Relative oxygenation of the Tithonian — Valanginian Vaca Muerta—Chachao formations of the Mendoza Shelf, Neuquén Basin, Argentina

Organic-rich sediments were deposited in the deeper sectors of the Neuquen Basin during the latest Jurassic and the Early Cretaceous. This paper presents the results of a detailed examination of these deposits in the northern-most extension of the basin, in the Mendoza Province, and explores their wider significance for palaeo-oxygenation studies. The Tithonian–Berriasian Vaca Muerta Formation, the primary source rock for the Neuquen Basin, comprises bituminous shales and interbedded limestones deposited during a major transgression. In the Valanginian, the beginning of a regressive phase enabled the development of shallow-marine carbonates to form the base of the Chachao Formation, which eventually led to extensive biohermal carbonates of the uppermost Chachao Formation. Along the length of the narrow N–S-trending Mendoza Shelf of the Neuquen Basin both units are well exposed, permitting detailed study of the stratigraphy, sedimentology, ichnology and palaeoecology. The analysis of the Tithonian–Valanginian succession in the Salado river valley shows that carbonate production increased up-section. Faunal associations are mostly limited to poorly diverse epibenthos and pseudoplankton in the lower part of the section (Vaca Muerta Formation), with increased diversity in the lower Chachao section, including shallow and deeper infaunal bivalves. A background level of laminated shales to Chondrites bioturbation is typical of anoxic–suboxic conditions. Micritic limestones and carbonate sandstones throughout the section commonly show the development of Thalassinoides suevicus. Relative oxygenation curves based on trace fossils and body fossils were developed and compared. There was a primary substrate control on trace fossil diversity and occurrence, with a primary oxygenation signal provided by body fossil evidence. Interpretation of the palaeo-oxygenation on the basis of trace fossil taxa alone, however, would lead to inaccurate results. This study, therefore, demonstrates the importance of integrated trace and body fossil analysis in the fuller understanding of black shales. Palaeoenvironmental analysis of oxygendeficient basins has been the subject of many detailed studies over the last 25 years, fuelled in part by the development of trace fossil models of relative palaeo-oxygenation following the initial seminal work by Rhoads & Morse (1971). In particular, Savrda & Bottjer (1986, 1987), Ekdale & Mason (1988), Allison et al. (1995) and Savrda (1995) have developed and synthesized models in which trace fossil diversity and burrow diameter are used as a proxy for relative oxygen levels within specific basins, with a direct relationship usually being recorded between increased diversity patterns/ burrow diameter and increasing levels of dissolved oxygen. This relationship has been tested in a number of ancient sedimentary basins following the development of the original Rhoads & Morse model, but little critical appraisal has been made of it. This is particularly true for the upper, oxygenated conditions, where the occurrence of wide burrow-diameter Thalassinoides From: VEIGA, G. D., SPALLETTI, L. A., HOWELL, J. A. & SCHWARZ, E. (eds) 2005. The Neuquen Basin, Argentina: A Case Study in Sequence Stratigraphy and Basin Dynamics. Geological Society, London, Special Publications, 252, 185–206. 0305-8719/05/$15.00 # The Geological Society of London 2005. and other diagnostic traces have been recognized as important indicators of increased oxygen levels. Other studies have concentrated on analysis of the diversity of body fossil macrobenthos in low oxygen environments (e.g. Duff 1974; Wignall 1990; Etter 1995). Together, body and trace fossil models have provided a useful guide to dissolved oxygen levels in specific basins. Recent studies have shown that the diversity response is more complex and difficult to determine solely with reference to oxygenation (e.g. Sageman & Bina 1997). Certain signals, such as the common development of opportunistic species and the adoption by benthos of pseudoplanktonic habit, are common in black shales (e.g. Wignall & Simms 1990; Doyle & Whitham 1991; Wignall 1993; Etter 1996). Such palaeoecological responses may reflect a primary low oxygen signal, as has been supposed in many studies. However, they may also indicate factors such as environmental instability and the presence of soupy substrates (Wignall 1993; Sageman & Bina 1997) or the capability of mass flow and other environmental factors in introducing oxygen-demanding trace makers in to otherwise anoxic basins (e.g. Follmi & Grimm 1990). The present study describes for the first time the palaeoenvironments of an important oxygen-deficient stratigraphic interval, the Vaca Muerta–lowermost Chachao formations of Tithonian–Valanginian age, in the Mendoza Shelf of the Neuquen Basin (Mendoza Province of Argentina) (Fig. 1). The Vaca Muerta Formation in particular is considered to be the primary source rock for the important Neuquen oilfield in Argentina, and yet it has not, to date, received extensive study in this field. Previous works on the palaeontology and stratigraphy of the Vaca Muerta Formation have been confined to the stratigraphy, aspects of particular macroinvertebrate and vertebrate fossil groups, and details of the sedimentology (e.g. Leanza et al. 1977; Leanza 1981; Leanza & Zeiss 1990; Gasparini et al. 1997, 1999, 2002; Spalletti et al. 1999). The Chachao Formation, and particularly its oxygen-deficient lower part, has received similarly scant attention (Mombru et al. 1978; Legarreta et al. 1981; Legarreta & Kozlowski 1981). The aim of this paper is to describe in detail the palaeoenvironments of the Vaca Muerta and lower Chachao formations from an examination of trace fossils and benthic macropalaeontology, and to compare the oxygenation signals provided by both data sources in order to test the veracity of the trace fossil oxygenation model. Late Jurassic–Early Cretaceous palaeogeography, stratigraphy and sequence stratigraphy of the