Extrinsic factors controlling the GOBE: new evidences from the slope facies of the Argentine Precordillera

VOLDMAN, G.G.

Durham

Simposio; IGCP 653 Opening Meeting; 2016

IGCP 653

The Great Ordovician Biodiversication Event (GOBE), the major diversication episodeof marine life in Earth history, is frequently considered as a combination of intrinsic(biological) and extrinsic (environmental) factors. Regarding the latter, the globaldistribution of Middle Ordovician megabreccias and debris flows has been related todestabilization of continental margins by meteorite impacts or earthquakes associatedto plate-tectonic processes, such as terrane accretions or riftings, which producedenvironmental perturbations that favoured the GOBE intrinsic mechanisms. Therefore, aprecise biostratigraphic scheme is essential to temporally adjust the disorganisedsedimentary deposits. As such, the age of the Los Sombreros Formation at thecontinental slope and rise of the Argentine Precordillera is controversial, frequentlyargued as Ordovician or either Devonian, based on the complex structure and theinherent reworked character of its components. Furongian, Tremadocian, and late Floianconodont elements have been recovered from the slope facies, frequently associated with middle Darriwilian specimens (Lenodus variabilis and Eoplacognathuspseudoplanus zones). The recent record of specimens referable to the Hirsutodontussimplex Subzone of the Cordylodus intermedius Zone (upper Furongian) and from theMacerodus dianae Zone (upper Tremadocian) in gravity-ow deposits withsynsedimentary deformation features from the Los Sombreros Formation imply that aslope connected the carbonate platform to the east with a deep-water (oceanic) basin tothe west at least since late Cambrian times. The new conodont data presented isconsistent with episodic extensional tectonic processes that controlled thedevelopment of the Precordilleran passive continental margin during most of theFurongian-Middle Ordovician, despite an elevated ux of extraterrestrial material thatcannot be neglected during the Darriwilian, as observed in the Precordillera platformand several thousand kilometres away from coeval horizons in Scotland, Sweden andcentral China.