Palynology and organic geochemistry of an early to Middle Devonian succession from TCB X-1001– tacobo borehole, sub-Andean basin, southern Bolivia

Abstract

Palynological and organic geochemical analyses have been performed on an Early to Middle Devonian clastic marine sedimentary succession of the TCB X-1001 – Tacobo borehole, in the Foothills of the southern sub-Andean ranges of Bolivia. The studied subsurface section encompasses shallow marine deposits of the Icla, Huamampampa and Los Monos formations. Eighteen from the twenty-three cutting samples available were productive, yielding variable abundance, diversity and preservation of organic-walled phytoplankton and miospores, with an increasing trend of terrestrial palynomorphs transpiring towards the younger beds. The palynological assemblage of the Icla Formation contains Bimerga nuda, Cordobesia cf. C. uruguayensis, Duvernaysphaera angelae, and Dibolisporites echinaceus. The Huamampampa Formation includes Polyedryxium sp. cf. P. condensum, Umbellasphaeridium deflandrei and Acinosporites eumammillatus. Arkonites bilixus, Maranhites mosesii, Polyedryxium “talus”, Stellinium comptun, Archaeozonotriletes variabilis, Chelinospora timanica, Convolutispora subtilis, Cymbosporites cyathus, Dibolisporites farraginis, Geminospora lemurata, Grandispora permulta, Samarisporites eximius and Verrucosisporites premnus are among the most relevant taxa of the Los Monos Formation. Based on the organic-walled phytoplankton and the miospores of the studied section, the Icla, Huamampampa and Los Monos formations are assigned to the late Pragian - Emsian, Emsian and Eifelian?- middle Givetian, respectively. The palynomorph distribution, relative abundance and preservation throughout the studied samples support the palaeoenvironment interpretation based on sedimentology and sequence stratigraphy. RockEval pyrolysis, TOC, HI and Tmax data were analysed to determine type and quality of the kerogen and hydrocarbon potential of the stratigraphic units.