DIVERSITY AND TURNOVER RATES OF CONODONT SPECIES FROM THE MIDDLE ORDOVICIAN OF THE ARGENTINE PRECORDILLERA

FERNANDA SERRA; GUILLERMO L. ALBANESI; NICOLÁS A. FELTES

Valencia

Simposio; FOURTH INTERNATIONAL CONODONT SYMPOSIUM; 2017

UNIVERSITY OF VALENCIA (SPAIN) IN COLLABORATION WITH: INSTITUTE OF GEOLOGY. ACADEMY OF SCIENCES (CZECH REPUBLIC), UNIVERSITY OF CAGLIARI (ITALY), UNIVERSITY OF GRAZ (AUSTRIA)

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. Lately, a recurring theme in conodont paleoecology has been the effect of eustatic sea level change in conodont community structures; for example, patterns of Middle Ordovician conodont diversity change were attributed mostly to oscillations in the sea level for the Argentine Precordillera. 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 occurs 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, total diversity and normalized diversity, and a quantitative stratigraphic approach with the CONOP9 program. 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. Detailed conodont taxonomy and occurrence records were performed in 4 stratigraphic sections from the Central Precordillera. Achieved information enabled the application of a quantitative stratigraphic approach (CONOP9), which led to the recognition and delimitation of the mentioned biozones. Different diversity measures showed similar trends for Darriwilian conodonts from the Argentine Precordillera. However, as shown by Hints et al. (2011), estimated MSD and normalized diversity tends to underestimate the CONOP9 diversity curve, while total diversity tends to overestimate it. These results show a positive trend in conodont diversification throughout the Middle Darriwilian. Both methodologies reveal a rapid conodont diversification through the L. variabilis and the Y. crassus zones. According to the estimated MSD and normalized diversity approach, the highest conodont diversity is reached in the E. pseudoplanus Zone. On the other hand, total diversity and CONOP9 curves show increased diversity values, both in the Y. crassus and E. pseudoplanus zones. In the former, an important origination pulse occurs (45%) and reveals a 69% turnover rate, while in the E. pseudoplanus Zone a 16% of species origination and 34% of species turnover take place. A marked drop in conodont diversity is documented at the end of the E. suecicus Zone by the CONOP9 approach. This bias is the result of all conodont species having their last appearance datum due to a biostratigraphic unconformity; nevertheless, a total of 37 species (Dnorm=18.5) are present in this interval. Here, the conventional measures seem to provide a more appropriate MSD estimation.This pattern of diversity appears to be related to local sea level fluctuations. Most of the conodonts that appear in the L. variabilis and Y. crassus zones have their records in the upper San Juan Formation at the 4 study sections; where 42 out of 51 species were documented. The uppermost part of the San Juan Formation represents the shallowest depositional environment from the units analysed, which is interpreted as distal ramp without storm influence. The facies transition between this formation and the overlying units at different localities is interpreted to have been caused by a flooding event over the carbonate platform. In these deeper environments the conodont species tend to disappear or to occur in low abundances, except for the middle and upper parts of the Las Chacritas Formation (E. pseudoplanus and E. suecicus zones), and the uppermost strata of the lower member of the Las Aguaditas Formation (E. pseudoplanus Zone), where a shallowing event is responsible for a high number of species (45 records) and specimens (over 9000 specimens). These evidences reveal the profound effect of oceanographic cycles in faunal changes.