Estudo dos gêneros dyris e tryonia (mollusca:gastropoda) da formação solimões: inferências paleoambientais e bioestratigráficas

Abstract

The Solimões Formation compresses the Solimões Basin Neogene strata, which owns a wide fossiliferous source. The studies about the fossil fauna of this unit provide important information about the evolution of paleoenvironments and hydrographic systems in West Amazonia during the Neogene. The West Amazonia fossil mollusks have been studied since XIX century and have showed potential for biostratigraphy and paleoenvironmental interpretation studies. However, most of those studies have been made using samples originated from outcrops, which limits the studied time range. Since the 70’s decade the exploration of coal and natural gas in West Amazonia opened possibilities to researches of wider time ranges thought boreholes. In the present study, 93 samples from the borehole 1AS-31-AM were analyzed, throughout 302,05 meters dept. The borehole is located by the edge of the Ituí River, in the Amazonas State. Seven of the studied samples presented gastropods in a big amount and high diversity of genuses, with Dyris and Tryonia, aim of this study, as the most abundant ones. Evolutionary events like radiations and extinctions were observed within these genuses. Some never yet found species of Dyris in Brazilian Amazonia had their paleobiogeographic distribution amplified. The presence of guide species such as Dyris megacarinatus, Dyris romeroi, Dyris renemai, Dyris ariei, Dyris microbispiralis and Tryonia scalarioides scalarioides has allowed to date the interval between 170,80m and 175,00m, where occurs the major concentration of gastropods, as Middle to Late Miocene (Serravalian to Tortonian) and relate it with the biozones of mollusks (MZ7 to MZ12), palinomorphs (Crassoretitriletes and Grimsdalea), and ostracods (Cyprideis caraione, Cyprideis minipunctata, Cyprideis obliquosulcata and Cyprideis cyrtoma) established in previous works. Based on the association of fresh and brackish to seawater genuses, the study area is interpreted as an environment which is conformable with a system of lakes close to lagoons.