The role of porosity in the development of authigenic dolomite and palygorskite. An example in siliceous sandstones of continental origin in Western Spain

Abstract

During the Tertiary, the Hercynian basement of the Iberian Peninsula was uneven due to Alpine tectonism which led to the development of a set of horsts and grabens forming its main geological structure. One of these grabens is the Ciudad Rodrigo Basin, located close to the Portuguese border. The south-western margin of this graben is filled with arkosic sandstones of continental origin that, once deposited, underwent important lowgrade diagenetic processes related to the surface processes. The main diagenetic processes were hydrolysis, replacement by carbonate, and argillization to palygorskite. These three processes led to the differentiation of three main levels within the outcrops studied: (1) the Lower Level, where the original clayey sandstone outcrops, displaying fairly hydromorphic traits; (2) the Middle Level, where part of the clayey matrix has been replaced by dolomitic carbonate; and (3) the Upper Level, where palygorskite is the new neoformed clay. The change in porosity seems to have been the controlling factor, since it governs the dynamics of the solutions inside the rocks. These conditions changed from being diffusion-driven, at the start of the diagenesis, to flowdriven as porosity increased.

Micromorphological investigation of pedogenic barite in Mormon Mesa petrocalcic horizons, Nevada USA: Implication for genesis

“…These pores are large enough to accommodate water and bacteria (Brady and Weil, 1999). The distribution of these micropores can be attributed to the original arrangement of fibers from growth (Varas-Muriel and Molina-Ballesteros, 2004) and from the secondary voids left amid fibers after dissolution of mineral grains (Arribas et al, 2004; Grillet et al, 1988). Therefore, fibrous clays in petrocalcic horizons may increase the neoformation of barite through sorption of water and other ions, and potentially providing areas suitable for microorganisms.…”

Role of canonical biplot method in the study of building stones: an example from Spanish monumental heritage

“…(mostly of a siliceous composition), while the other four samples are more strongly related to CaO, MgO and volatiles (H 2 O and CO 2 ) (mainly of carbonate composition). These differences indicate an important diagenetic process (first diagenesis) related to replacement of the original silicates by dolomitic carbonates (Varas et al, 2002;Varas and Molina, 2004).…”

“…These changes also influenced the porosity of the stone. The total porosity ranges from 30.40% (siliceous sandstones) to 17.22% (carbonate sandstones) (Varas et al, 2003;Varas and Molina, 2004). Similar diagenetic processes have been studied for tertiary materials from the Arabian Gulf (Khalaf, 1990;Khalaf and Abdal, 1993), and more recently for the Palaeogene of Portugal (Pimentel, 2002).…”

“…The brown variety also departs, showing a second step in the diagenesis process (second diagenesis) which these materials undergo. Such a step coincides with development of palygorskite (Mg and Al silicate) (Varas and Molina, 2004). The three sandstone varieties formed upon this second diagenetic evolution (striped, nodular and white varieties) show similar contents of silica and alumina.…”

“…The two quarries define a weathering profile with three levels of diagenetic transformation (Figure 2) (Varas and Molina, 2004):…”