5 research outputs found
Revisi贸n estratigr谩fica y litofacial de la Formaci贸n La Silla (Ordov铆cico Inferior) en la Precordillera Oriental de San Juan, Argentina
La Formaci贸n La Silla (C谩mbrico Superior - Ordov铆cico Inferior) fue definida en 1994 por Keller et al. a partir de los niveles inferiores de la Formaci贸n San Juan. Esta Formaci贸n consiste en una sucesi贸n distintiva de calizas claras, con escasos f贸siles y muy explotadas por su alta pureza. Basados en estudios detallados en la secci贸n tipo cercana a J谩chal y siete secciones en el 谩rea sur cercana a la ciudad de San Juan, proponemos una subdivisi贸n formal para la Formaci贸n La Silla en tres miembros. La divisi贸n de la unidad est谩 basada en la ubicaci贸n preferencial de bancos dolom铆ticos en el sector central de la Formaci贸n mientras que el sector superior e inferior son calizas mon贸tonas. Los miembros se denominan, de base a techo: Miembro R铆o del Agua, Miembro R铆o Blanco y Miembro R铆o Salado (85, 140 y 81 m, respectivamente). Las dolom铆as en el Miembro R铆o Blanco son de dos tipos: macizas, potentes y generalmente asociadas a chert; y dolom铆as con laminaci贸n planar y/o entrecruzada, ambas f谩cilmente distinguibles a simple vista, otorg谩ndole a este Miembro un aspecto bandeado debido a la alternancia entre calizas y dolom铆as. Esto contrasta con el aspecto macizo y mon贸tono de los Miembros restantes con estratificaci贸n tabular y a veces suavemente ondulante. Se reconocen las facies de grainstone peloidal, grainstone ool铆tico, rudstone intracl谩stico, packstone peloidal, mudstone, microbiales laminadas y trombol铆ticas. Adem谩s se describen las nuevas facies de dolom铆as laminadas y dolom铆as macizas. De la misma manera que lo observado por Keller et al. (1994), la Formaci贸n La Silla representa una fase distinta en la sucesi贸n cambro-ordov铆cica, con las dolom铆as perimareales de la Formaci贸n La Flecha por debajo y las calizas fosil铆feras de plataforma abierta de la Formaci贸n San Juan por encima
Review of the Cambrian Stratigraphy of the Argentine Precordillera
providing a new stratigraphic interpretation. The lithostratigraphic assemblages provide evidence that the Precordilleran Cambrian Basin was a rimmed platform. The lithostratigraphy record enables to distinguish three large areas corresponding to an inner platform, an outer platform to slope environment and a mixed zone where both platforms and the rim interfinger. The biostratigraphic proposal based on trilobites include restricted shelf biozones to the East, characterized by sparse, endemic and low diversity polymeroid faunules. Moreover, to the West outer shelf biozones are characterized by widespread, high diversity, mixed endemic and cosmopolitan polymeroid and agnostoid faunas. The lithostratigraphic and biostratigraphic data prove the existence of an important hiatus near the Lower-Middle Cambrian boundary, which is correlated to the Hawke Bay regressive event recorded along the eastern margin of Laurentia. The Olenellus Zone assemblage trilobites underlies the Glossopleura or the Ehmaniella Zone. The early Middle Cambrian zones (from Plagiura-Poliella to Albertella) are lacking in the whole Precordillera
Trilobite-based biostratigraphic model (biofacies and biozonation) for the Middle Cambrian carbonate platform of the Argentine Precordillera
A preliminary biostratigraphic model that combines trilobite biofacies and biozonation is proposed for the early Marjumian (Middle Cambrian) of the Precordillera of western Argentina. This interval involves a variety of paleogeographic锟絧aleoenvironmental settings across the carbonate platform, ranging from its interior to exterior areas beyond the platform margin and including both open-marine and near-shore environments. This model is based on three stratigraphic sections of the basal part of the Alojamiento Formation, which crops out in the Cord贸n del Alojamiento (Provincia de Mendoza) and Cord贸n de Santa Clara (Provincia de San Juan); and three sections of the Soldano Member of La Laja Formation in the Sierra Chica de Zonda (Provincia de San Juan).
Three biofacies were recognized on the basis of trilobite associations that show a marked environmental distribution: 1) The Ptychagnostidae Biofacies is represented by an assemblage exclusively dominated by agnostoids, most of them belonging to Ptychagnostus; this assemblage occurs in mudstones and calcareous shales deposited below storm wave-base seaward of the platform margin; 2) the Peronopsidae Biofacies also developed in the external areas but it occurs in limestones deposited between fair-weather and storm wave-base; this assemblage is likewise dominated by agnostoids, but instead belonging to Peronopsis, with the rest of the fauna comprising polymeroids (Dolichometopidae, Oryctocephalidae, Zacanthoididae and Alokistocaridae); 3) the Alokistocaridae Biofacies occurs in grainstones, mudstones and calcareous shales deposited between fair-weather and storm wave-base under changeable conditions located near the platform margin and in the interior. It is dominated by alokistocarids whose relative abundance increases landward from 52% to 58% and to 95%. The remaining components belong to Dorypygidae, Zacanthoididae and Oryctocephalidae; Eodiscidae and Peronopsidae comprise minor proportions in the most seaward location. Due to the contrasting, environmentally mediated associations, these sections belong to three essentially synchronous biozones erected previously for the Middle Cambrian of western Laurentia, the Ptychagnostus gibbus, Oryctocephalus and Ehmaniella Biozones
Cambrian agnostoid trilobites from Mendoza, Argentina: A systematic revision and biostratigraphic implications
Chapter 19 Global Cambrian trilobite palaeobiogeography assessed using parsimony analysis of endemicity
<p>Palaeobiogeographical data on Cambrian trilobites obtained during the twentieth century are combined in this paper to evaluate
palaeoceanographic links through <em>c.</em> 30 myr, once these arthropods biomineralized. Worldwide major tectonostratigraphic units are characterized at series intervals
of Cambrian time and datasets of trilobite genera (629 for Cambrian Series 2, 965 for Cambrian Series 3, and 866 for the Furongian
Series) are analysed using parsimony analysis of endemicity. Special attention is given to the biogeographical observations
made in microcontinents and exotic terranes. The same is done for platform-basinal transects of well-known continental margins.
The parsimony analysis of endemicity analysis resulted in distinct palaeogeographical area groupings among the tectonostratigraphic
units. With these groupings, several palaeobiogeographical units are distinguished, which do not necessarily fit the previously
proposed biogeographical realms and provinces. Their development and spatial distributions are broadly controlled by Cambrian
palaeoclimates, palaeogeographical conditions (e.g. carbonate productivity and anoxic conditions) and ocean current circulation.
</p