THE GENUS HARPAGODES GILL (GASTROPODA, STROMBOIDEA) IN THE LOWER CRETACEOUS OF THE NEUQUÉN BASIN, ARGENTINA

CATALDO, S.C. Y LAZO, D.G.

Neuquén

Congreso; XVIII Congreso Geologico Argentino; 2011

Asociación Geológica Argentina

In the Agrio Formation of the Neuquén Basin, the family Pleurotomariidae is represented by a single species, Pleurotomaria gerthi Weaver, 1931. Here, the species is redescribed and reillustrated on the basis of newly collected specimens from the Pilmatué Member of the Agrio Formation sampled at nine localities in the Neuquén province, Argentina (Fig. 1), including its type locality. Our aims are as follows: (1) to revise the taxonomy of P. gerthi, (2) to date accurately the studied material based on a detailed ammonoid zonation, in order to establish the stratigraphic range of P. gerthi, (3) to analyse the taphonomy and facies relationships of the studied material to reconstruct the associated palaeoenvironments, (4) to assess the palaeobiogeographic distribution of the genus Pleurotomaria during the Early Cretaceous and estimate possible affinities of P. gerthi with the pleurotomariids of other Cretaceous marine basins. The emended diagnosis of P. gerthi is as follows: shell gradate-trochiform, slightly umbilicate, with angulate spire whorls and biangulate last whorl, the lower angulation corresponding to the periphery; upper angulation sculptured with rounded, heavy and transversely elongate nodes, forming a shoulder that delimits a wide and steeply sloping sutural ramp; suture moderately abutting; periphery with rounded nodes; selenizone broad and flattened, just below shoulder and slightly above mid-whorl; fine spiral cords and very thin growth lines complete the sculpture; aperture nearly oval, obliquely elongate. The studied specimens were compared to the holotype and found to conform in morphology. P. gerthi is clearly distinct from all other Early Cretaceous records of Pleurotomaria. P. gerthi-bearing levels are dated using a detailed ammonoid zonation (Aguirre-Urreta et al., 2007) as being of Late Valanginian to Early Hauterivian age. Taphonomic and facies-relationships studies carried out at three of the nine sections of the Pilmatué Member (Bajada Vieja, Cerro Birrete and Cerro Marucho, Fig. 2) show that this species inhabited shallow marine settings above storm weather wave base. Associated palaeoenvironments are interpreted as inner shelf and middle carbonate ramp deposits under the influence of storm currents and waves. P. gerthi lived on well-oxygenated soft sandy, fine carbonate and patchy bioclastic substrates, under warm-temperate waters of normal marine salinity, within the photic zone. It was part of a high diverse macrofaunal assemblage including cephalopods, bivalves, other gastropods, serpulids, echinoids and coral colonies. We infer that P. gerthi was a generalist regarding substrate composition, as it is recorded in a variety of substrates ranging from sandy to fine carbonate and patchy-bioclastic. Our study shows that its preferred habitat was the middle carbonate ramp, below the fair-weather wave-base, with low to moderate energy from episodic storm waves and currents. Other works reveal similar environmental findings for Pleurotomaria (e.g., Kiel y Bandel, 2004; Begg and Grant-Mackie, 2003). P. gerthi is endemic to the marine Andean basins of the western margin of southern South America, being present in the Neuquén Basin (west-central Argentina) as well as in the Aconcagua-Central Chile and Chañarcillo (northern Chile) Basin (Biró-Bagóczky, 1964; Pérez y Reyes, 2000). The oldest record of P. gerthi is from the Tithonian of the Aconcagua-Central Chile Basin (Lo Valdés Formation), whilst in the other two depocentres, Neuquén and Chañarcillo, it appears during the Late Valanginian and Hauterivian, respectively. Therefore, it seems that this species first differentiated in populations inhabiting central Chile, and later migrated northwards into Chañarcillo and south-eastwards into Neuquén. The absence of P. gerthi earlier in the Neuquén Basin (during the Tithonian-Barremian interval) is due to the development of very different facies there in comparison to those present in central Chile. Whilst in the Aconcagua-Central Chile Basin clastics and carbonates accumulated in littoral and sub-littoral settings, in central Neuquén and southern Mendoza provinces (Argentina) deep neritic shales and marls predominated (Hallam et al., 1986). This environmental condition must have precluded the development of pleurotomariid fauna in this area until the Early Valanginian. In the southern part of the Neuquén province a shallow-water carbonate depositional system developed during the Tithonian-Early Barremian interval. Here, other pleurotomariid species has been recorded: Bathrotomaria cf. provincialis and an internal mould of Pleurotomariidae gen. et sp. indet. appeared in Tithonian beds (pers. obs.). Despite its endemism, species showing the closest morphological resemblance to P. gerthi are those from the Tethys. The genus Pleurotomaria has a pandemic distribution during the Berriasian-Barremian interval, with records in peri-Tethyan basins in Europe and India, as well as in north and south temperate marine basins in England and the western margin of South America. In spite of its endemism, P. gerthi shows the closest morphological resemblance to contemporary Pleurotomaria species from the Tethyan Realm.         Aguirre-Urreta, M.B., Amaro Mourgues, F., Rawson, P.F., Bulot, L.G. and Jaillard, E. 2007. The Lower Cretaceous Chañarcillo and Neuquén Andean Basins: ammonoid biostratigraphy and correlations. Geological Journal 42: 143-173. Begg, J.G. and Grant-Mackie, J.A. 2003. 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