Palynostratigraphy of the late Palaeozoic of Uruguay, Paraná Basin

Abstract

A biozonation for the Permian strata of Uruguay is proposed based on published data from boreholes 201, 221, CLSC 3, 4, 11, 13 and 24 drilled by Dirección Nacional de Minería y Geología of Uruguay (DINAMIGE), as well as two superficial samples and the samples from borehole 254, which are described here. Eighteen samples were retrieved from borehole 254 (DINAMIGE), extending from the San Gregorio to the Yaguarí Formation. One hundred and eighty-four taxa were identified. From their vertical distribution in the column, two assemblages were recognised: a lower assemblage that encompasses the San Gregorio Formation up to the lower part of the Melo Formation (Assemblage I), and an upper assemblage that includes the upper part of the Melo Formation (Assemblage II). The boundary between these two assemblages lies in the Mangrullo Member of the Melo Formation. In ascending stratigraphic order, the zones proposed are the Cristatisporites inconstans- Vittatina saccata Assemblage Zone ( IS) and the Striatoabieites anaverrucosus-Staurosaccites cordubensis Assemblage Zone ( AC). In the IS Zone identified in the San Gregorio and Tres Islas Formations and in the basal part of the Melo Formation (Fraile Muerto Member), the assemblages are characterised by trilete spores and monosaccate pollen and, to a lesser extent, by non-taeniate bisaccate, taeniate bisaccate and plicate pollen grains. An early Cisuralian age is proposed for this Zone. The AC Zone was identified in the Melo Formation, in the Mangrullo and Paso Aguiar Members. It is characterised by assemblages that are dominated by bisaccate (both taeniate and non-taeniate) pollen grains. These palynological assemblages also include a few plicate and monosaccate pollen grains and trilete and monolete spores. A late Cisuralian–Guadalupian age is proposed for this zone.