The Pennsylvanian palynoflora of the Pando X-1 Borehole, northern Bolivia

Abstract

Palynological analysis of ten core samples obtained from 867-729 m depth in the Pando X-1 Borehole (11° 36′07″ S, 67° 56′45″ W), northern Bolivia, is presented. Palynomorphs are fairly well-preserved although many contain abundant pyrite. The whole assemblage is composed of 48 species of which six are spores. Among the pollen grains, 21 are monosaccate and 19 bisaccate (11 striate species). Scarce scolecodonts, microforaminifera and algae are also present. Two assemblages are recognized on the basis of the stratigraphic distribution of species, mainly of the striate pollen grains. The lower assemblage (A1) is dominated by amorphous organic matter and scarce monosaccate pollen grains, trilete spores and microforaminifera. The upper assemblage (A2) is dominated by tracheids and brown and black phytoclasts and characterized by more diverse palynomorphs, mainly striate and non-striate pollen grains. The geographical and stratigraphic distribution of taxa reveals that many species are recorded from Pennsylvanian and Permian palynofloras elsewhere in Gondwana and Euramerica. The presence of Lahirites segmentatus, Limitisporites scitulus, Lunatisporites onerosus and Vittatina species, common with Brazilian palynofloras, supports a Moscovian age for the assemblage 2. This age is also supported by calcareous microfossils previously found in the same interval of this borehole. Twenty two species are first records in the Pennsylvanian of Bolivia (e.g., Florinites eremus, Protohaploxypinus varius, Striatopodocarpites antiquus, S. gondwanensis, S. solitus, Vittatina sp.). Palynofacies features of both assemblages are indicative of a shallow marine palaeoenvironment. Qualitative changes on the floral composition between A1 and A2 may reflect broadly local palaeoclimatical changes. The dominance of Cordaitales and Coniferales in A1 indicates a relatively more humid local condition than during A2 time, which is characterized by the appearance of abundant and diverse striate pollen grains related to new groups of Pteridosperms. These features are in agreement with a lower humidity or seasonally arid conditions prevailing in the terrestrial landscape near to the Copacabana marine palaeoenvironment.