Sciency Thoughts: Neuquén Basin

Showing posts with label Neuquén Basin. Show all posts

Saturday, 8 April 2023

Oceanic Phylloceratid Ammonites from the Middle Jurassic Neuquén Basin of Argentina.

The Neuquén Basin is a sedimentary basin to the east of the Andes in western Argentina, covering about 120 000 km². Here a series of deposits record repeated marine transgressions from the Jurassic well into the Tertiary, and is a significant oil reserve as well as a famous fossil-producing region. During the Middle Jurassic the basin was cut off from the Palaeopacific Ocean by a volcanic arc following the line of the modern Andes. In theory, this enclosed basin should have been devoid of open ocean organisms, such as Phylloceratid Ammonites, however, members of this group are occasionally reported from Middle Jurassic deposits within the Neuquén Basin, suggesting that a connection to to the open ocean was at least sporadically present.

In a paper published in the journal Revue de Paléobiologie, Genève on 15 January 2023, Bernard Jolly of Beaugency in France, Horacio Parent of the Laboratorio de Paleontología, at the Universidad Nacional de Rosario, and Alberto Garrido of the Museo Provincial de Ciencias Naturales 'Prof. Dr. Juan A. Olsacher', and the Centro de Investigación en Geociencias de la Patagonia of the Universidad Nacional del Comahue, describe Phylloceratid Ammonites belonging to three species from an exposure of the Middle Jurassic Los Molles Formation in Picún Leufú Department of Neuquén Province, Argentina.

The outcrop from which the specimens were collected lies about 7 km to the west of the point where National Road-40 crosses the Arroyo Picún Leufú stream. The rocks here are of Aalenian Age (between 174.1 and 170.3 million years old), with most of the sequence being covered, but several layers of black shale and fine, calcarious sandstone exposed.

(A) The Neuquén Basin with indication of the study area. (B) Geological map of the study area with indication of the studied section. Jolly et al. (2023).

The first three specimens described are juvenile phragmocones (chambered sections of the shell, where the Animal lived before reaching maturity) assigned to the species Phylloceras perplanum, which has previously been recorded from Hungary and France. These are compressed and very involute Ammonites, with smooth, slightly rounded sides. The Argentinian specimens are very close to the French and Hungarian specimens in both form and dimensions.

The next three specimens are three more juvenile phragmocones, referred to the genus Calliphyloceras, and considered to show affinities to the species Calliphyloceras achtalense, which has previously been recorded from Georgia (eastern Europe) and France. These are compressed specimens with constrictions perfectly matching specimens of Calliphyloceras achtalense from what would have been the ancient Tethys Ocean, although slightly smaller and with with a narrower umbilicus. The European/Tethyan specimens are also slightly younger, dating from the Bajocian (170.3 to 168.3 million years ago).

The final specimen described is an entire conche (outer shell) assigned to the genus Costiphylloceras, and assigned to a new species, Costiphylloceras limayense, in reference to the Rio Limay, into which the Picún Leufú flows. The new species is erected on the basis of the occurrence of pronounced ribs on the upper third of the flanks, particularly on the ventral side.

Phylloceratid Ammonites from the lower Aalenian of Picún Leufú; Los Molles Formation. (A)-(C): Phylloceras perplanum; juvenile phragmocones; (A) MOZ-PI-6640; (B) MOZ-PI-6641; (C) MOZ-PI-6642). (D)-(F) Calliphylloceras aff. achtalense; phragmocones; (D) MOZ-PI-6637; (E) MOZ-PI-6638; (F) MOZ-PI-6643). (G) Costiphylloceras limayense; phragmocone (MOZ-PI-6639). Jolly et al. (2023).

Monday, 27 December 2021

Patagoniapteris artabeae: A new species of Dipteridacean Fern from the Triassic of Argentina.

The Family Dipteridaceae today contains ten species of Ferns divided into two genera, found in tropical and subtropical areas of Asia, Australia, and the Pacific Islands. However, in the past they were much more widespread and diverse, with fossils known from all over the world. The oldest examples appear in the fossil record in the Middle Triassic, leading to speculation that they may have first evolved in the Early Triassic or even the Permian, and they reached their maximum diversity in the Late Triassic and Early Jurassic, after which they appear to have declined throughout the remainder of the Mesozoic, a pattern also seen in many other Fern groups.

The Dipteridaceae have distinctive bilobed, fan-shaped fronds, born on long stipes (leaf stems), with toothed margins and reticulate (meshed) veination. Sori (spore-bearing bodies) are scattered across the undersides of these fronds.

In a paper published in the journal Acta Palaeontologica Polonica on 29 November 2021, Silvia Cristina Gnaedinger of the Área de Paleontología at the Centro de Ecología Aplicada del Litoral of the Consejo Nacional de Investigaciones Científicas y Técnicas, and Ana María Zavattieri of the Departamento de Paleontología at the Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales, describe a new species of Dipteridacean Fern from the Paso Flores Formation of Neuquén Province in Argentina.

The Paso Flores Formation is found within the Neuquén Basin, a rift valley to the east of the Andes that formed in the Late Triassic and Early Cretaceous by extensional rifting on the western margin of the Gondwanan supercontinent. The formation is a sedimentary system recording a braided river system and alluvial fan. It overlies a late Palaeozoic volcanic system unconformably, and is itself overlain unconformably by Early Jurassic volcanic rocks. This formation has produced a wide range of Late Triassic fossil Plants, including three previous Dipteridacean Ferns.

Generalised outline of the Neuquén Basin, showing the location of the studied area of the Paso Flores Formation, Neuquén Province, Patagonia, Argentina. Gnaedinger & Zavattieri (2021).

The new species is named Patagoniapteris artabeae, where 'Patagoniapteris' means 'Patagonian Fern' and 'artabeae' honours Argentinian palaeobotanist Analia Artabe for her important contributions to the knowledge of the Triassic and Jurassic floras of Argentina. The species is described from a series of sterile and fertile frond fragment impressions. These fronds are dissected into two equal and opposite rachial arms, each one having more than 18 primary segments. The primary segments are fused together for the lower third of their length, the remaining parts are free and lance-shaped, with undulating to deeply dissected margins, The primary and secondary veins are simple, with the tertiary veins forming an irregular polygonal mesh.

Dipteridacean Fern Patagoniapteris artabeae from the upper Norian–Rhaetian, Upper Triassic Paso Flores Formation, Neuquén Province, Argentina. (A) MCF-PBPH 066, part of the frond showing fused and dissected portions. (A₁), (A₂) basal portion of a rachial arm with the lamina of the primary segments fused. (A₃) detail showing primary segments with the dissected portion of the lamina. (B) MCF-PBPH 415, primary segments of fertile frond; (B₁), (B₂) deeply dissected lobes. (C), (D) Primary segments of sterile frond. MCF-PBPH 067d (C) and MCF-PBPH 069 (D). (C), (D₁) deeply dissected lobes. (D₂) detail of a lobe of the primary segment, showing secondary and tertiary veins. Scale bars 10 mm. Gnaedinger & Zavattieri (2021).