New Ordovician chitinozoans from the Central Andean Basin, Argentina

DE LA PUENTE, G. SUSANA; ASTINI, RICARDO A.

Lille

Congreso; Virtual Annual Meeting of IGCP 653 and IGCP 735, and Meeting of the Subcommission on Ordovician Stratigraphy; 2021

IGCP 653 and IGCP 735, and the Subcommission on Ordovician Stratigraphy

New micropaleontological data from eastern most localities (Santa Ana and Caspalá) in the Cordillera Oriental of Argentina are herein examined. The Ordovician strata in these localities exceed 1000 m thick and constitute one of the most complete lower Palaeozoic sections within the Central Andean Basin of South America. The chitinozoan record from the lowest to the uppermost Ordovician strata in the Santa Ana and Caspalá areas is shown.The Ordovician siliciclastic succession includes: 1) a dark gray massive shally-interval, 2) a thick sandy-rich rhythmic shelfal and shallow-marine interval, 3) a purple-reddish, possibly fluvio-estuarine interval, 4) a greenish thoroughly bioturbated transgressive package, and 5) an glacially related massive and thin-bedded diamictites. The chitinozoan association recovered from the dark gray interval contains Euconochitina paschaensis and Lagenochitina sp.cf. longiformis. A late Tremadocian age is suggested by the chitinozoan association, supported by the records ofthe Araneograptus murrayi and Hunnegraptus copiosus graptolite zones and the lower part of the Acodus deltatus-Paroistodus proteus conodont Zone. The above rhythmic olive to grayish-green sequence contains in addition Euconochitina aff. paschaensis and a short form of Eremochitina sp.The last two chitinozoan species are also present in the green bioturbated levels above the purple-red interval. Lagenochitina sp. cf. longiformis and Desmochitina sp., interpreted as a reworked material, are also observed in a level of this interval. Eremochitina brevis form A and Velatachitina veligera characterize the middle-upper part of the succession, with Conochitina spp. This assemblage indicates a Floian age. The upper part of the greenish package contains, in addition, Lagenochitina sp. A. A Floian to Dapingian age has been also reported in the area according to the record of the Baltoniodus cf. triangularis conodont Zone. The bioturbated upper part of the studied section is characterized by an early Katian association composed of Pistillachitina comma, Belonechitina gr. micracantha, Belonechitina robusta, and long forms of Belonechitina. The uppermost rich-chitinozoan level contains a late Katian-early Hirnantian assemblage including Tanuchitina cf. fistulosa, Conochitina elegans, and Belonechitina sp. cf. llangrannogensis. Within the uppermost glacial interval, thin interbedded shally partitions were barren. Its sharp basal boundary together with its barren nature made indicate a lacustrine or highly restricted origin for this Hirnantian interval. The Ordovician chitinozoan assemblages indicate Northern Gondwana affinities. B. llangrannogensis has been only recorded in Avalonia.