Palynology and paleoenvironment of the Jurassic lacustrine Cañadón Asfalto Formation at Cañadón Lahuincó locality, Chubut province, central Patagonia, Argentina

VOLKHEIMER, WOLFGANG; QUATROCCIO, MIRTA; CABALERI, NORA; GARCÍA, VIVIAN

REVISTA ESPANOLA DE MICROPALEONTOLOGIA

Instituto Geológico Minero de Esapaña

Lugar: La Rioja (España); Año: 2008 vol. 40 p. 77 - 94

0556-655X

In the context of the Jurassic extension of southwestern Gondwana, the Cañadón Asfalto Formation filled the semigrabens located in the central area of the Chubut province. The principal outcrops are in the middle Chubut river valley, where volcanites, biochemical, pyroclastic and epiclastic deposits alternate. The palynologic and palynofacial study of dark gray pelites of the lower part of the Cañadón Asfalto Formation at the Cañadón Lahuincó locality, Cerro Cóndor Depocenter, is presented. The abundance of amorphous organic matter (60-90%) in all the palynologic samples studied is characteristic of stagnant bottom conditions. The material mostlycorresponds to destroyed palynomorphs, including microplancton and Botryococcus. The scarce opaque phytoclasts consist of black, equidimensional, structureless material. The presence of Botryococcus indicates shallow lake levels and probably saline conditions.Diagenetic processes such as degradation, corrosion and the absence of pyrite in all the samples indicate that bottom conditions were mostly aerobic. The Cheirolepidiaceae dominate the spectrum (up to 80% of Classopollis spp.), associated with Araucariaceae (up to20%). Warm and relatively humid climatic conditions are indicated by these high percentages of the thermophilic Cheirolepidiaceae, together with the Araucariaceae that grow under relatively humid conditions. Also, the presence of Bryophyta (Nevesisporites cf. radiatus) and pteridosperms (Alisporites similis) suggest probable local humid conditions for the lower part of the Formation. The stratigraphic range of the palynomorphs indicates a Middle Jurassic (Bajocian to early Bathonian) age for the lower part of the Cañadón Asfalto Formation at the Cañadón Lahuincó locality.