The relationship between the continental deposits of a lowstand wedge and the tectonic and magmatic processes at the pacific continental margin: example from the Upper Jurassic of the Neuquén Basin, Argentina.

SPALLETTI, L.; COLOMBO PIÑOL, F.; MATHEOS, S. D.; QUERALT, I.

Barcelona (España)

Simposio; 6th International Symposium on Andean Geodynamics; 2005

ISAG - Universitat de Barcelona

The Upper Jurassic fill of the backarc Neuquén Basin includes a lowstand wedge known as the Tordillo Formation. The studied deposits crop out along a N-S oriented belt that runs parallel to the Andean magmatic arc. They are limited to the south by the east-west oriented positive structure of the Huincul arch formed as a result of Upper Jurassic tectonic inversion.  The Tordillo deposits were formed in an arid fluvial-dominated system characterised by systematic downstream changes in architectural style. A gravelly and sandy bedload fluvial system is recognised in the southern upstream sector. The reduced thickness and the coarse grain size suggest steep gradients, excess of bedload supply and a low subsidence rate. The Tordillo sediments are characterised by moderate SiO2 contents, variable abundances of K2O and Na2O, and a relatively high proportion of ferromagnesian elements. The degree of chemical weathering in the source areas, expressed as the chemical index of alteration (CIA), was low to moderate. The major element geochemistry and the Th/Sc, K, Rb, Co/Th, La/Sc and Cr/Th values point to a significant input of detrital volcanic material of calcalkaline felsic and intermediate composition. However, major element geochemistry was not useful to interpret tectonic setting. Discrimination plots based on immobile trace elements, such as Ti, Zr, La, Sc and Th, showed that most data lie in the field for active continental margin. Yet, geochemical information was not sufficiently sensitive to differentiate between the two different source areas recognised by petrographic and modal analyses of conglomerates and sandstones.