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, F; MATHEOS, S; QUERALT, I.

Barcelona

Simposio; 6th International Symposium on Andean Geodynamics; 2005

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. Ticker and finer-grained deposits prevail to the north and northeast. They were formed under arid conditions in a wadi-sand flat-playa fluvial system. This distal facies association indicates increased accommodation owing to high rates of subsidence relative to coarse siliciclastic sedimentation rates. These low-gradient deposits are characterised by cyclic alternations of mud-dominated and sand-dominated packages interpreted as high- and low-accommodation systems tracts. The overall fining upward stacking pattern of the Tordillo Formation suggests a change towards higher accommodation rates. This is accompanied by frequent development of soil horizons and darker primary and reworked pyroclastic deposits. These attributes indicate a stronger explosive volcanic activity associated with increased precipitation and high water table emplacement towards the end of the Tordillo lowstand wedge. The pebble composition of conglomerates, the framework composition of sandstones and pelitic rocks, and the major-element and trace-element geochemistry of sandstones, mudstones and prymary pyroclastic deposits are evaluated to determine the provenance and tectonic setting of the sedimentary basin. Conglomerates and sandstones derived almost exclusively from volcanic sources. The stratigraphic sections located to the south show a clast population of conglomerates dominated by silicic volcanic fragments and predominance of feldlitharenites. This framework composition records erosion of Triassic-Jurassic synrift volcaniclastic rocks and basement rocks from the Huincul arch, exhumed as a result of late Jurassic inversion. Towards the northwest, the conglomerates show a large proportion of mafic and acidic volcanic rock fragments, and the sandstones are characterised by a high content of mafic volcanic lithoclasts and plagioclase. These data suggest that the source of these sandstones and conglomerates was primarily the Andean magmatic arc located to the west of the Neuquén basin. The clay mineral assemblage is interpreted as the result of combination of a complex set of factors, such as source rock, climate, transport and diagenesis. Post-depositional processes produced significant variations with respect to original compositions, especially in fine-grained deposits. 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.