CONICET | Buscador de Institutos y Recursos Humanos

Facies analysis of the upper member of the Agrio Formation (Hauterivian - Barremian) in Loma La Torre, central Neuquén Basin allowed the definition of an open marine ramp, mainly characterized by the accumulation of fine-grained sediments of the basinal to outer ramp settings, with subordinated mid to inner ramp silts, carbonate sands and carbonate buildups. Accumulation conditions for the fine-grained facies match with a low energy environment, below storm-wave base level, poor to null oxygenation of the substrate, and alternating conditions of siliciclastic input and micritic carbonate production. Carbonate-rich facies (wackestones, packstones, grainstones and biolitites) as well as siltstones and heterolithic intervals may have accumulated under more oxygenated and moderate energy conditions, with some evidence of distal orbital flows. An oxygenation curve was traced from the combined analysis of ichnofacies and sedimentary facies. This curve shows that diversity of ichnofossils is controlled not only by the oxygenation of the substrate, but also by its nature, or even the energy of the environment. Vertical association of facies and stratal geometry allowed the definition of five stratigraphic sections, which were grouped into three depositional sequences related to third order cycles of sea-level change. Each sequence is characterized by a basal transgressive interval (TST) followed by a highstand systems tract (HST). The TSTs are relativelly thick and show an aggradational stacking of outer ramp to basinal deposits. The HSTs are characterized by shallower deposits that may show either a strong progradational arrangement, evidenced by low angle clinoforms, or a sub-horizontal stratal pattern with a shallowing upward trend. The progradational arrangement of HSTs indicates low rates of accommodation/siliciclastic input or accommodation/carbonate production. This contrasts with transgressive intervals, in which the increase of these rates cause a starved stage especially in the deepest portions of the basin. The relatively shallower position of the HST deposits allowed the development of high frequency cycles which seem to be related with high-order eustatic oscillations. In theses cycles, transgressive periods, characterized by anoxic conditions and saturated substrates, pass vertically into highstands represented by more oxygenated and looser substrates in which benthic organisms spread, supplying significant volumes of carbonate sand.

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