Sedimentologic and sequence stratigraphic model of a Neocomian marine carbonate-siliciclastic ramp: Neuquen Basin, Argentina

Facies analysis of the upper member of the Agrio Formation (Hauterivian-Bar remian) in Loma La Torre, central Neuquen Basin, allowed the definition of an open marine ramp mainly characterised by the accumulation of fine-graine d sediments of the basinal to outer ramp settings, with subordinated mid to inner ramp silts. carbonate sands. and carbonate buildups. Accumulation co nditions for the fine-grained facies match with a low energy environment, b elow storm-wave base level, poor to null oxygenation of the substrate, and alternating conditions of siliciclastic input and micritic carbonate produc tion. Carbonate-rich facies (wackestones, packstones, grainstones, and biol itites) as well as siltstones and heterolithic intervals may have accumulat ed under more oxygenated and moderate energy conditions, with some evidence of distal. orbital flows. An oxygenation curve was traced from the combine d analysis of ichnofacies and sedimentary facies. This curve shows that div ersity of ichnofossils is controlled not only by the oxygenation of the sub strate, but also by its consistency 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 characterised by a basal transgressive interval (TST) followed by a high stand systems tract (HST). The TSTs are relatively thick and show an aggrad ational stacking of outer ramp to basinal deposits. The HSTs are characteri sed by shallower deposits that may show either a strong progradational arra ngement, evidenced by low angle clinoforms, or a subhorizontal stratal patt ern with a shallowing upward trend. The progradational arrangement of HSTs indicates low rates of accommodation/siliciclastic input or accommodation/c arbonate production. This contrasts with transgressive intervals, in which the increase of these rates cause a starved stage, especially in the deepes t portions of the basin. The relatively shallower position of the HST depos its allowed the development of high-frequency cycles that seem to be relate d to high-order eustatic oscillations. In theses cycles, transgressive peri ods, characterised by anoxic conditions and saturated substrates, pass vert ically into highstands represented by more oxygenated and looser substrates in which benthic organisms spread, supplying significant volumes of carbon ate sand. (C) 2001 Elsevier Science Ltd. All rights reserved.