Silurian palynomorphs from the Precordillera basin, western Argentina: biostratigraphy and diversity trends

PHILIPPE STEEMANS; CLAUDIA VIVIANA RUBINSTEIN; VICTORIA JOSEFINA GARCÍA

Salvador de Bahía

Congreso; XIV International Palynological Congress (IPC) and the X International Organisation of Palaeobotany Conference (IOPC); 2016

Asociación Latinoamericana de Paleobotánica y Palinología

The Siluriansuccession of the Precordillera, western Argentina, constitutes a typicalforeland basin deposit. The middle-upper Silurian to Lower Devonian (Wenlock toLochkovian) is represented in the Los Espejos Formation. It is composed of siliciclastic,mainly fine-grained shelf deposits with a gradual upward thickening and coarseningand evidences of subaerial exposures in its upper part. Its thicknessdiminishes from north to south and the younger levels are only present in thenorthern outcrops. In this contribution, the organic-walled phytoplankton andthe miospores from two sections of the Los Espejos Formation, Quebrada Anchaand Cerro La Chilca, are presented. A total of 21 productive samples, in whichmarine palynomorphs exhibit a clear predominance in almost all the samples inboth sections, were obtained. The only productive samples that evidence apredominance of miospores over marine phytoplankton are the two uppermost ones ofthe Quebrada Ancha section and the uppermost sample of Cerro La Chilca section.Fimbriaglomerelladivisa, Leiofusa bernesga, Melikeriopalla polygonia, Muraticavea wenlockia, Oppilatala ramusculosa, Ozotobrachion palidodigitatus, Pterospermellamartini and Schismatosphaeridium perforatum were among the mostrelevant phytoplankton species recorded in the Quebrada Ancha section. The phytoplankton species support, for thewhole section, the Gorstian?-Ludfordian to Pridoli age based on the miospores. Chelinospora poecilomorpha var. verucataMorphon, Stellatispora inframurinata var. inframurinata, Chelinosporacf. hemiesferica in Richardson et al. (2001), Synorisporites tripapillatus, Brochotriletesfoveolatus, Leonispora argovejae and Amicosporites streeliiare the more biostratigraphically relevant miospores species recorded in thissection. The Cerro La Chilca section contains a phytoplankton assemblagecomposed of species such as Schismatosphaeridium algerense, Fimbriaglomerelladivisa and Ozotobrachion palidodigitatus. The stratigraphic range ofthe phytoplankton species support the Homerian?-Ludfordian age based onmiospores such as the hilate cryptospore Hispanaediscus lamontii and thetrilete spores Chelinospora cf. cantabrica and Chelinospora sanpetrensis. The lowerpart of the Los Espejos Formation displays the highest phytoplankton diversityand the better preserved palynomorphs of the whole formation in both sections.This peak of diversity corresponds to the early Ludfordian and could consequentlyindicate a particularly favorable environmental condition for this time interval.The diversity tends to diminish towards the upper part of the formation inagreement with the transition to storm-dominated shelf and shorefaceenvironments and subaerial exposures that probably prevent the preservation ofpalynomorphs.