CONODONT BIOSTRATIGRAPHY OF THE LOWER ITAITUBA FORMATION (ATOKAN, PENNSYLVANIAN), AMAZONAS BASIN, BRAZIL

This study reports on the conodonts from an outcrop region in the south Amazonas Basin that comprises the lower part of the Itaituba Formation. The studied area consists of two limestone quarries, informally named 1 and 2, owned by CAIMA, a cement industry. The conodont association includes Pa elements of Idiognathodus incurvus, Idiognathoides sinuatus, Neognathodus roundyi, N. medadultimus, N. bassleri, N. atokaensis, Adetognathus lautus, Hindeodus minutus, Diplognathodus coloradoensis, and Idioprioniodus sp. It is herein proposed one informal taxon-range zone and one subzone, respectively: Idiognathoides sinuatus zone and Neognathodus roundyi subzone. The conodonts occur mainly in the packstones, and subordinately in the wackestones and mudstones. The Itaituba Formation is the richest interval containing normal marine organisms in the Pennsylvanian of Amazonas Basin. The associated fauna includes brachiopods, crinoids, echinoids, bryozoans, gastropods, foraminifers, sponges, ostracodes, trilobites, scolecodonts, bivalves and fish teeth and scales. The studied interval is characterized by moderate to high levels of bioturbation. Burrows are common in the packstones and wackestones. The predominant trace fossil observed was classified as Thalassinoides, occurring mainly in the packstones and wackstones. Zoophycos occurs less commonly, in the mudstones and packstones. Because of the presence at the conodonts Neognathodus atokaensis and Idiognathodus incurvus, it is possible to suggest an Atokan age for the analyzed interval. The associated fauna, bioturbation, and lithology suggests shallow, nearshore waters, with mudstones of low-energy, wackestones and packstones of moderate-energy, and grainstones of relatively high-energy marine conditions.