Facies, Bioaccumulations and Macrofauna Drom The Volcancito Formation (Late Cambrian, Filo Azul Member), Famatina Range, Argentina

ESTEBAN, SUSANA BEATRIZ; TORTELLO, M.F.

Catalogue of Paleozoic reefs and Bioaccumulations.

Lugar: París; Año: 2006;

The Filo Azul Member of the Volcancito Formation crops out in the Volcancito river section (Famatina range, La Rioja Province, northwestern Argentina). The lower to middle part of the member is composed of carbonate-rich fine-grained sediments and sandstones of late Cambrian age, whereas the upper part is characterized by siliciclastic rocks (shales and subordinated sandstones) assignable to the early Ordovician. The late Cambrian sediments are represented by marlstones showing thick horizontal lamination (Facies 1) and wavy lamination (Facies 2). In addition, there are intercalations of carbonate sandstones (Facies 3) in the lower part of the section, and massive carbonate sandstones (Facies 4) and fine sandstones with hummocky cross stratification (Facies 5) in the middle part.The sandy facies are interpreted as formed by turbidity currents, probably induced by storms. In contrast, the laminated muddy facies mostly indicate suspension settling. Facies 1 is produced by decantation alternating with bottom flowing currents, whereas Facies 2 can be explained by benthic microbial mat development. The wavy lamination shows clear undulation surfaces at megascopic scale. In thin section, it is composed of alternating black organic-rich laminae less than 0.2 mm thick, and lighter colored carbonate-rich clay and fine silt laminae(0.1-0.2 mm thick).SEM examination showed that microbial mats are represented by calcite filaments up to 20 mm in diameter, which are interpreted as filamentous microorganisms. The presence of black, organic-rich laminae in the fine-grained sediments, and the absence of bioturbation in the lower to middle part of the section, constitute evidence in favor of a low-oxygen environment. Latest Cambrian trilobites of the Parabolina (Neoparabolina) frequens argentina Biozone occur in Facies 1, 2 and 3. Agnostoids [Lotagnostus, Micragnostus vilonii Harrington and Leanza, Micragnostus calviformis Harrington and Leanza, Strictagnostus micropeltis (Harrington and Leanza), Pseudorhaptagnostus (Machairagnostus) tmetus Harrington and Leanza, P. (M.) corrugatus (Suárez-Soruco), Gymnagnostus bolivianus (Hoek), and G. perinflatus (Harrington and Leanza)] and olenids [Parabolina (Neoparabolina), Angelina hyeronimi (Kayser), Parabolinella, Plicatolina] dominate the assemblages, followed by shumardids (Conophrys), ceratopygids (Onychopyge), asaphids (Asaphellus) and hapalopleurids (Rhadinopleura). Remains are represented by both articulated and disarticulated exoskeletons. The filamentous microorganisms reported from Facies 2 are probably sulfide-oxidazing bacteria. Agnostoids and olenids are.especially diverse in Facies 1 and 2. The abundance of nutrients in the sea bottom must have favoured the development of these faunas, which were especially adapted to live under quite, deep, low-oxygen conditions. In contrast, biodiversity decreases with Facies 4 and 5 in the middle part of the section, where small fragments of unidentifiable trilobites and brachiopods are documented. black, organic-rich laminae in the fine-grained sediments, and the absence of bioturbation in the lower to middle part of the section, constitute evidence in favor of a low-oxygen environment. Latest Cambrian trilobites of the Parabolina (Neoparabolina) frequens argentina Biozone occur in Facies 1, 2 and 3. Agnostoids [Lotagnostus, Micragnostus vilonii Harrington and Leanza, Micragnostus calviformis Harrington and Leanza, Strictagnostus micropeltis (Harrington and Leanza), Pseudorhaptagnostus (Machairagnostus) tmetus Harrington and Leanza, P. (M.) corrugatus (Suárez-Soruco), Gymnagnostus bolivianus (Hoek), and G. perinflatus (Harrington