The Middle Darriwilian (Middle Ordovician) d13C isotopic event (MDICE) recognized in the Argentine Precordillera

ALBANESI, G. L., BERGSTRÖM, S. M., SCHMITZ, B., F. SERRA, N. FELTES, G. VOLDMAN

Australia

Congreso; EGU General Assembly 2012; 2012

European Geological Union

The middle Dariwilian isotope carbon excursion (MDICE) has been documented in Baltoscandic, Chinese, andNorth American successions suggesting that it has a global distribution. In an attempt to discover it also in SouthAmerica, we examined the carbon isotope chemostratigraphy of the MDICE interval at two localities in the centralSan Juan Province in the Precordillera of Argentina, namely the Las Chacritas River and Las Aguaditas sections.Both successions consist of dominating limestones which rest on the San Juan Limestone and represent distal rampor platform margin deposits. More than 60 samples provided a detailed conodont biostratigraphy similar to that inBaltoscandia. At the former locality, 56 isotope samples were collected through the approximately 54 m thick LasChacritas Formation. Samples from the lower 35 m of this unit yielded d13C values -1h which are consideredbaseline values. In the overlying, 19 m thick, part of the formation, an interval representing the Eoplacognathuspseudoplanus and lowermost E. suecicus conodont zones, there is a modest but recognizable isotope excursionwith d13C values -0.5hB˙ ecause its biostratigraphic position and magnitude are very similar to those of theMDICE in Baltoscandia and China, it is identified as the MDICE. This first confirmed record of MDICE in SouthAmerica provides a high-resolution tool for middle Darriwilian intercontinental correlations. In the Las Aguaditassection, we collected more than 150 isotope samples through the lower 170 m of the Las Aguaditas Formation,an interval corresponding to the Yangtzeplacognathus crassus, Dzikodus tablepointensis, E. suecicus, Pygodusserra, and P. anserinus conodont zones. The resulting d13C curve may be subdivided into two parts. The lowerpart, representing the basal 50 m of the formation, has d13C values of -1h and there is no clear evidence of thepresence of the MDICE within the biostratigraphic interval where it occurs elsewhere in the world. The upper partof the Las Aguaditas d13C curve shows uniform d13C values of +1h and is closely similar to upper Darriwilianand lowermost Sandbian d13C curves from other continents. The reason for the apparent absence of the MDICE inthis biostratigraphically well-controlled and closely sampled succession, in which there is no published evidenceof a stratigraphic gap, is currently not obvious and requires further study.