Geology and geomorphology of Serra Geral do Tocantins ecological station

Abstract

This work presents a basic description of Serra Geral do Tocantins Ecological Station, located in the eastern part of Tocantins State and western part of Bahia State, Central Brazil. Two main factors of physical environment are treated: geology, composed by sedimentary litotypes, and the relief forms, due to predominance of mesetas and inselbergs in the region, famous for Jalapão Dunes in the north. This interaction, sculpted under typical savanna climate, has turned out into a mosaic series with immense landscape peculiarities. Serra Geral do Tocantins is a sedimentary plateau sculpted at Sedimentary Basin of Parnaiba. The Ecological Station's area is featured by sedimentary packs mainly continental, subsequent from basaltic extrusion that occurred during Wealdenian Reactivation in the Jura-Cretaceous period. It is composed by sandstones whose colors changes from pale to red ones, extremely friable, originated from dry paleoclimate accumulations (arid to semi-arid ones) of Mesozoic. It is possible to be related to contemporary dry Mesozoic period of Parana's Basin (Botucatu Desert), though upper layers can be from fluvial-lake environment. The relief forms involve mesetas and level sequences characterized by structural features with retreated edges and step levels into erosive forms. The mesetas are filled with cretaceous sediments and configure great plateau relief units penetrated by pedimented valleys, with scarped edges and large amphitheatres. Its attack by post-cretaceous erosion resulted into step levels whose softened surface produced pediplains that run to Tocantins River course. The very existence of considerable numbers of residual landforms contributes to the geomorphologic contrast. The predominance of the mechanical morphogenetic process can be divided into four sculptural phases resulting in differential dissection. Based on relief compartments it is possible to point out some considerations about the geographical distribution of the fauna in the Ecological Station's area and its surroundings, because at least part of it can be related to the topographic step-levels forms originated from geomorphological units. The biggest geographical barrier was created during Neoproterozoic, dividing the step-levels relief forms; the Mesozoic tilting produced the mesetas which by mechanical erosion formed pediments that suffered coalescence from Upper Cretaceous to Upper Tertiary. This process resulted into residual landforms, whose action in combination with ancient faults reactivations helped the isolation of biological compartments; continuing during Quaternary due to alternations of semi-arid to humid periods, mechanical erosion was initiated again and fluvial plains were formed.