Near-surface K-feldspar precipitation in Cretaceous sandstones from the Potiguar Basin, northeastern brazil

The Cretaceous fluvial sandstones of the Acu Formation, Potiguar Basin, northeastern Brazil, contain abundant, very early, authigenic K-feldspar overgrowths around detrital orthoclase and microcline grains. Detailed optical and cathodoluminescence petrography, scanning electron microscopy, and electron microprobe analyses provided information about the conditions and timing of precipitation of these overgrowths, as well as of the associated diagenetic phases. The overgrowths are optically discontinuous with the detrital grains, untwined, nonluminescing, and were formed clear on grain surfaces under phreatic conditions. The overgrowths commonly show a rhombohedral (sanidine) habit, range in thickness from about 0.4 to 1.0 mm, are chemically pure KAlSi3O8, and constitute up to 6% of the whole rock. The extremely early origin of the K-feldspar overgrowths is indicated by their coating of mechanically infiltrated smectitic clays. Authigenic smectite rims, booklet and vermicular aggregates of kaolinite, and precompactional calcite cement also cover, and thus postdate, the overgrowths. The K, Si, and Al ions needed to form the K-feldspar overgrowths were probably derived from the hydrolysis (dissolution and replacement by clay minerals) of detrital feldspars and micas in the coarse, alluvial Acu Formation deposits along the basin margins. Meteoric fluids with high (alphaK(+))/(alphaH(+)) and alphaSiO(2aq) evolved through evaporative concentration along the groundwater circulation pathways throughout the basin. Relatively high pH and (alphaK(+))/(alphaH(+)) ratios were required to precipitate authigenic K-feldspar within the K2O-Na2O-Al2O3-SiO2-H2O system at around 30degreesC and 1 atm. Subsequent smectite and kaolinite authigenesis occurred under decreasing pH and (alphaK(+))/(alphaH(+)) ratio, related to K+ consumption, episodic meteoric infiltration, and climatic shifts toward more humid conditions during the deposition of the Acu Formation. The on-course radiometric dating of the early diagenetic K-feldspar overgrowths offers the possibility to solve the depositional age of the Acu Formation and other continental clastic sequences, for which biostratigraphic dating is imprecise.