Diversity and turnover rates of conodont species from the Middle Ordovician of the Argentine Precordillera.

N. FELTES; G. L. ALBANESI; F. SERRA

FOURTH INTERNATIONAL CONODONT SYMPOSIUM. ICOS IV ?PROGRESS ON CONODONT INVESTIGATION?

INSTITUTO GEOLÓGICO Y MINERO DE ESPAÑA

Año: 2017; p. 277 - 281

Studies on taxonomic diversity through time constitute useful tools for understanding evolutionary processes and patterns. In this sense, conodonts are particularly suitable due to their abundance, continuity in their stratigraphic record and excellent preservation (Sweet and Donoghue 2001). The paleoecology of conodonts received much attention after the pioneering work of Seddon and Sweet (1971), Barnes and Fåhraeus (1975), among others. Recently, the high diversity of Ordovician conodonts was considered to propose a combined ecological model, where conodonts occupied a variety of available ecological niches with different modes of life (Zhen and Percival, 2003). Lately, a recurring theme in conodont paleoecology has been the effect of eustatic sea level change in conodont community structures (Wu et al., 2014, and referred papers); for example, patterns of Middle Ordovician conodont diversity change were attributed mostly to oscillations in the sea level for the Argentine Precordillera (Albanesi and Bergström, 2004).The Precordillera of western Argentina is characterized by a thick Cambro-Ordovician succession (ca. 2,200 m) of marine limestones that interdigitate with clastic slope deposits toward the west. Spanning a range of depositional environments, from shallow intertidal to marginal shelf and deep ramp settings, the Precordillera is the only lower Paleozoic basin of South America with a carbonate platform. A significant succession of black shales developed after the drowning of the carbonate platform associated with a rapid sea level rise during the early Middle Ordovician. At the Las Chacritas and Las Aguaditas localities occur the only remnants of carbonate deposits in the Precordillera concomitant with the referred black shales succession, and the Las Aguaditas Formation is the only unit that shows an environmental transition from platform to slope in the Middle Ordovician. The equivalent Gualcamayo Formation conformably overlies the San Juan limestones and is characterized by black shales interbedded with calcareous strata, deposited on the outer platform (Astini, 1995).Conodont species diversity is herein analyzed by means of conventional measures: estimated MSD (Foote, 2000), total diversity and normalized diversity (Cooper, 2004), and a quantitative stratigraphic approach with the CONOP9 program (Sadler and Cooper, 2003). These measures are widely used in paleoecology and are discussed elsewhere (e.g., Foote, 2000, and referred works). The biostratigraphic intervals that span the Lenodus variabilis, Yangtzeplacognathus crassus, Eoplacognathus pseudoplanus and Eoplacognathus suecicus zones are interpreted from 4 sections in the Argentine Precordillera from the San Juan Province: Las Chacritas type section, Las Chacritas-south section, Las Aguaditas Creek and Cerro La Chilca sections.