Early diagenetic calcite replacement of evaporites in playa lakes of the Quiricó Formation (Lower Cretaceous, SE Brazil)

Abstract

Evaporites are abundant in modern playa lakes, where efflorescence, inorganic and/or bioinduced precipitation coexist with siliciclastic deposition. Overall, the sensibility of salts to slight physicochemical changes results in partial to complete dissolution of evaporites in surface and shallow subsurface. However, in some cases, salt minerals are pseudomorphically replaced, providing a snapshot to depositional settings and diagenetic processes of evaporite-bearing successions. This is recorded in the Quiricó Formation (Lower Cretaceous), Sanfranciscana Basin, southeastern Brazil. This studied succession portrays a playa lake system subdivided in saline flat and saline lake deposits, whose diagenesis was marked by ubiquitous cementation and replacement reactions. Petrographic and geochemical techniques were undertaken to characterise primary and secondary textures, mineralogy and distribution of mudstone- and sandstone-hosted pseudomorphs. The pseudomorphs frequently underlie the stratigraphic contact of playa lake-aeolian beds. Pseudomorphs occur as coalescent nodules and as chaotically-distributed chevron crystals, composed of inequicrystalline calcite mosaics. Nodular pseudomorphs have cubic/sub-cubic “ghost” inclusions and displacive contacts with the matrix, whereas the contacts of chevron pseudomorphs and the matrix are undisturbed. Both types exhibit morphological replication of precursor minerals and non-luminescent euhedral relicts. Stable isotopic signatures are covariant and range between −1.6–0.6 ‰ δ13C and 1.4–2.5 ‰ δ18O for nodular pseudomorphs, and −2.2 to −1.5 ‰ δ13C and 0.3–1.3 ‰ δ18O for chevron pseudomorphs. Nodular crystals reflect subaerial and intrasedimentary crystallisation of Ca-sulphates in saline flats, whereas chevron-shaped pseudomorphs indicate bottom-growth halite in shallow lakes. Textural evidences from pseudomorphs and host lithologies constrain the diagenetic overprint to surface and shallow burial settings (syndepositional/eodiagenetic stages). Despite multiple generations of calcite, the constant isotopic signatures of pseudomorphs reflect water chemistry fractionation in a closed-lake basin. Widespread calcitisation of playa lake sediments was caused by calcium-rich groundwaters, influenced by limestones from the Precambrian basement. The playa lake deposits record the onset of aridification processes that culminated in the Thermal Cretaceous Maximum.