Diego A. Kietzmann

Evidence of precessional and eccentricity orbital cycles in a Tithonian source rock: The mid-outer carbonate ramp of the Vaca Muerta Formation, northern Neuquen Basin, Argentina

Basin to mid-ramp cyclic facies of the Tithonian Vaca Muerta Formation are exposed in the Loncoche Creek section of the Neuquen Basin, Mendoza province, Argentina. This unit is characterized by a decimeter-scale rhythmic alternation of marls, shales and limestones and extends from the lower Tithonian to the upper Berriasian. Cyclostratigraphic studies based on a detailed facies analysis allowed the identification of cyclic patterns with frequencies within the Milankovitch band. According to biostratigraphic data, the dominant cycle in the studied section has a period of 20 k.y., which correlates with the Earths axis precession element. Spectral analysis based on a series of compacted and decompacted cycle thickness identified a subordinate frequency of about 90 to 120 k.y., which we interpret as the modulation of the precessional cycle caused by the Earths orbital eccentricity. The strength of the precession signal, together with the absence of a well-defined cyclicity attributable to the obliquity orbital cycle (i.e., 40 k.y.), is in agreement with previous data from the Northern Hemisphere.

Crustacean microcoprolites from the Upper Jurassic-Lower Cretaceous of the Neuquén Basin, Argentina: Systematics and biostratigraphic implications

As a result of a microfacial study in the outer and middle ramp deposits of the Vaca Muerta Formation (lower Tithonian-upper Valanginian), four ichnotaxa of crustacean microcoprolites are described: Palaxius azulensis Kietzmann isp. nov., Palaxius caracuraensis Kietzmann isp. nov., Helicerina? isp. A. aff. Helicerina siciliana and Helicerina isp. B. They represent one of the first records of crustacean microcoprolites for the Neuquén Basin and Argentina. Helicerina is reported for the first time from the Upper Jurassic and Lower Cretaceous of South America. It is inferred that Palaxius ichnospecies were produced by callianassids, while Helicerina ichnospecies could be produced by decapods of Mecochiridae, Erymidae, and/or Nephropidae affinity. Two assemblages of crustacean microcoprolites are recognised, a middle Tithonian to lower Berriasian Palaxius-dominated assemblage and an early to late Valanginian Helicerina-dominated assemblage.

The Vaca Muerta–Quintuco system (Tithonian–Valanginian) in the Neuquén Basin, Argentina: A view from the outcrops in the Chos Malal fold and thrust belt

The Vaca Muerta–Quintuco system (uppermost lower Tithonian–lower Valanginian) is a thick shallowing-upward sedimentary cycle consisting of dark bituminous shales, marlstones, limestones, and sandstones, cropping out in the Neuquen Basin, west–central Argentina. This paper analyzes three outcrop sections in Chos Malal area, northem Neuquen province. Detailed facies analysis allows us to differentiate six facies associations, representing basinal to proximal outer ramp facies of a homoclinal carbonate ramp system (Vaca Muerta Formation) and basinal to shoreface facies of a mixed carbonate–siliciclastic shelf system (Quintuco Formation), prograding westward from the eastern margin of the basin. Two sequence hierarchies were recognized: 5 composite depositional sequences (third order) and 15 high-frequency sequences (fourth order). Fluctuations in organic matter content within the Vaca Muerta Formation suggest a relationship with depositional sequences, finding the highest values associated with transgressive systems tract, whereas the transition to the Quintuco Formation shows a strong decrease in total organic carbon. The x-ray diffraction studies show an increase of clay minerals and quartz in the transgressive systems tract of the Vaca Muerta Formation and an increase in the content of calcite in highstand systems tracts. This pattern is reversed in the Quintuco Formation. Our sequence stratigraphic approach contributes to the understanding of the relationship between organic matter, clay minerals, facies, stacking pattern, and relative sea level changes in this exceptional shale oil and shale gas unconventional reservoir. This study may be helpful for a better postulate of petrophysical and geomechanical models for unconventional exploration.