Ammonite biostratigraphy and bioevents in the Jurassic e Cretaceous boundary of central Chile

Abstract

Ammonites are abundant and well preserved in the Tithonian-Valanginian (Upper Jurassic eLowerCretaceous) sedimentary successions of the central Chilean Andes, known as the Lo Valdes and Ba~nos delFlaco Formations. A total of 1429 ammonites were collected from these stratigraphic units and thefollowing index zones based on ammonites are here proposed, from base to top: Virgatosphinctes mex-icanus/Pseudolissoceras cf. zitteli,Windhauseniceras internispinosum,Micracanthoceras microcanthum/Corongoceras alternans,Substeueroceras koeneni (Berriasella fraudans and Groebericeras rocardi sub-zones)and Thurmanniceras thurmanni/Argentiniceras fasciculatum.Our analyses indicate that “Ammonite Richness”is high in the middle and upper Tithonian and intothe lower Berriasian part of the succession, but decreases towards the lower Valanginian. Highest sim-ilarity values are established for the upper Tithonian, coincident with a high diversity; similarity declinesin the lower Berriasian. The main faunal turnover occurred between the early and late Berriasian fol-lowed by a secondary bioevent between the late Berriasian and early Valanginian