and Leanza)] and olenids [Parabolina (Neoparabolina), Angelina hyeronimi (Kayser), Parabolinella, Plicatolina] dominate the assemblages, followed by shumardids (Conophrys), ceratopygids (Onychopyge), asaphids (Asaphellus) and hapalopleurids (Rhadinopleura). Remains are represented by both articulated and disarticulated exoskeletons. The filamentous microorganisms reported from Facies 2 are probably sulfide-oxidazing bacteria. Agnostoids and olenids are.especially diverse in Facies 1 and 2. The abundance of nutrients in the sea bottom must have favoured the development of these faunas, which were especially adapted to live under quite, deep, low-oxygen conditions. In contrast, biodiversity decreases with Facies 4 and 5 in the middle part of the section, where small fragments of unidentifiable trilobites and brachiopods are documented. black, organic-rich laminae in the fine-grained sediments, and the absence of bioturbation in the lower to middle part of the section, constitute evidence in favor of a low-oxygen environment. Latest Cambrian trilobites of the Parabolina (Neoparabolina) frequens argentina Biozone occur in Facies 1, 2 and 3. Agnostoids [Lotagnostus, Micragnostus vilonii Harrington and Leanza, Micragnostus calviformis Harrington and Leanza, Strictagnostus micropeltis (Harrington and Leanza), Pseudorhaptagnostus (Machairagnostus) tmetus Harrington and Leanza, P. (M.) corrugatus (Suárez-Soruco), Gymnagnostus bolivianus (Hoek), and G. perinflatus (Harrington and Leanza)] and olenids [Parabolina (Neoparabolina), Angelina hyeronimi (Kayser), Parabolinella, Plicatolina] dominate the assemblages, followed by shumardids (Conophrys), ceratopygids (Onychopyge), asaphids (Asaphellus) and hapalopleurids (Rhadinopleura). Remains are represented by both articulated and disarticulated exoskeletons. The filamentous microorganisms reported from Facies 2 are probably sulfide-oxidazing bacteria. Agnostoids and olenids are.especially diverse in Facies 1 and 2. The abundance of nutrients in the sea bottom must have favoured the development of these faunas, which were especially adapted to live under quite, deep, low-oxygen conditions. In contrast, biodiversity decreases with Facies 4 and 5 in the middle part of the section, where small fragments of unidentifiable trilobites and brachiopods are documented. mm in diameter, which are interpreted as filamentous microorganisms. The presence of black, organic-rich laminae in the fine-grained sediments, and the absence of bioturbation in the lower to middle part of the section, constitute evidence in favor of a low-oxygen environment. Latest Cambrian trilobites of the Parabolina (Neoparabolina) frequens argentina Biozone occur in Facies 1, 2 and 3. Agnostoids [Lotagnostus, Micragnostus vilonii Harrington and Leanza, Micragnostus calviformis Harrington and Leanza, Strictagnostus micropeltis (Harrington and Leanza), Pseudorhaptagnostus (Machairagnostus) tmetus Harrington and Leanza, P. (M.) corrugatus (Suárez-Soruco), Gymnagnostus bolivianus (Hoek), and G. perinflatus (Harrington and Leanza)] and olenids [Parabolina (Neoparabolina), Angelina hyeronimi (Kayser), Parabolinella, Plicatolina] dominate the assemblages, followed by shumardids (Conophrys), ceratopygids (Onychopyge), asaphids (Asaphellus) and hapalopleurids (Rhadinopleura). Remains are represented by both articulated and disarticulated exoskeletons. The filamentous microorganisms reported from Facies 2 are probably sulfide-oxidazing bacteria. Agnostoids and olenids are.especially diverse in Facies 1 and 2. The abundance of nutrients in the sea bottom must have favoured the development of these faunas, which were especially adapted to live under quite, deep, low-oxygen conditions. In contrast, biodiversity decreases with Facies 4 and 5 in the middle part of the section, where small fragments of unidentifiable trilobites and brachiopods are documented.