ABSTRACT

Abundant authigenic K-feldspar is present in continental sandstones of the Lower Cretaceous Areado Formation in Sao Francisco Basin, central Brazil. Feldspar authigenesis occurred in two major episodes. (1) In the Early Cretaceous K-feldspar precipitated as overgrowths, healing of microfractures, and replacement of detrital K-feldspar and plagioclase at very shallow depths. Authigenesis was related to meteoric dissolution of detrital volcanic feldspars, volcanic vitric rock fragments, and micas in a hot semiarid climate. (2) In the Late Cretaceous, while still at shallow depths, K-feldspar precipitated as abundant fine crystals associated with opal-CT, hematite, and mixed-layer illite/smectite. The feldspar has extensively filled the intergranular and, to a lesser extent, intragranular pores, and replaced mud intraclasts and volcanic rock fragments. The authigenesis in this second episode was related to percolation of meteoric waters through the overlying Upper Cretaceous alkaline volcanic rocks of the Mata da Corda Formation, as indicated by the δ¹⁸O_SMOW of K-feldspars from 19.4 to 21.8‰.