Mid- Ordovician faunal assemblages and events in the Villicúm range, Argentinian Precordillera.

BERESI M.S. AND KRÖGER B.

Lille, Francia

Congreso; Paleozoic Climates. IGCP 503: “Ordovician Paleogeography and Paleoclimate; 2008

CNRS, Univ. Ccias. y Tec. de Lille (USTL )y Univ. Católica de Lille

MID ORDOVICIAN FAUNAL ASSEMBLAGES AND EVENTS IN THE VILLICÚM RANGE, ARGENTINIAN PRECORDILLERA. Matilde Sylvia Beresi¹ and Björn Kröger² ¹ IANIGLA-CONICET. Av. Adrián Ruiz Leal s/n, Parque Gral. San Martín, 5500 Mendoza, Argentina; mberesi@lab.cricyt.edu.ar; ² USTL, Cité Scientifique SN5 F-59655,Lille,France ; Bjorn.Kroger@univ-lille1.fr.; ² USTL, Cité Scientifique SN5 F-59655,Lille,France ; Bjorn.Kroger@univ-lille1.fr.   In the Villicúm Range, the eastern thrust belt of the Precordillera of San Juan Province, westernArgentina, a warm carbonate platform that corresponds to the San Juan Formation (380 m thick) occurs. The upper part of the formation (Eoplacognathus pseudoplanus Zone) is composed of a lower nodular wackestones; a middle thick-bedded packstone-wackestone and upper fossiliferous wackestones and mudstones. Ash beds ( k-bentonites) are intercalated with the uppermost beds.The record of these bentonites evidence the volcanism through the early to middle Ordovician in the Precordillera, as part of the extensive “Mid Ordovician mantle superplume event” (Barnes, 2003). After this event a marked change occurs. A rhythmically bedded limestone-marlstone (12 m thick) with scarce breviconic orthocones, cyrtocones and trilobites overlies the limestones along a hardground surface. Widespread graptolite-rich deep water black shales of the Gualcamayo/Los Azules Formation cover the carbonates in many localities of the Precordillera basin. These intervals of black shales evidence a major transgressive event of the Middle Ordovician. n the Villicúm Range, the eastern thrust belt of the Precordillera of San Juan Province, westernArgentina, a warm carbonate platform that corresponds to the San Juan Formation (380 m thick) occurs. The upper part of the formation (Eoplacognathus pseudoplanus Zone) is composed of a lower nodular wackestones; a middle thick-bedded packstone-wackestone and upper fossiliferous wackestones and mudstones. Ash beds ( k-bentonites) are intercalated with the uppermost beds.The record of these bentonites evidence the volcanism through the early to middle Ordovician in the Precordillera, as part of the extensive “Mid Ordovician mantle superplume event” (Barnes, 2003). After this event a marked change occurs. A rhythmically bedded limestone-marlstone (12 m thick) with scarce breviconic orthocones, cyrtocones and trilobites overlies the limestones along a hardground surface. Widespread graptolite-rich deep water black shales of the Gualcamayo/Los Azules Formation cover the carbonates in many localities of the Precordillera basin. These intervals of black shales evidence a major transgressive event of the Middle Ordovician.  In the uppermost unit of the carbonate sequence (9 m thick), burrowed bioclastic wackestones with a diversified fauna of trilobites, sponges, articulated brachiopods, gastropods, nautiloids, ramose bryozoans, algae and conodonts occurs immediately below and during the deposition of bentonites, during the early Darriwilian. This fauna exhibits some degree of community succession. In this unit can be distinguished three characteristic assemblages, based on the percentages of fossil fauna (from bottom to top): Assemblage I: composed of trilobites (Annamitela and Illaenus), asaphids, articulated brachiopods (Paralenortis, Tritoechia, among others), gastropods (Lophospira, Hormotoma, Helicotoma and Maclurites), crinoids, ramose bryozoans and small-sized demosponges. Assemblage II: consists predominantly of lithistid sponges of dishes, domical, cup and bowl forms (Psarodictyum, Patellispongia, Hudsonospongia and Calycocoelia) comprising about 60% of the total biological volume. Calcified cyanobacteria (Girvanella problematica) and microproblematica (Rothpletzella sp., Halysis monoliformis and Nuia sibirica, Beresi, 2003) are present and are locally abundant. Assemblage III: dominated by a diversified and abundant nautiloid fauna of early Darriwilian orthoceroidans (Braulioceras sanjuanense, Eosomichelinoceras baldisii, Gangshanoceras villicumense, Rhynchorthoceras minor; Kröger et al, 2007) and endocerids.These faunas grow in shallow subtidal environments within the photic zone, where organic nutrients are most abundant and are situated below the fair-weather wave base. The presence of lithistids in life position implies a low-energy level environment in which this assemblage developed. The algae and cyanobacteria indicate deposition within the photic zone. The fauna suggests low-energy, subtidal conditions on an open carbonate platform under warm temperatures during this interval of the San Juan Formation deposition. So, during the Early and Middle Ordovician times the Precordillera should have been located at low latitudes under a warm temperate climate.  At the top of the San Juan Formation there is a laterally persistent, crinoidal grainstone, representing a high-energy shallow carbonate setting. On the bedding planes, large orthoconids show a preferential alignment on the sea floor. The stop of the carbonate fabric and the change to graptolitic shales occur within the E. pseudoplanus Zone, in the Villicúm Range. In other places of the Precordillera, however, the cessation of the carbonate fabric was diachronic.   Sea-level changes and epeirogenic subsidence were possibly the mechanisms responsible for producing the diachronic end of the carbonate fabric in the Precordillera basin.