Abstract
Eighteen core samples from PETROBRAS well 1-JD-1-AM, comprising the upper part of the Jutai Formation, Jandiatuba area, Solimoes Basin, northwestern Brazil, have been studied for marine microphytoplankton. Forty-seven taxa of acritarchs and prasinophytes have been identified. The presence of Demorhethium lappaceum, Duvernaysphaera actinota, Riculasphaera fissa, Thysanoprobolus polykion, Winwaloeusia distracta, ?Cymatiosphaera florida, and numerous Schizocystia spp., among others, suggests a Lochkovian age for the assemblage. Moreover, the occurrence of Polyedryxium fragosulum and Pterospermella chiquitina indicates a possible late Lochkovian age, in agreement with previous age determinations based on chitinozoans and miospores. The significance of Schizocystia as a biostratigraphic marker for the beginning of the Devonian in Gondwana is discussed. The Solimoes Basin assemblage shows close compositional similarity with coeval assemblages from North America, north Gondwana, and the peri-Gondwana region.
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