Paleoclimatic implicatons (Late Cretaceous-Paleogene) from micromorphology of calcretes, palustrine limestones and silcretes, southern Paraná Basin

TOFALO, O. R Y PAZOS, P. J

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

Elsevier

Año: 2010 vol. 29 p. 665 - 675

0895-9811

Sedimentologic and petrographic analyses of outcroping and subsurface calcretes, palustrine carbonates, and silcretes were carried out in the ParanáBasin (western Uruguay) and in the Santa Lucía Basin (southern Uruguay). Since these rocks contain significant information about palaeoenvironmental and palaeoclimatic evolution, the aim of this work is to provide their microfabric and genesis, through detailed analyses.                 The main calcrete and silcrete host rock (Mercedes Formation) is represented by a fluvial thinning upward succession of psephitic and psammitic deposits, with isolated pelitic intervals. Most of the limestones here studied are macroscopically massive and they have micromorphological features of alpha fabric, originated by a displacive growth of calcite in the host clastic material, due to evaporation, evapotranspiration and degassing. Micromorphologycally calcretes indicate an origin in the vadose and phreatic diagenetic environments. Micrite is the principal component, and reflects a rapid precipitation in the vadose zone from supersaturated solutions. The abundance of microsparite and secondary sparite is regarded to dissolution and re-precipitation processes. Subordinate brecciated calcretes were recognized. They are common in vadose diagenetic environments, where taking place the alternation between cementation and non-tectonic fracturing conditions. All of these processes originated episodes of fragmentation, brecciation and cementation. Fissures are filled with clear primary sparitic calcite, formed by precipitation of extremely supersaturated solutions in a phreatic diagenetic environment. The micromorphological characteristics indicate that calcretes resulted by vertical and lateral migration of the groundwater. The lack of undisputable evidence of beta fabric, which is recognised by the dominance of biogenic structures, suggest they were formed in zones of little biological activity or that the overimposed processes related to the water table movements produced intense re-crystallization and the biogenic fabric was completely obliterated. The limestones containing gastropod are ever situated to the top of the limestone successions, but they are geographicaly restricted. They exhibit brecciate and peloidal-intraclastic textures, lacking lamination, edaphic structures, aggregates and vertical rhizoliths, which indicates they correspond to low-energy palustrine deposits. They were generated in shallow, local and ephemeral ponds developed in topographic depressions, and are thus less regional extended than calcretes. When the levels of the water table droped, the palustrine deposits were exposed, it accounts for the presence of terrestrial gastropods, seeds and insect nests. The combination of calcretes and palustrine carbonates indicates periods and areas with a reduced clastic input and a predominantly semiarid climate, with well-defined humid and dry seasons. The later development of massive and nodular horizons of silcretes, preserving the internal structure of host rocks, the small areal extent, the formation of massive lenses, complex pore infillings, and the lack of a columnar upper section, indicates that silcretes were generated by silicification from groundwaters. Every silcretized horizon indicates different positions of the water table and relates to the dissection of palaeosurfaces. The age of calcretization and silcretization is bracketed between the late Cretaceous (Campanian-Mastrichtian) and de early Eocene. The last one represented by Ultisoils intensely lateritized. Paleoclimate indicates changing conditions from warm and humid (end of Cretaceous) to warm and dry (Palaocene) and subtropical and seasonal in the early Eocene.