Preferred orientation of phyllosilicates in Gulf Coast mudstones and relation to the smectite-illite transition

Development of preferred orientations of illite-smectite (I-S) has been stu died using X-ray diffraction (XRD) texture goniometry to produce pole figur es for clay minerals of a suite of 16 mudstone samples from a core from the Gulf Coast. Samples represent a compaction-loading environment in which th e smectite-to-illite (S-I) transition occurs. In five shallow, pre-transiti on samples, there is no significant preferred orientation for smectite-rich I-S. Development of preferred orientation of I-S, although weak, was first detected at depths slightly less than that of the S-I transition. The degr ee of preferred orientation, which is always bedding-parallel, increases ra ther abruptly, but continuously, over a narrow interval corresponding to th e onset of the S-I transition, then continues to strengthen only slightly w ith increasing depth. The degree of post-transition preferred orientation i s also dependent on lithology, where the preferred orientation is less well -defined for quartz-rich samples. Previously obtained transmission electron microscope (TEM) data define text ures consistent with the change in orientation over many crystallites. The smectite in pre-transition rocks consists largely of anastomosing, "wavy" l ayers with variable orientation and whose mean orientation is parallel to b edding, but which deviate continuously from that orientation. This results in broad, poorly defined peaks in pole figures. Post-transition illite, by contrast, consists of thin, straight packets, with most individual crystall ites being parallel or nearly parallel to bedding. This results in pole fig ures with sharply defined maxima. By analogy with development of slaty clea vage in response to tectonic stress during metamorphism, the S-I transition is marked by dissolution of smectite and neocrystallization of illite or I -S locally within the continuous "megacrystals" of smectite. The transition is inferred to have some component, of mechanical rotation of coherent ill ite crystals within a pliant matrix of smectite. The data suggest that chan ge in orientation and coalescence of clay packets plays an important role i n the formation of the hydraulic seal required for overpressure generation.