THE GENUS STEINMANELLA CRICKMAY (BIVALVIA) IN THE TRANSITION BETWEEN THE VACA MUERTA AND MULICHINCO FORMATIONS, EARLY VALANGINIAN, NEUQUÉN BASIN, ARGENTINA
Abstract
The genus Steinmanella Crickmay is recorded in several Lower Cretaceous marine units of the Neuquén Basin. It is particularly abundant at the transition between the Vaca Muerta and Mulichinco formations. This paper presents a taxonomic revision of this fauna, discusses the stratigraphic range of each species, and describes and interprets the associated lithofacies based on detailed field sections, newly collected specimens and revision of previous fossil collections. Four sections were measured in northern Neuquén comprising the upper beds of the Vaca Muerta Formation and the lower ones of the Mulichinco Formation. The Steinmanella fauna was dated accurately on the basis of a precise ammonoid zonation. The species were found in the Neocomites wichmanni and Lissonia riveroi zones of early Valanginian age. Specimens were collected in situ from dark grey shales, alternating siltstones, wackestones and mudstones, and rudstones. Three species were identified: Steinmanella curacoensis (Weaver), S. quintucoensis (Weaver), and S. subquadrata sp. nov. Steinmanella curacoensis has an oval outline, poor demarcation of carinae, flank and corselet ribs partially intercalated at the median part of the valve, and oval escutcheon. Steinmanella quintucoensis has a furrow below the marginal carina, subrectangular outline, robustly tuberculated flank ribs, and lanceolate escutcheon. Steinmanella subquadrata sp. nov. has a subquadrate outline, well-marked carinae, and oval escutcheon ornamented with thin ribs and small elongated tubercles. Resumen. EL GÉNERO STEINMANELLA CRICKMAY (BIVALVIA) EN LA TRANSICIÓN ENTRE LAS FORMACIONES VACA MUERTA Y MULICHINCO, VALANGINIANO TEMPRANO, CUENCA NEUQUINA, ARGENTINA. El género Steinmanella Crickmay se registra en distintas unidades marinas del Cretácico Inferior en la Cuenca Neuquina. En particular, es muy abundante en la transición entre las formaciones Vaca Muerta y Mulichinco. Este trabajo presenta una revisión taxonómica de esta fauna, con una discusión del rango estratigráfico de cada una de las especies halladas y la descripción e interpretación de las litofacies asociadas, sobre la base de nueva información de campo disponible y revisión de colecciones previas. Se midieron cuatro perfiles en el norte de Neuquén, abarcando los últimos metros de la Formación Vaca Muerta y los primeros de la Formación Mulichinco. La fauna de Steinmanella fue datada con precisión mediante una zonación de amonoideos. La fauna fue hallada en facies de lutitas grises oscuras, limolitas, wackestones y mudstones alternantes, y rudstones. Se identificaron tres especies: Steinmanella curacoensis (Weaver), S. quintucoensis (Weaver), y S. subquadrata sp. nov. Las mismas fueron registradas en las zonas de Lissonia riveroi y Neocomites wichmanni, correspondientes al Valanginiano temprano. Steinmanella curacoensis presenta un contorno oval, carenas pobremente marcadas, costillas del flanco intercaladas con las del área y un escudete oval. Steinmanella quintucoensis presenta un surco bajo la carena marginal característico, contorno subrectangular, costillas del flanco con tubérculos robustos y un escudete lanceolado. Steinmanella subquadrata sp. nov. presenta un contorno subcuadrado, carenas bien demarcadas y un escudete oval con costillas delgadas y tubérculos elongados y pequeños.Downloads
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