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Diversidad taxonómica y distribución de los morfotipos de braquiópodos en la Zona de Ahtiella argentina (Ordovícico Medio), Formación San Juan, Precordillera Argentina
| Content Provider | Semantic Scholar |
|---|---|
| Author | Sorrentino, Leonor Benedetto, Juan Luis Carrera, Marcelo G. |
| Copyright Year | 2009 |
| Abstract | BRACHIOPOD TAXONOMIC DIVERSITY AND MORPHOTYPE DISTRIBUTION IN THE AHTIELLA ARGENTINA ZONE (MIDDLE ORDOVICIAN), SAN JUAN FORMATION, ARGENTINA. The upper part of the San Juan Formation in the Cerro Viejo area (NE of Jachal city) was deposited in a middle to distal carbonate ramp setting during the lower Darriwilian. This interval, c. 24 meters thick, shows a general deepening upwards trend, ranging from bioclastic packstones-wacke- stones in the base to nodular mudstones in the upper part. Most of the fossiliferous levels have yielded rich rhyn- chonelliformean brachiopod assemblages. The dominant forms are the plectambonitoids Ahtiella argentina (the in- dex fossil of the biozone), Leptella (Petroria) rugosa, Taffia (Chaloupskia) anomala, and the orthoids Paralenorthis vul- garis and Orthidium geniculatum. Other taxa, such as Productorthis cienagaensis, Martellia mesocosta, Inversella (Reinversella) arancibiai and Sanjuanella plicata are less common and display more restricted stratigraphic ranges. In order to evaluate the correlation between the mode of life of brachiopods and lithofacies (reflecting a set of envi- ronmental parameters), species were grouped into four general morphotypes: pedunculate erect, pedunculate re- cumbent, semi-infaunal, and liberosessile. The umbo-down, permanent-attached pedunculate species are distrib- uted throughout the section, whereas liberosessile, semi-infaunal and recumbent forms are dominant or common in the nodular mudstones and wackestones. This implies that the low-attached permanent pedunculate forms (i.e Paralenorthis) were more ubiquitous ecologically than the liberosessile and semi infaunal forms, which inhabited preferably soft-bottom, low-energy substrates. Overall brachiopod species richness attains higher values in the nodular mudstone levels, which were deposited during a high stand sea level episode. Such an increase of richness may have resulted from a pulse of immigration coupled with newly evolved species during this interval. Resumen. Los niveles cuspidales de la Formacion San Juan en el cordon del Cerro Viejo (NE de Jachal) correspon- den a depositos de rampa carbonatica media a distal. Este intervalo, del Darriwiliano inferior, muestra una ten- dencia general transgresiva variando desde packstones-wackestones microbioclasticos en la parte inferior hasta muds- tones nodulares en la parte superior. La mayoria de las capas contienen una rica fauna de braquiopodos rinconelli- formes. Las formas dominantes son los plectambonitoideos Ahtiella argentina, Leptella (Petroria) rugosa y Taffia (Cha- loupskia) anomala y los ortoideos Paralenorthis vulgaris y Orthidium geniculatum. Otros taxones, tales como Produc- torthis, Martellia, Inversella (Reinversella) y Sanjuanella plicata, son menos abundantes y estan restringidos a ciertos niveles estratigraficos. Para evaluar las posibles relaciones entre el modo de vida de los braquiopodos y las litofa- cies que los contienen se agruparon en cuatro morfotipos basicos: pedunculados erectos, pedunculados reclinados, semi-infaunales y liberosesiles. Las especies que vivian erectas adheridas por el pedunculo estan distribuidas en todo el intervalo estudiado, mientras que las formas liberosesiles, semi-infaunales y recumbentes son dominantes o mas frecuentes en los wackestones y mudstones nodulares. Los pedunculados permanentes (e.g. Paralenorthis) eran formas ecologicamente mas ubicuas mientras que las restantes tenian preferencia por substratos blandos y bajos niveles de energia. La riqueza de los braquiopodos alcanza sus valores mas altos en el intervalo de mudstones nodu- lares depositados durante un evento de mar alto. Este aumento seria producto de un pulso migratorio coincidente con la profundizacion de la rampa carbonatica sumado a la aparicion de nuevas especies que evolucionaron a par- tir de taxones preexistentes en la cuenca. |
| Starting Page | 481 |
| Ending Page | 493 |
| Page Count | 13 |
| File Format | PDF HTM / HTML |
| Volume Number | 46 |
| Alternate Webpage(s) | https://ri.conicet.gov.ar/bitstream/handle/11336/55397/CONICET_Digital_Nro.dd4970bf-4f92-4e51-8a8f-78e93bd6253b_A.pdf?isAllowed=y&sequence=2 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
