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Abstract: Paleoenvironmental Interpretation and Organic Geochemistry of the
Agua de la Mula Member (Agrio Formation) in the Pampa Tril Area,
Neuquen Basin, Argentina
The scope of this work is to broaden the existing paleoenvironmental and stratigraphic knowledge of the Agrio Formation using sedimentology and organic geochemistry in a sequence-stratigraphy framework. Eleven facies (F1 to F11) were recognized in the 264-m thick Late Hauterivian limestones of the Agua de la Mula Member (Agrio Formation), in the Pampa Tril area (Neuquén Province).
Outcrop facies analysis enabled the interpretation of specific paleoenvironments of deposition within a mixed carbonatesiliciclastic marine ramp. Basal facies (F1) belong to a low-energy marine environment with limited siliciclastic input evidenced by wackestones and mudstones with collapsed ammonites. In F4, the abundance of organic matter (~2% total organic carbon or TOC) and bivalves (Neocomiceramus curacoensis), which lived on substrates under restricted oxygen levels suggests oxygen-stratified water conditions during sedimentation. F6 is composed of low organic matter content shales with alternating levels of calcareous concretions interpreted as being deposited in a low-energy anoxic environment with high siliciclastic input. In contrast, the uppermost facies (F9 to F11), which occupy higher stratigraphic positions and have lower organic matter content, show evidence of higher energy, likely associated to a wave-dominated and well oxygenated marine environment close to the shoreline.
The organic matter analysis performed along the stratigraphic column revealed TOC contents ranging from 0.54% to 3.07%. The hydrogen index (HI) values permitted the discrimination of three sample groups that plot in the trends of kerogens types I, II, and III. The microscope study of each kerogen trend reinforced the former division and allowed the recognition of specific organic matter components (amorphous, liptinite, woody, and coaly). The abundance of amorphous organic matter in group 1 samples, from Facies F4, demonstrates oxygen-stratified water conditions. The samples of the second group, belonging to F6, show a significant content of woody and coaly organic matter components, which represent an important terrestrial input. The thermal maturity estimation derived from measured vitrinite reflectance (%Ro) and the thermal alteration index (TAI) places the studied interval in the early oil-generation window. These results correlate to similar values obtained in nearby study areas.
The entire succession shows three cycles: two complete transgressiveregressive cycles overlying a third, incomplete, cycle at the base of the local stratigraphic column, within a succession deposited in an external mixed marine ramp setting. Each cycle comprises an initial set of retrogradational parasequences, followed by a maximum flooding surface (MFS) and a subsequent progradational parasequence set. The MFS was recognized on the basis of parasequence stacking patterns and the abundance of amorphous organic matter components, which indicate oxygen-stratified marine conditions and limited sediment input. The uppermost cycle is truncated by an erosive surface, which defines a sequence boundary, below the fluvial facies of the Lower Troncoso Member (Huitrín Formation).