Abstract
Two stratigraphic sections of carbonate sediments with significant thickness differences and without appreciable tectonic deformation were studied near the trough and on a threshold zone at the Álava Trough. Such characteristics make them appropriate to analyze the influence of a slow progression of the diagenesis over the original clay suite. X Ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and Analytical Electron Microscopy (AEM) techniques were applied in natural and alkylammonium-treated samples. Diagenesis slightly modified the clay mineralogy, the disappearance of smectite, and the variation in the content and ordering of the I/S mixed layer, with burial being the most noteworthy process. The total charge in the 2:1 expandable layers of smectite and I/S shows a slight increase, preferentially located on tetrahedral sheets, with depth. The data suggest a moderate diagenesis grade for the studied materials. The combination of techniques allowed identification of several types of detrital micaceous phases, as well-crystallized K-rich micas, Na-K micas, mica-chlorite stacks, and illites, with an expandable behaviour after the alkylammonium treatment. The total charge of illites did not change with diagenesis, suggesting their detrital origin. This research shows that the detrital assemblage masks the diagenetic evolution in the basin, which indicates the importance of the combination of different techniques to infer correctly the diagenetic grade in a sedimentary basin.
Highlights
Smectite illitization is the most common mineral diagenetic process that occurs in the burial of pelitic rocks
36 marly samples were collected (Figure 1). Their clay mineralogy was confirmed by conventional X Ray Diffraction (XRD) methods [26] and compared to that determined on the cuttings analysed in the above mentioned work
The analysis shows no correlation between the Fe/(Fe + Mg)/AlVI − AlIV ratio and all have a high number of interlayer cations: 0.12–0.58 (Table 2)
Summary
Smectite illitization is the most common mineral diagenetic process that occurs in the burial of pelitic rocks. The process comprises a series of chemical reactions and structural changes that give rise to coherent domains intermediate between smectite and illite. These are the so-called illite/smectite (I/S) mixed-layer minerals, which consist of different stacking sequences of two kinds of TOT (tetrahedral-octahedral-tetrahedral) layers, illitic (I) and smectitic (S), whose main difference is the layer charge, due to different compositions [1,2,3]. In a stage of advanced diagenetic evolution, the layer stacking is defined by sequences with the R > 1 type, in which the amount of illite in the coherent domains is >80% and S layers are always separated by at less two illitic layers. Bay of of Biscay, which is Thegeological geologicalhistory historyofofthis thisbasin basinisisrelated relatedtoto the opening and closing Bay
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