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Geological Society, London, Special Publications; 2005; v. 252; p. 231-250;
DOI: 10.1144/GSL.SP.2005.252.01.11
© 2005 Geological Society of London

Sedimentary Geology and Cyclostratigraphy in Offshore Deposits

Hemipelagic record of orbitally-induced dilution cycles in Lower Cretaceous sediments of the Neuquén Basin

Guillermina Sagasti

Centro de Investigaciones Geológicas, CONICET - UNLP, Calle 1 No. 644, B1900TAC La Plata, Buenos Aires, Argentinagsagastic{at}repsolypf.com

The lower Valanginian-lower Barremian Agrio Formation in the Mendoza region of the Neuquén Basin (Argentina) accumulated in a hemipelagic outer ramp/basin setting. The formation records the alternation of periods of fine-grained carbonate accumulation 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, Al and K, and less than 60% CaCO3, whereas carbonate hemicycles contain relatively low quantities of detrital elements (Fe, Al and K), and more than 60% CaCO3. Fluctuations in terrigenous supply to the basin occurred in response to the alternation 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 hemicycles. The ultimate mechanism that controlled the alternation of the dark and light intervals was orbital forcing, as the latitudinal shifting of climate zones was driven by the c. 21 000 years precessional cycle.