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Pedogenesis in Lutitic Cr Horizons of Gypsiferous Soils

Dep. of Soils and Irrigation, Laboratorio asociado de Agronomía y Medio Ambiente (DGA-CSIC), P.O. Box 727, 50080 Zaragoza, Spain

View larger version (38K): [in a new window]   Fig. 1. Location of the two study Areas A and B, and weather stations, in the Tertiary Basin of the Ebro Valley. Sketches (not to scale) of the distribution of the pedons on a cross-section of the landscape of Areas A and B are also shown.

View larger version (120K): [in a new window]   Fig. 2. (a) Incomplete infilling of the voids (V) between lutitic fragments (L) by lenticular gypsum crystals (G). Thin section of Pedon B8 at the 75- to 90-cm depth (Cry2 horizon) under cross-polarizers (length of bar: 1 mm). (b) The increase in concentration of gypsum crystals (G) in the voids (V) leads to isles fabric, that is, isles of lutitic material (L) separated by gypsum. Thin section of pedon B3 at the 115- to 200-cm depth (2Cry horizon) under cross-polarizers (length of bar: 1 mm). (c) An advanced stage in isles fabric shows lutitic isles (L) on a groundmass of lenticular gypsum (G) Thin section of the pedon B7 at the 35- to 65-cm depth (By2/Cry horizon) under cross polarizers (length of bar: 1 mm).

View larger version (109K): [in a new window]   Fig. 3. This thin section is of the Pedon A2 at the 65- to 95-cm depth (Cry horizon). Several coalescent queras are shown (a) under plane-polarized light, and (b) under cross-polarizers (length of bar: 1 mm). The upper half of micrograph shows a transverse cross-section of queras, whereas the lower half shows a longitudinal cross-section of a quera. (c) Sketch of upper left corner of the above photographs illustrating the components of the queras. (d) Detail of another field in the same thin section showing a transverse section of the quera with the quesparite grains, or quemosaic (QM), infilling the central void, and the decalcified area, or quedecal (QD) (length of bar: 400 µm).

View larger version (54K): [in a new window]   Fig. 4. Backscattered electron microscope micrograph of the Cry1 horizon of Pedon B1 at the 10- to 25-cm depth (length of bar: 50 µm) shows a quemosaic fragment containing a quesparite grain with a central void (CV). The EDS spectrum shows the predominance of Ca, O, and Mg in the composition of quesparite grains.

View larger version (120K): [in a new window]   Fig. 5. This organic residue (OR) contains quesparite crystals (Q) within residues of cell walls (CW). This thin section is of Pedon A1 at the 37- to 45-cm depth (Bwy horizon) (a) under cross-polarizers, and (b) under polarizers at 45° (length of bar: 100 µm).

View larger version (80K): [in a new window]   Fig. 6. Electron micrographs of Pedon B1 at the 10- to 25-cm depth (Cry1 horizon). (a) EDS dot maps showing spatial distribution of Si, Al, Mg, S, and Ca in the contact between a lutitic fragment (left) and the residue of a quemosaic (right). The length of each frame is 750 µm. (b) The diagonal marked by the arrows is the contact between the unweathered lutitic material (lower half) and the quedecal (upper half). The BSE image shows that the quedecal is much more porous than lutite, because of the decalcification process (length of bar: 100 µm).

View larger version (107K): [in a new window]   Fig. 7. Photomicrographs of the 2Cry horizon of Pedon B3 (115-200 cm) under crossed polarizers. (a) Carbonate rings (CR) forming an incomplete infilling in lutitic material with a decalcified area (D), (length of bar: 200 µm). (b) Detail of (a), (length of bar: 100 µm).

View larger version (185K): [in a new window]   Fig. 8. Photomicrographs of carbonate rings. (a) Fragments of the carbonate rings (CR) dispersed in both the gypsic and the carbonatic-silicatic groundmass of the 2Cry horizon of Pedon B3 (115–200 cm) under crossed polarizers (length of bar: 200 µm). (b) Stained thin section under plane-polarized light of the 4Cry1 horizon of pedon B9 at 160-190 cm, showing carbonate rings (CR) dispersed in gypsic (G) groundmass (length of bar: 200 µm).

View larger version (100K): [in a new window]   Fig. 9. Sketch of the proposed pedogenic model, illustrated in four stages from (a) to (d). The development of gypsum crystals and queras are the main weathering agents, transforming the lutite in a looser and coarser material, which can become a B horizon.