Abstract
The initial syn-rift infill (Upper Triassic-Lower Jurassic) of the Neuquen Basin involved a complex stacking of lava, pyroclastic and sedimentary units, in which volcanic rocks compose up to 72% of the infill. The high-quality outcrops of the southern Sanico depocentre, which was uplifted and exposed during the Andean orogeny, were studied in order to determine the lithofacies arrangements, the tectonostratigraphic framework and the main controls over the syn-rift succession. The tectonostratigraphic study made it possible to determine three sections characterized by the predominance of effusive, pyroclastic or sedimentary deposits. These sections indicate three different evolutionary stages of the volcanic rifting. The onset of this rift stage was dominated by effusive rocks with minor ignimbrite and alluvial volcaniclastic units, which indicates the occurrence of composite volcanoes at the boundaries of the depocentre. The mid-rift stage was characterized by the development of fault-controlled, large ignimbrite deposits intebedded with volcaniclastic alluvial deposits and lava units, which suggests a volcanotectonic depression related to asymmetrical and incremental subsidence. The end of the volcanic rifting event was marked by a decrease in the volume of the volcanic rocks and the occurrence of low-relief effusive aggradational volcanic edifices and lava fields in close relation with alluvial, deltaic and lacustrine units. The analysis of the synrift sequence in the Sanico depocentre showed the complex relationships between tectonic and volcanic controls which occurred in different magnitudes and times during the rifting evolution.
Highlights
The Neuquén Basin is located in the Andean margin between 32 and 40°S latitude (Fig. 1)
Most of the features of the volcanic and sedimentary units defined were described for many composite volcanoes in the world, such as the compositional range from andesitic to rhyodacitic products dominated by thick andesitic lava successions (Cas and Wright, 1987), the pyroclastic current deposits that filled valleys (Davidson and De Silva, 2000), major degradational events composed of volcanic debris avalanches (Schneider and Fisher, 1998; Reubi and Hernández, 2000; Belousov et al, 1999; Clavero et al, 2002; Shea et al, 2008), large channelized volcaniclastic alluvial units (Palmer et al, 1993; Zanchetta et al, 2004) and minor acid lava domes and necks that worked as satellite vents (Davidson and De Silva, 2000)
The Sañicó depocentre is a extensional half-graben with a NW-SE orientation and oblique internal tectonic features that were active during the early rifting of the Neuquén Basin
Summary
The Neuquén Basin is located in the Andean margin between 32 and 40°S latitude (Fig. 1). The aims of this study are to carry out a tectonostratigraphic analysis of the volcanic syn-rift deposits cropping out in the Sañicó depocentre -to the south of the Neuquén Province, Argentina- focusing both on the depositional processes and on the relationship between tectonic and volcanic controls in the development of the sedimentary sequence, and to devise an evolution scheme of the depocentre. The stratigraphy of the initial syn-rift of the basin is complex, involving a continental volcanosedimentary succession known as the Precuyano Cycle (Gulisano et al, 1984; Gulisano, 1993), which is constituted by different lithostratigraphic units (Franzese and Spalletti, 2001). These deposits were grouped into the Piedra Pintada Formation, which is composed of coeval siliciclastic and carbonate deposits (Fig. 2B; Herbst, 1966; Stipanicic et al, 1968; Stipanicic, 1969) of Pliensbachian age (Damborenea et al, 1975; Damborenea and Manceñido, 1993), which were interpreted as shallow-marine siliciclastic and carbonate deposits that change laterally and upward into off-shore deposits (Gulisano and Pando, 1981; Gulisano et al, 1984)
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