Gigaimages / virtual microscope


A complex genetic problem.

2BCtk Horizon is a Calcic Palexeral decapitate. 

Thin section RNS16-5. This preparation corresponds to the horizon inmedaitley below the horizon RNS16-4.


Low/medium magnifications
Low/medium magnifications


Horizon interpretation

I suggest to first browse, observe, describe and then, deduce from these images. I already did it and this is my interpretation.


It is evident the high heterogeneity of this horizon. It is composed of three different materials: sand/silt, extremely clayey and highly abundat carbonates areas.  Click on images for zoom in
The sand/silt areas show thin yellow clay coating in the packing voids of the sand grains.
When analysing the extremely clayey areas with the polarizer only, we can observe a yellow color, quite transparent, very clean and with a very homogeneous aspect. When using the analyzer, we can see strong sepic orientations that usually have yellow coating clay in their pores.
The carbonate areas are micritic. They are frequently related with the pores, forming more or less dense coatings  being massive in other areas.   
The carbonate coatings are present in both, the sand/slit and the extremely clayey areas.  
Sometimes, the micritic carbonates (Cm) show spatic recrystallizations (Ce) and the voids of these spatic crystals are infilled with clay (Ai).    
The carbonate coatings of the pores are also covered by a yellow illuviation clay.
The yellow illuvial clay coatings are soemtimes covered with orange illuvial clay coating.




The sand/silt areas show micromorphological features identic to the ones showed in the horizon below (hor 2CBtk)

The extreme clayey area presents similar micromorphological features to the ones appearing in two Bt horizons above (2Bt2 y 2Bt3). Transparency, cleaness, homogeneity, strong orientations with crossed nicols that usually present remainings of fragmented illuvial coating clay (papules) integrated in the basal mass, such that it is not possible to differentiate them from the remaining clay of the surrounding plasma of the soil.   

The general characteristics describe the soil situated in a fluvial terrace.

According to the analytical data in the table, it is remarkable the high number of gravels in the two most superficial horizons whilst they rarely appear in the lower packet.

Micromorphologically speaking, horizon Bt1 is completely different to 2Bt2.

The augering core sample has been used for sampling from two to four meter, at the bottom of calicata, and a packet of sandy texture horizons was found that presents clay illuviation with no carbonates. These horizons were named 2CBt2 (205-250 cm), 2CBt3 (250-310 cm), 2CBt4 (310-350 cm) y 2CBt5 (350 a +400 cm). Due to its great thickness, calling them transition horizons might not be appropriate. They could have been named as Bt because of its intense illuviation clay, but the soil structure is not developed. They were also named like this in order to differentiate them from the upper Bt horizons (clearly different to those ones). Whatever name they get, the important feature is the existence of such a thick packet with such string illuviation clay below a highly calcareous horizon.   2CBt2
(205-250 cm)
(350-400 cm)



Both, the yellow and the orange clay coatings present the typical microfeatures of illuvial clay accummulations (located in the macropores, clear boundaries, clean -not including silt or sand-, strong orientations, sometimes microlaminated, ...).
The carbonates also come from dragging  (dissolved in this case) the upper horizons. They start accumulating in the wall of pores, swelling and finally, merging with the carbonates of the neighboring pores creating an undifferentiated mass. As many of them are covered by illuvial clay, we can conclude that cabornatation is not a process ocurring at present time, it is sealed by illuviation.
The yellow illuviation clay is also a fosil process because some of the accummulations are covered by orange illuviation clay.   

It is difficult to find carbonates in the horizon inmediately below, and the existing ones present coatings so that we can conclude they have an illuvial origin. There are no carbonates in the deepest packet (2CBt1 al 2CBt5), thus, we can conclude that the carbonates were not present in the sand and silt that create this palaeosoil .

All the previously mentioned statements lead into the following genesis.

The soil is composed of two packets with different origins. Horizons 2Bt2, 2Bt3, 2BCtk and 2CBtk belong to the lower packet. This soil is the preterrace (before terrace) that existed before the area was taken by the waters of the river Tormes. When the path of the river Tormes arrived, it dragged the upper horizons of the soil  (Ah, E and the upper part 2Bt2 we can see now) and at the same time the waters deposit detritic material in its riverbed (upper packet). Later, when the waters of the river Tormes change its path, the abandone riverbed soil forming start and the upper horizons A and Bt1 are formed. Carbonates arrive with the waters and illuviate, accummulating in the lower horizons sealing the yellow primitive illuviation. Additionally, the new environment of the soil can be more aerated and the higher degree of oxidantion makes that the clay illuviating now in the new soil has more reddish colors.

When the lower horizons are buried by the packet upper to its terminology, we have to add a "b": 2Bt2b, 2Bt3b, 2BCtkb y 2CBtkb.

An ilustrative diagram of this geneis can be also found in another soil of the same terraces in section "Escrito en el suelo", specifically in this web site

The thin section RNS16-4 also belongs to this soil.


Soil data

The profile

The landscape

General features

Morphology, physical and chemical analysis

Micromorphologic images: Hor Ap      Hor Bt1     Hor 2Bt2     Hor 2Bt3     Hor 2BCtk     Hor 2CBtk     comparative study



This soil was presented in XIII Reunión Nacional de Suelos in Salamanca in 1985