La transición Cámbrico-Ordovícico en la Formación Volcancito (sierra de Famatina, La Rioja, Argentina)

Abstract

The "Lower Member" (seneu Turner) of the Volcancito Formation outcrops in the Volcancito river section (Fama tina range, La Rioja Province, northwestern Argentina). The lower part of the section is composed of outer shelf marls and intercalations of muddy sandstones. Its abundant trilobites, which were described by previous authors and referred to the Parabolina [requens argentina Biozone, are assigned to the latest Cambrian. Based on new collections, information on the systematics of agnostoids is given: descriptions of Strictagnoetusl micropeltis (Harrington and Leanza) and Gymnagnostlls perinjlatus (Harrington and Leanza) are completed, and Lotagnosius sp., Micragnostus tnlonii Harrington and Leanza, Pseudorhaptagnosius (lviachairagnostus) d. imeius Harrington and Leanza, and Pseudorhaptagnostus (Mudiairagnostus) sp. are described from La Rioja for the first time. In the middle part of the mernber, there are inner shelf to shoreface massive sandstones and mudstones with hummocky cross stratification. Many leveis of sandstone contain small fragments of unidentifiable trilobites and brachiopods. The upper part of the mernber is mainly composed of outer shelf green shales with intercalations of very fine grained sandstones. Occurrence of the trilobite Jlljllyaspis keideli Kobayashi and the graptolites Rhabdinopora flabellifor- 111isd. socialis (Salter) and R. f. d. noroegica (Bulman) indica te an earliest Tremadocian age. This evidence reveals for the Famatina range a fact previously documented in the Argentinian Cordillera Oriental: the first record s of J. keideli and R.flabelliformis are above the first appearence of the trilobite Parabolina [requens argentina. Therefore, only the upper part of the P. [requens argentina Biozone is assignable to the Ordovician.