriously, so they show almost normal high Th/K. If identifying paleosol only by abnormal high Th/K, many paleosol layers with low and middle maturity will be lost. Anyway, paleosol with different maturity will become poriferous after weathering, showing higher porosity and lower density than before (of course, the density may become bigger because of calcification). It is a physical change which is not relate to mineral composition.
On the base the above analysis, correlation analysis among CNL/DEN, Th/K, DEN, CNL, K and Th are carried on. The result shows the high correlation of CNL and Th/K, CNL/DEN and Th/K, K and Th/K, which indicates high CNL and CNL/DEN are good indexes to identify paleosol.
After the CNL and CNL/DEN threshold value to identify paleosol is confirmed, they are applied in the 8th Member, Lower Shihezi Formation in Sulige area, Ordos Basin. A lot of paleosol layers are identified and a sequence stratigraphic framework is built in fluvial strata.