Edsel D. Brussa

Ordovician and Silurian graptolite assemblages from Cerro del Fuerte, San Juan Province, Argentina

A description is given of three transects through the Silurian strata of Cerro del Fuerte, San Juan Province, Argentina. All are in the Precordillera structural belt; the Silurian strata probably have conformable boundaries with the Ordovician and Devonian, and there is a regional angular unconformity at the base of the Carboniferous. The Silurian includes two formations, the La Chilca below and the Los Espejos above. Both formations exhibit a coarsening upwards sequence: the Los Espejos Formation spans the Wenlock, Ludlow and, probably, Přidoli epochs, the first being of relatively fine sediments and the latter two of coarser sediments. The third transect also includes Devonian sediments which seem to represent, albeit incompletely, a third coarsening upwards cycle. The graptolite faunas have been extensively collected and form the basis of the systematic section of the paper. Late Ordovician, Llandovery and Ludlow faunas are described.

The Ordovician Quebrada Grande Formation, Cordón de Lila (Antofagasta Region, northern Chile): stratigraphicand paleogeographic significance

The Cordon de Lila is located immediately to the south of the Salar de Atacama, in northern Chile. The geology of the Cordon de Lila is characterized by extensive outcrops of Early Paleozoic volcanic and sedimentary rocks (Cordon de Lila Igneous and Sedimentary Complex; CISL) that form the ‘Arco magmatico occidental’ (AMO) which is intruded by a multiple suite of Middle Ordovician to Lower Silurian granitoids. In this contribution we report the results of a sedimentological and paleontological study of a recently discovered 1,600 m thick marine Ordovician sedimentary sequence (Quebrada Grande Formation) which unconformably overlies the CISL. The Quebrada Grande Formation comprises three mayor facies associations. In stratigrafic order, these facies associations are: matrix-supported conglomerates (1), interbedded sandstones and siltstones (2) and clast-supported conglomerates (3). Facies association (1) is interpreted as debris flow deposits accumulated in a proximal marine fan-delta setting while the overlying sandstones and siltstones facies (2) represent open-marine platform deposits. Clast-supported conglomerates (3) are the record of high-energy sedimentary episodes during which coarse sediments reached the distal part of the fan-delta. The detrital fraction of the Quebrada Grande Formation is derived from the erosion of the Cambrian?-Lower Ordovician? CISL volcanic arc, the plutonic roots of the arc and its continental basement. The age of this formation is well constrained by the occurrence of brachiopods (Paralenorthis sp., Monorthis transversa Benedetto, and Mollesella? sp.) near the base and graptolites (Tetragraptus sp. and Cryptograptus? sp.) towards the middle of the succession. Both brachiopods and graptolites indicate an Arenigian to early Llanvirnian (Darriwilian) age. The brachiopod assemblage from Cordon de Lila displays affinities with the Famatina basin fauna of western Argentina. The absence of taxa in common between the AMO and the nearby ‘Faja Eruptiva de la Puna Occidental’ (FEPO) of NW Argentina may result from some kind of environmental barrier or some geographic separation from the Gondwana margin. According to the latter hypothesis, the Quebrada Grande succession developed on the allochtonous Arequipa-Antofalla Terrane.

La biozona de Paraglossograptus tentaculatus (Graptolithina) en la Formacion Sierra de La Invernada, Precordillera Occidental, Argentina

The Paraglossograptus tentaculatus Biozone from the Sierra de La Invernada Formation, western Precordillera of Argentina, is recognized. This biozone is divided into a lower part and an upper part based on their associations. The outcrops suggest an Upper Arenig - Lower Llanvirn age. The vertical distribution of the taxa, fossil associations, local and intercontinental correlation, and the palaeogeographic aspects are discussed.