Journal of Conference Abstracts

The Chachil Limestone and its Significance in the Early Evolution of the Neuquen Basin (Cerro Chachil, Neuquen, Argentina)

Irene Gomez-Perez (irene.gomez@casp.co.uk)¹ & Juan Franzese (franzese@cig.museo.unlp.edu.ar)²

¹ CASP (Cambridge Arctic Shelf Programme), Department of Earth Sciences, West Building, UK

² CIG (Centro de Investigaciones Geologicas), Calle 1, n. 644, 1900 La Plata, Argentina

The Chachil Formation includes shallow marine platform limestones and sandstones of Pliensbachian age (Early Jurassic). It crops out around the Cerro Chachil mountains, in the SW part of the Neuquen Basin, west central Argentina. The Chachil Formation lies on continental to restricted marine early Jurassic volcaniclastic deposits, or on Palaeozoic igneous or Triassic volcanic rocks. Its basal boundary can be both conformable or unconformable. It shows a gradual transition to overlying basinal deep marine shales and turbiditic sandstones.

The Chachil Formation includes a basal sandstone unit, representing proximal clastic shallow-water marine environments, and an upper limestone unit representing carbonate ramp environments. A basal conglomerate locally represents a transgressive lag. The limit between both units is transitional to locally unconformable, and the general sequence is deepening upward. The basal siliciclastic platform environments (lower unit) include high-energy troughs where cross-bedded fossiliferous carbonate sandstones were deposited, and low-energy areas with shale deposition. Carbonate ramp environments (upper unit) include: 1) inner ramp: high-energy mixed siliciclastic-carbonate environments, represented by sandy rudstones and grainstones and calcareous sandstones, and low-energy paralic environments, represented by mudstones and carbonaceous shales; 2) mid ramp: low-mid energy carbonate environments, represented by bioturbated mudstones; and 3) outer ramp: low-energy environments, represented by bioturbated mudstones and shales and turbiditic packstones.

The Chachil limestone represents the first open marine transgression into this area of the Neuquen Basin, and an important change in the subsidence pattern of the basin: the underlying volcaniclastic rocks are restricted to tectonically controlled depocentres, but the Chachil Formation covers areas that were previously tectonic highs. A general tilting to the SW is shown by facies and thickness distribution. Although some minor tectonic activity occurred during the deposition of the Chachil Formation, and younger sediments onlap basement palaeohighs, this formation represents the change from the syn-rift to the post-rift stage in the evolution of the SW area of the Neuquen Basin.

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