Taxonomia de ostracodes e fácies sedimentares das formações Irati e Estrada Nova, nas Regiões de Alto Garças (MT) e Portelândia (GO)

Abstract

This monograph is a requirement for the MSc’s degree. This work presents the taxonomy, paleoecology and paleoenvironmental interpretation of the ostracods, as well as a sedimentary facies approach from Estrada Nova and Irati formations (Passa Dois Group) in the Paraná Basin. This approach allows us to subdivide these formations into five facies associations named as 1 to 5, which might be observed in outcrops located in the regions Alto Garças (MT) and Portelândia (GO). The facies analysis gives rise the opportunity to the study the ostracod content of the Permian in the southerwestern Mato Grosso State. Unfortunetely, the ostracod content has not been recovered in the Irati Formation (facies association 1 and 2) which could be explained as a consequence of the silica cementation. Only one individual was recovered in the Irati Formation. However, the poor preservation became this individual unidentifiable. Herein, only mesossaurs fossils could be illustrated. The ostracod content recovered in this work is restricted to Estrada Nova Formation (facies association 4 and 5), in which fifteen species of marine ostracods was illustrated: Basslerella sp. 1, Praepilatina sp. 1, ?Praepilatina sp. 2, Silenites sp. 1, Silenites sp. 2, Bythocypris sp. 1, Bairdiacypris sp. 1, ?Microcheilinella sp. 1, ?Miltonella sp. 1, Langdaia sp. 1, Gen. 1 sp. 1, Gen. 2 sp. 1, Gen. 3 sp. 1, Gen. 4 sp. 1, Gen. 5 sp. 1. These species have been distributed in the family Bairdiacyprididae, Pachidomellidae, Bairdidae, Miltonellidae, and Knoxitidae, (besides Incertae family, with ostracods open nomenclature), which illustrate a sprectrum of variations in salinity, habit, substrate and depth. Such variations suggest these families have lived into a probably marine plataform which is near to the coastline. The illustrated contents of ostracods and other microfossils as well as macrofossils have not been in situ, which might be interpreted from the presence of: a) articulated and disarticulated shells with surface fragmentation; b) fragmentation of teeth and fish scales of paleonisciformes fishes; and c) disarticulated and semiarticulated bones the mesossaurs. Besides of the ostracod study, facies analysis describes five facies associations which are related to a paleoenvironmental and/or paleobathymetric reconstruction. The interpretation of the most important facies features in these associations lead to interpret a mixed shallow marine shelf, dominated by a wave carbonate setting gradually becaming more coastal with deltaic tendency. This interpretation is suggested by the presence of marine ostracods, mesossaur remains, passage of carbonate facies to siliciclastics toward the top of the succession, as well as plants fragments. Together, these features do evidence a wide process of inland. So, it might be possible to ascertain the association of facies 1to the shoreface subenvironment into that previously interpreted shelf, which was suggested by the predominance of limestone in addition to wave structures. The facies association 2, siliciclastic and carbonate, characteristic of shale facies, is interpreted as a lagoon, due to structures as bird-eyes and stromatolite in addition to shale nature. The facies association 3 embraces interblended carbonate and calcareous pelite, partially silicified, attributed to palustrine environment. The facies association 4 and 5 are siliciclastic and presents beds of sandstones, siltstones and cm-scaled layers of calcirudite. These deposits are respectively interpreted as prodelta (platform>) and distal sandy bars of the prograding foreset in a deltaic setting.