Faunal assemblages from the Lower Ordovician carbonate sequence, at the Villicum Range, Eastern Precordillera, Argentina.

BERESI, MATILDE S.

Potsdam, Germany

Conferencia; 19th Colloquium on Latin American Geosciences.; 2005

Fundación Alfred Wegened, Geo Union; German Science Foundation (DFG)

  The eastern and central belts of the Precordillera of northwest Argentina (29º to 33ºS) is characterized by a thick Lower-Middle Ordovician platform carbonate succession. In the eastern Precordillera of San Juan Province, these deposits are best exposed in Sierra de Villicúm and belong to the San Juan Formation (Arenig-lower Llanvirn). The Quebrada Gustavo section exposes the upper member of the formation, and the boundary with the overlying Llanvirn clastic sediments. The lower boundary of the SJF is marked by the appearance of open marine fauna, whereas the upper boundary is characterized by a transitional facies consisting of an alternation of thin dark mudstones and black shales. The upper member (9 m) is composed of well-bedded argillaceous lime mudstones, fossiliferous wackestones, K-bentonites (fossil volcanic ash layers) and grainstones. At the top, calcareous strata contain conodonts of the E. suecicus Zone (Lower Llanvirn). The uppermost levels of the underlying nodular gray wackestones contain an association of gastropods (macluritids), articulate brachiopods (orthids) and longiconic nautiloids (endocerids).The upper member (subtidal facies) exhibits a distinct paleocommunity succession based on vertical taxonomic distribution. Three faunal assemblages occur in the first 5 m of this unit, consisting from bottom to top of: A.I: trilobites (Annamitella, Illaenus), big asaphids, articulate brachiopods (Paralenortis, Tritoechia, Orthidium), crinoids, small-sized sponges, gastropods (Ozarquispira, Lophospira, Hormotoma), ramose bryozoans, calcareous algae and cyanobacteria (Nuia, Girvanella).A.II: predominantly composed of longiconic orthocones (endocerids, orthoceratids ellesmerocerids) of medium and short length, and scarce breviconic orthocones and cyrtocones. A.III: lithistid sponges as the dominant group in this assemblage: Psarodyctium, Patellispongia, Hudsonospongia, Lissocoelia, Calycocoelia, Archaeoscyphia and crinoids. Above this unit occurs barren interval (6 m) characterized by argillaceous mudstone interlayered with thin K-bentonite layers. The upper member is capped by a distinct package of thick grainstone beds with crinoids, and representing a high-energy shallow setting. On bedding planes occur a accumulations (shell pavements) of orthocone, predominantly endoceratids (lengths up to 0.60m), with low diversified and preferential alignments on the sea floor. Faunal assemblage diversity reached its zenith in the Lower Llanvirn, in a warm-water setting, attributable to the position of the Precordillera in the equatorial region. The subsequent, drastic but gradual faunal change is correlated to the eustatic transgressive event of the Lower Llanvirn which produced the drowning of the carbonate platform followed by the deposition of deeper black shales. E. suecicus Zone (Lower Llanvirn). The uppermost levels of the underlying nodular gray wackestones contain an association of gastropods (macluritids), articulate brachiopods (orthids) and longiconic nautiloids (endocerids).The upper member (subtidal facies) exhibits a distinct paleocommunity succession based on vertical taxonomic distribution. Three faunal assemblages occur in the first 5 m of this unit, consisting from bottom to top of: A.I: trilobites (Annamitella, Illaenus), big asaphids, articulate brachiopods (Paralenortis, Tritoechia, Orthidium), crinoids, small-sized sponges, gastropods (Ozarquispira, Lophospira, Hormotoma), ramose bryozoans, calcareous algae and cyanobacteria (Nuia, Girvanella).A.II: predominantly composed of longiconic orthocones (endocerids, orthoceratids ellesmerocerids) of medium and short length, and scarce breviconic orthocones and cyrtocones. A.III: lithistid sponges as the dominant group in this assemblage: Psarodyctium, Patellispongia, Hudsonospongia, Lissocoelia, Calycocoelia, Archaeoscyphia and crinoids. Above this unit occurs barren interval (6 m) characterized by argillaceous mudstone interlayered with thin K-bentonite layers. The upper member is capped by a distinct package of thick grainstone beds with crinoids, and representing a high-energy shallow setting. On bedding planes occur a accumulations (shell pavements) of orthocone, predominantly endoceratids (lengths up to 0.60m), with low diversified and preferential alignments on the sea floor. Faunal assemblage diversity reached its zenith in the Lower Llanvirn, in a warm-water setting, attributable to the position of the Precordillera in the equatorial region. The subsequent, drastic but gradual faunal change is correlated to the eustatic transgressive event of the Lower Llanvirn which produced the drowning of the carbonate platform followed by the deposition of deeper black shales.