MULTIPLE AUTHIGENESIS OF K-FELDSPAR IN SANDSTONES - EVIDENCE FROM THECRETACEOUS AREADO FORMATION, SAO-FRANCISCO BASIN, CENTRAL BRAZIL

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 o f microfractures, and replacement of detrital EC-feldspar and plagiocl ase at very shallow depths. Authigenesis was related to meteoric disso lution of detrital volcanic feldspars, volcanic vitric rock fragments, and micas in a hot semiarid climate. (2) In the Late Cretaceous, whil e still at shallow depths, K-feldspar precipitated as abundant fine cr ystals associated with opal-CT, hematite, and mixed-layer illite/smect ite. The feldspar has extensively filled the intergranular and, to a l esser extent, intragranular pores, and replaced mud intraclasts and vo lcanic rock fragments. The authigenesis in this second episode was rel ated to percolation of meteoric waters through the overlying Upper Cre taceous alkaline volcanic racks of the Mata da Corda Formation, as ind icated by the delta(18)O(SMOW) of K-feldspars from 19.4 to 21.8 parts per thousand.