Sara Ballent

Hettangian and Sinemurian (lower Jurassic) biostratigraphy of Argentina

Abstract On the northern bank of the Rio Atuel, Mendoza province, Argentina, the marine fossiliferous Mesozoic is now known to begin with Hettangian-lower Sinemurian strata. Traditionally, the marine Jurassic sequence in the area was believed to start with upper Sinemurian levels. Representatives of the genera Psiloceras Hyatt, Caloceras Hyatt, Alsatites Haug, Waehneroceras Hyatt, Schlotheimia Bayle, Sulciferites Spath, Badouxia Geux and Taylor, Vermiceras Hyatt, Coroniceras Hyatt, Agassiceras Hyatt, Euagassiceras Spath, and Arnioceras Hyatt thus indicate the presence of beds equivalent to the Planorbis, Liasicus, Angulata, Bucklandi, Semicostatum, and Turneri zones of the standard international chronostratigraphic scale. Hence, in Argentina, the marine Hettangian, Sinemurian, and lowermost Pliensbachian have been proved only in the neighborhood of the Rio Atuel region, where marine Triassic could also be represented. Elsewhere in Argentina, the oldest Jurassic ammonite-bearing levels usually belong to the upper lower or the upper Pliensbachian. Regionally, these findings imply a substantial modification to previous paleogeographic reconstructions for the Early Jurassic of South America.

The distribution of the Mesozoic ostracod genus Procytherura Whatley: palaeogeographical implications with special reference to Argentina

A review is undertaken of the nine species of Procytherura known to occur in Argentina and a new species, Procytherura serangodes sp. nov. is described. The global distribution of the genus indicates that it was more or less equally diverse and widely distributed in both hemispheres during most of the Lower and Middle Jurassic, but that in the Upper Jurassic and in the Lower Cretaceous, it became progressively restricted to the Southern Hemisphere. The widespread distribution of several Jurassic species of Procytherura, which occur in both Great Britain and Northwest Europe and in Argentina, is shown to be related to the availability of important migration routes, including the Tethys and the Hispanic Corridor. In the Lower Cretaceous, the very widespread distribution of Procytherura in the Southern Hemisphere, is used to support the existence of important routes along the eastern and western seaboards of Africa, the latter associated with the opening of the South Atlantic.

The composition of Argentinian Jurassic marine ostracod and foraminiferal faunas: Environment and zoogeography

The composition of Jurassic marine ostracod and foraminiferal assemblages from the Neuquen Basin is analysed. The absence of these microfossils from certain levels is attributed to a number of causes, such as inimical facies for their existence or for their preservation. Evidence is produced demonstrating that the absence of ostracods and the virtual absence of Foraminifera from such important Middle Jurassic sections as that at Chacay Melehue is due to their lower bathyal or abyssal palaeodepth. The relative success of filter and deposit feeders is shown to be probably related to palaeoxygen levels, with filter feeders being able to tolerate lower levels of oxygen concentration than their competitors. The palaeoecology of the various ostracod assemblages is compared and contrasted with that of their contemporary Foraminifera. While in the Liassic and the early Middle Jurassic there is a considerable general similarity between ostracod assemblages wordwide, throughout the remainder of the Jurassic there is evidence of the progressive isolation of southern South America, with consequent generic impoverishment and the absence or extreme rarity of many of the major families and genera of NW Europe. Conversely, there are no genera or supra-generic taxa endemic to Argentina.

Middle Jurassic Biostratigraphyof Argentina

Abstract The Middle Jurassic of west-central Argentina comprises several stratigraphic sequences characterized on thebasis of ammonoids, bivalves, brachiopods, foraminifers and ostracods. Aalenian and Bajocian ammonoids belong in the Bredyia manflasensis, “Zurcheria” groeberi Puchenquia malarguensis, Pseudotoites singularis, Emileia giebeli, Stephanoceras humphriesianum and Megasphaeroceras rotundum Zones. Three independent series of four biozonal units each, respectively based on bivalves, brachiopods and microfauna, are recognized in this interval. Bathonian and Callovian ammonoids are allocated to the Cadomites--Tulitidae, Lilloettia steinmanni, Eurycephalites vergarensis, Neuqueniceras (Frickites) bodenbenderi, Hecticoceras proximum Zones and Rehmannia (Loczyceras) patagoniensis Horizon. At the same time, further biostratigraphic subdivisions are distinguished by other taxa, totalling two for bivalves, three for brachiopods and one for calcareous microfossils. Diversity trends of the different faunal groups and sequence distribution are explained by changes in relative extension of platform and basinal areas.