INTRACLASTIC BRECCIAS (TSUNAMITES) ON THE TOP OF SAN JUAN FORMATION (DARRIWILIAN), SAN JUAN PRECORDILLERA, ARGENTINA. MICROFACIES AND CONODONTS.-This paper describes for the first time intraclastic breccias beds in the last meter of the San Juan Formation, Central Precordillera (San Juan, Argentina). The microfacial features, lithofacial relations and variations in conodont population linked with the conglomerate allow us to defineinterpreting that these deposits were produced by a great eroding event, possibly generated by a tsunami or storms out of series which has affected the carbonate ramp during the Middle Darriwilian. Resumen: BRECHAS INTRACLASTICAS (TSUNAMITAS?) EN EL TOPE DE LA FORMACION SAN JUAN (DARRIWILIANO), PRECORDILLERA DE SAN JUAN, ARGENTINA. MICROFACIES Y CONODONTES.-Se describen por primera vez niveles de brechas intraclasticas en el ultimo metro de la Formacion San Juan, Precordillera Central (San Juan-Argentina). Las caracteristicas microfaciales, relaciones litofaciales y las variaciones en las poblaciones de conodontes asociadas a las mismas permiten interpretarlas como depositos producidos por eventos erosivos de gran magnitud que habrian sido generados por tsunamis o tormentas fuera de serie que afectaron la rampa carbonatica durante el Darriwiliano medio.
Abstract This contribution deals with the record of the Ordovician genus Pygodus in Cuyania, Argentina. Several classical sections have been sampled for conodonts in the Precordillera and the San Rafael Block, involving diverse sedimentary deposits: coarse clastic rocks with carbonate cement, carbonate beds intercalated in fine clastic deposits, green shale, and black carbonate deposits. The recovered species of this genus are Pygodus lunnensis Zhang, P. anitae Bergström, P. serra (Hadding), P. protoanserinus Zhang and P. anserinus Lamont & Lindström. These key conodonts indicate middle to upper Darriwilian – lower Sandbian age for the bearing strata. The species P. protoanserinus is recorded for the first time from the Precordillera. Detailed observations were made on specimens of P. anitae , P. lunnensis and P. protoanserinus ; the two last species are described for the first time from Cuyania.
THE ORDOVICIAN AT THE SIERRA DE LA HIGUERA (MENDOZA): CONODONTS AND MICROFACIES. At the southern end of the Sierra de La Higuera, northern of Mendoza’s Precordillera, crops out an eopaleozoic succession tectonically affected. This is represented by an N-S strip of limestone that dips to the west and is partly folded. It is assigned to the San Juan Formation (Lower Ordovician). The maximum thicknesses of the carbonate deposits are about a 70 meters. The conodont fauna of the middle and upper part of the section suggests a Floian age. The carbonate sequence was interpreted on microfacies terms according to textural, compositional, fossiliferous and diagenetic features. It has been recognized the following microfacies: mudstones, bioclastic-pelletoidal wackestones, packstones with intraclasts, grainstones, crinoidal grainstones and rudstones. At the lower member there are sedimentary structures that indicate low to medium energy. In the upper Member, the lower section is characterized by microfacies showing a strong mechanical distribution of bioclasts, suggesting sedimentary structures associates with medium to high energy. Towards the top, carbonate microfacies reflect low energy (mudstones, and pelletoidal wackestones). The San Juan Formation is transitionally underlying with a mixed carbonate-shally unit of dark colors, six meters thick, which can be interpreted as the Gualcamayo Formation (Floian-Dapingian). Resumen: EL ORDOVICICO DE LA SIERRA DE LA HIGUERA (MENDOZA): CONODONTES Y MICROFACIES. En el extremo austral de la Sierra de La Higuera, norte de la Precordillera de Mendoza, aflora una sucesion eopaleozoica afectada tectonicamente y representada por una franja de calizas con rumbo meridiano y buzamiento de alto angulo al Oeste, en parte rebatida y plegada, correspondiente a la Formacion San Juan (Ordovicico Inferior). El espesor maximo de los depositos calcareos es ca. 70 metros. La asociacion de conodontes presentes en los tramos medios y superiores sugiere edad floiana. La secuencia carbonatica fue interpretada en terminos de microfacies de acuerdo a caracteristicas texturales, composicionales, fosiliferas y diageneticas. Se han reconocido las siguientes microfacies: mudstones, wackestones pelletoidalbioclasticos, packstones con intraclastos, grainstones, grainstones crinoidales y rudstones. Para el Miembro inferior se identifican microfacies que se asocian a estructuras sedimentarias que indican energia baja a media. En el Miembro superior, la seccion inferior esta caracterizada por microfacies que muestran una distribucion exclusivamente mecanica de los bioclastos, asociadas a estructuras sedimentarias de media a alta energia. Hacia el tope las microfacies reflejan menor energia (mudstones y wackestones pelletoidal). La Formacion San Juan pasa de forma transicional a otra unidad mixto calcareo-pelitico de colores oscuros y de 6 metros de espesor en afloramiento que permite ser interpretada como la Formacion Gualcamayo de edad floiana-dapingiana.
Ordovician conodont collections from several Argentinian basins including the Eastern Cordillera, Famatina and Precordillera allow recognition of a group of conodonts that comprise a new genus here named Condorodus n. gen. Species of this genus have an apparatus composed of six elements recovered so far: Pa, Pb, Sb1, Sb2, Sc and Sd. The differences mainly between the P elements support recognizing three species, from the older to younger: C. diablensis n. gen., n. sp., C. gracielae n. gen., n. sp. and C. chilcaensis n. gen., n. sp., that appeared in the upper Floian (Lower Ordovician) and vanished in middle Darriwilian time (Middle Ordovician). The Eastern Cordillera is here assumed as the place of origin of the Condorodus n. gen. lineage during the late Floian, and then this genus dispersed through the western margin of Gondwana, reaching the Precordillera in the early Darriwilian, from there it could have dispersed to different regions of Gondwana, Perigondwana and Laurentia during the late Darriwilian, and probably give rise to conodont apparatuses of similar morphology in the Late Ordovician.
Fil: Soria Pures, Tatiana Belen. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Tucuman. Instituto Superior de Correlacion Geologica. Universidad Nacional de Tucuman. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de Geologia. Catedra Geologia Estructural. Instituto Superior de Correlacion Geologica; Argentina. Universidad Nacional de San Juan. Facultad de Ingenieria; Argentina
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