Portail de Conférences / Conference Portal, JK2018

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Ammonites from the Tithonian – Hauterivian ‎(upper Jurassic – lower Cretaceous) Lo Valdés Formation in central Chile
Christian Salazar, Wolfgang Stinnesbeck

Building: Muséum d'Histoire Naturelle de Genève
Room: Amphithéâtre
Date: 2018-12-06 12:00 PM – 12:20 PM
Last modified: 2018-11-26

Abstract


The Lo Valdés Formation in the Andes of central Chile contains abundant and well preserved ammonites, indicating a Tithonian-Hauterivian age (late Jurassic-early Cretaceous) for this unit. In the type locality at Lo Valdés, also in Cajón del Morado and Cruz de Piedra sections, a total of 1206 ammonites were collected, and 39 species were distinguished referred to 22 genera. This faunal assemblage is here described and discussed for the first time. Aulacosphinctes proximus, Micracanthoceras spinulosum, Corongoceras cf. koellikeri, Substeueroceras koeneni, Argentiniceras fasciculatum, Pseudofavrella angulatiformis, Crioceratites andinum, and Cr. diamantense were informally recorded previously from the Lo Valdés Formation. Frenguelliceras magister is a new record for the unit and for central Chile. Pterolytoceras exoticum, Aspidoceras rogoznicense, Micracanthoceras microcanthum, M. vetustum, Corongoceras lotenoense, C. mendozanum, Spiticeras acutum, Sp. pricei, Sp. spitiense, Groebericeras rocardi, Berriasella (Berriasella) jacobi, Malbosiceras malbosi, Chigaroceras bardensis, Tirnovella kayseri, Thurmaniceras thurmanni, Crioceratites perditum, and Bochianites sp. are first registers for Chile. Lytohoplites paredesi n.sp., L. zambranoi n.sp., L. varelae n.sp., and L. rauloi n.sp. are new species. Parodontoceras is here considered a junior synonym of Substeueroceras. Other lectotypes were designated for Micracanthoceras spinulosum, M. vetustum, Spiticeras acutum, Substeueroceras calistoide, Argentiniceras fasciculatum, Tirnovella kayseri, Crioceratites andinum, Cr. diamantense, and Cr. perditum. Spiticeras acutum is considered a morphologically variable taxon. Berriasella "jacobi fraudans" is considered a synonym of B. jacobi; nevertheless B. jacobi is a widely used index taxon for the base of the Berriasian and the name should be kept.

The following upper Tithonian to upper Hauterivian index fossils are used to subdivide the Lo Valdés Formation: Micracanthoceras microcanthum (lower upper Tithonian), Corongoceras alternans, Berriasella jacobi, Groebericeras rocardi, Substeueroceras koeneni (lowermost Berriasian), Thurmanniceras thurmanni, Argentiniceras fasciculatum, and Crioceratites diamantense.