Devonian chitinozoans from the Los Monos Formation in the Ramos borehole, northwestern Argentina

CAMINA, SONIA CLARA; RUBINSTEIN, CLAUDIA VIVIANA; VERGANI, GUSTAVO; PEREIRA, MARTÍN

Kentucky

Congreso; 55th Annual Meeting of AASP-TPS Joint with the Commission Internationale Microflore Paléozoique (CIMP); 2023

Cortland Eble, Kentucky Geological Survey, University of Kentucky, Lexington, KY

The Ramos R1011 borehole, located in the north-eastern Salta province of Argentina, waspalynologically analysed. It comprises rocks from the Tarija basin in northwestern Argentinawhich is considered to be connected with the Silurian-Devonian sub-Andean basin from SouthBolivia. Preliminary palynological results of Middle Devonian samples from this borehole suggesta marine platform paleoenvironment for the upper samples with some fluctuations to shallowerdeposits in the deepest samples. The organic-walled marine phytoplankton and miospores wouldalso indicate a possible Emsian - Givetian age for this section.Nineteen cutting samples were processed and a moderately well-preserved chitinozoanassemblage was recognized from the Los Monos Formation in the upper and middle parts of thestudied section. Five genera and twenty-one chitinozoan species were recognised from the 323chitinozoan specimens analysed with SEM.Three new chitinozoan species were recognised. Two of them (Ramochitina candelariaensis andRamochitina durandi) were previously recorded as a nomen nudum for Pérez-Leytón (2007).Furthermore, another two species that were previously recorded from the Tacobo borehole(Ancyrochitina aff. morzadeci and Lagenochitina cf. pirum) are well represented herein, whichsuggest that they could also be new species themselves. The assemblage yielded species that arerestricted to Western Gondwana such as Ancyrochitina biconstricta, Ancyrochitina parisi andRamochitina boliviensis. However, well-known species with worldwide distribution such asAncyrochitina cornigera, Ancyrochitina fragilis and Fungochitina pilosa, were also recognised.The Ramos and Tacobo borehole chitinozoan assemblages share 50% of the species and bothresemble strongly to the typical middle Devonian chitinozoan fauna from Western Gondwanaand Bolivia.The topmost part of the Ramos assemblage can be assigned to the early Givetian postdesmeayeseraebiozone from South Bolivia (Pérez-Leytón, 2007) and the Fungochitina microespinosa-Ancyrochitina taouratinensis biozone from Western Gondwana (Grahn, 2005). Furthermore, thesamples from the middle part of the section can be assigned to the late Eifelian – early Givetiancandelariaensis informal biozone from South Bolivia, which correlates with the Ramochitinastiphrospinata biozone from Western Gondwana. The Middle Devonian Ancyrochitina cornigeraglobal biozone (Paris et al., 2000) can be assigned to the entire assemblage.Summarizing, the samples studied from the Ramos borehole yielded a typical Middle DevonianWestern Gondwana assemblage that can be assigned to the late Eifelian – early Givetian.