Ocorrência inédita de Ancyrochitina Fragilis Erevis (Chitinozoa) na área do Rio Mapuera, Para, Brasil

Abstract

By order of Eletronorte (Centrais Elétricas do Norte do Brasil S.A.), several mixed (rotational/percussive) shallow wells have been drilled by Enge-Rio (Engenharia e Consultoria S.A.) in the Mapuera River region, State of Pará (northern rim of the Amazon Basin). Core samples from wells SM-2001, SM-2003, SM-2004, SM-2005 and SM-2006, submitted to palinological investigation, revealed chitinozoans herein assigned to Ancyrochitina fragilis brevis. This form ranges from the Silurian through the Lower Devonian in the International Chronostratigraphic Column. Taxa associated to the Brazilian occurrence of Ancyrochitina fragilis brevis include certain acritarchs (Tyligmasoma alargadum, Leiofusa estrecha, Veryhachium valiente and Duvernaysphaera tessella), besides other chitinozoans like Linochitina errática and Margachitina catenaria tenuipes. Although the Paleozoic lithostratigraphy of the Mapuera River area deserves thorough revision, the chitinozoan-bearing sediments under discussion will probably turn out to belong to the Manacapuru Member of the Trombetas Formation. The age of these sediments is interpreted to be within the Silurian-Early Devonian rance, based solely on the occurrence of Ancyrochitina fragilis brevis. Thus these easily recognizable forms may eventually prove very useful as regional index fossils for post-Ashgillian, pre-Eifelian sedimentary rocks of the Amazon Basin. The association of Ancyrochitina fragilis brevis with other chitinozoan and acritarch species is indicative of Early Devonian age for specific sections of wells SM-2001, SM-2003, SM-2004 and SM-2005.