Paleontological and microfacial features of the Las Chacritas formation (Middle Ordovician), Central Precordillera, San Juan Province, Argentina
- Autores: M. S. Beresi, S. Heredia, S. Peralta
- Localización: I Congresso Ibérico de Paleontologia. XVI Jornadas de la Sociedad Española de Paleontologia: "Paleontologia em Portugal: História e colaboraçoes Espanha-Portugal". VIII Reuniao Internacional de PICG nº 421 Bioeventos mediopaleozoicos en Gondwana Norte/ Modelosbiogeográlicos em relaçao com a dinimica crustal. / coord. por José Bienvenido Díez Ferrer, Ausenda Cáceres Balbino, 2000, ISBN 972-778-026-1, pág. 38
- Idioma: inglés
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Resumen
The Ordovician carbonate sediments of the Las Chacritas river section, outcrop in the Cordón de la Trampa, western margin of the Central Precordillera of San Juan province, western Argentina. The Upper Arenig-Lower Llanvim sequence ofthe Las Chacritas river section, is called Las Chacritas Formation. This sequence of 55 m in thickness, is composed mainly of limestones, marly limestones ánd several K-bentonite levels. These carbonate deposits overlain the top of the Arenig limestones ofthe San Juan Formation (hardground surface). The Las Chacritas Formation is overlain in paraconcordance ( omission surface) by the graptolite black shales of Lower Caradoc age.
At the Las Chacritas succession, the Lower Member (38 m thick) consists of deep mudstones, spiculites and calcisiltites. The microfacies analysis indicates that the mudstones are characterized by a fine larnination, dark colour and scarce fossils ( crinoids, gastropods and undeterrnined rnicrobioclasts ). At the base of this member severa! thin yellow brownish Kbentonites ( fossil volcanic ash layers) occur interlayered with argillaceous limestones. These litho and biofacial features are typical of an open deep subtidal deposition ( outer shelf).
The Upper Member (17 m thick) is composed predorninantly of fossiliferous (alga! and crinoidal) wackestones, rare packstones and a few interlayered spiculite mudstones. Microfacies analysis indicate that the wackestones are characterized by fine-grained matrix and a diversity of organisms, where sorting is not visible. Fossils found in these wackestones are stenohaline.
The abundant and diversified fauna is composed by articulate and inarticulate brachiopods, sponges, trilobites, crinoids, bivalves, briozoans, calcareous algae (Nuia), ostracods, gastropods, nautiloids, and conodonts. These wackestones and their fossil fauna were deposited in an open marine subtidal shelf environment with warm water, normal marine salinity, low-energy and below the wave base of storms.
Conodont assemblages have been collected in the medium and upper part ofthe sequence. In the Upper Member the Paroistodus horridus Subzone (Lenodus variabilis Biozone) occurs. This biozone indicates the Upper Arenig-Lower Llanvim.
The fossil content of the LCHF increases towards the top of the formation where there are intercalations of light gray pelmatozoan grainstones with coarse-grained texture. They are almost exclusively composed of the dissociated parts of crinoid skeletons packed closely together without any sort of orientation. These grainstones suggest a shoal-water sand body.
The Upper Member is overlain by black shales of Caradoc age. The top of the Las Chacritas Formation coincides with a transgressive surface.
The litho and paleontological facies of the Las Chacritas Formation suggest an open-marine deep subtidal to shallow shelf deposition. The gradual replacement of deep-water dark mudstones by the shallow-water wackestones is consistent with this paleoenvironmental interpretation.
The Ordovician carbonate sedimentation on the platform of the Argentine Precordillera stopped during the early Llanvirn because of the regional drowning and the subsequent deposition of graptolite black shales.
In this paper we focus on the paleontological and microfacial features of the carbonate sequence ofthe Las Chacritas Formation and on the paleoenvironmental interpretation.
