Asociaciones palinológicas en las formaciones Los Monos (Devónico) e Itacua (Carbonífero Inferior) en Balapuca (Cuenca Tarija), sur de Bolivia. Parte 2. Asociación de la Formación Itacua e interpretación estratigráfica y cronología de las formaciones Los

DI PASQUO, MARÍA DE LAS MERCEDES

REVISTA GEOLOGICA DE CHILE

Servicio Nacional de Geología y Minería

Lugar: Santiago; Año: 2007 vol. 34 p. 163 - 198

Palynological assemblages present in Los Monos (Devonian) and Itacua (Lower Carboniferous) formations at Balapuca (Tarija basin), southern Bolivia. Part two. Itacua Formation assemblage and stratigraphical interpretation and chronology of the Los Monos and Itacua formations. The palynological result of three fertile samples of the Itacua Formation is presented. This unit crops out at Balapuca located in the Bolivian margin of the Bermejo river, in the mountain range of San Telmo. The stratigraphic extension of the species determined in this association allowed to identify two sets of palynomorphs, one indigenous composed of 61 species of spores and few colonies of Botryococcus, and another set of reworked species conformed by spores, acritarchs, prasinophytes, chloroficean algae, cryptospores and chitinozoans, in its majority coming from the Devonian layers and subordinated spores of the Tournaisian (Carboniferous) ones. Two indigenous spore species are proposed as new combinations. The stratigraphical distribution of indigenous key species allowed to assign this association to the early Viséan (Early Carboniferous). This is the first work where indigenous palynomorphs of this age are illustrated along with a selection of reworked species, and are obtained of an in situ deposit of the Itacua Formation cropping out in the southern Tarija Basin. Based on the age attributed to the Los Monos and Itacua formations in this study, a hiatus among them is defined, characterized by an erosive phase during the Frasnian-Tournaisian and another non-depositional one specially during the latest Famennian ("Strunian"). This is interpreted from the presence/absence of reworked species with restricted stratigraphical range in the Itacua Formation assemblage. On the other hand, the Macharetí Group overlies this unit, and it was dated as early Late Carboniferous based on palynology. Between both units a new hiatus is registered that spans the late Viséan-Serpukhovian partly interpreted like non-depositional. In addition, a discussion about a possible tectonic and glacioeustatic origen for both hiatuses is included.