First record of a Devonian palynological assemblage from the Zenta range, Eastern Cordillera, northwestern Argentina

NOETINGER, S., DI PASQUO, M.M.

Florianopolis, Brasil

Simposio; 12º Simposio de Paleobotánica y Palinología; 2008

Universidade do Rio Grande do Sul

Devonian surface and subsurface deposits are recognized between Bolivia, Paraguay and Argentina. In the latter, the Silurian- Devonian units, represented by shale and sandstone facies deposited mainly in a shallow marine environment are well developed in the Eastern Cordillera, the Subandean Range and the subsurface of the Chaco-Salteño Plain. This study presents a preliminary palynological survey from the Abra Límite locality (S 23º 10,858’, W 65º 0,343’) in the Zenta Range at the Eastern Cordillera, Jujuy Province. The outcrop comprises heterolithic facies at the base, followed by thick packages of sandstones interbedded with less thick beds of conglomerates and shales to fine-grained sandstones (ca. 250 m thick) and finishes with a greenish-grey diamictitic bed (ca. 50 m thick) in unconformable contact. The section below the unconformity yielded poorly preserved palynological material due to relatively high level of thermal maturity. The basal part is characterized mainly by chitinozoans, such as Cingulochitina serrata (Taugourdeau and de Jekhowsky), Angochitina chlupaci Paris and Laufeld, Hoegisphaera sp. cf. H. glabra Staplin, Ancyrochitna tomentosa Taugordeau and de Jekhowsky, and several species left in open nomenclature due to the rather poor preservation (e.g. Ramochitina sp.). These taxa are accompanied by few species of phytoplankton (e.g., ?Multiplicisphaeridium robertinum (Cramer) Lister, Diexallophasis remota Deunff emend. Playford, Multisphaeridium ramusculosum Deflandre emend. Lister, Veryhachium sp.) suggesting an offshore palaeoenvironment for this association. Two samples of the upper section yielded spores and phytoplankton. The stratigraphic distribution of species like Cingulochitina serrata, Ancyrochitina tomentosa, Angochitina chlupaci and Hoegisphaera sp. cf. H. glabra suggests an Early Devonian (?Late Lochkovian) age for the lowest part of the studied section represented by the BAFC-Pl 2088 and 2089;  whereas a late Eifelian to Givetian age is given to the upper section (BAFC-Pl 2086 and 2092) mainly based on the appearance of Diboliporites farraginis McGregor and Camfield. The levels in between (BAFC-Pl 2090 and 2091) are delimited by the ages given above. Therefore, the proposed transgression for the Early Devonian, which marks the origin of the basin expansion, is registered in this area. In contrast, the upper part of the succession denotes a clear environmental change in accordance with the lithology, the lack of marine phytoplankton and chitinozoans, and the occurrence of some fresh or brackish water taxa, such as Quadrisporites variabilis, Quadrisporites granulatus; ?coenobial algae, and sporomorphs.