Abstract: The lower Valanginian-lower Barremian Agrio Formation in the Mendoza

region of the Neuqutn Basin (Argentina) accumulated in a hemipelagic outer amp/basin setting. The formation records the alternation ofperiods of fine-grained carbonate accumu-lation and periods of fine-grained clastic deposition, which resulted in a strongly rhythmic succession in which dark clastic hemicycles alternate with light carbonate hemicycles. Clastic and carbonate hemicycles show distinct geochemical signatures that reflect cyclic fluctuations in terrigenous influx to the basin. Clastic hemicycles have relatively high percentages of Fe, A1 and K, and less than 60 % CaCO3, whereas carbonate hemicycles contain relatively low quantities of detrital elements (Fe, AI and K), and more than 60% CaCO3. Fluctuations in terrigenous supply to the basin occurred in response to the alterna-tion between two contrasting climate regimes and resulted in the formation of dilution cycles. Warm temperate (winter-wet) climate conditions led to the accumulation of clastic hemicycles, whereas arid conditions promoted the deposition of carbonate hemi-cycles. The ultimate mechanism that controlled the alternation ofthe dark and light intervals was orbital forcing, as the latitudinal shifting of climate zones was driven by the c. 21 000 years precessional cycle