SHELL-RICH PEBBLY RESISTATES: RECOGNIZING THE FOSSILS AND INTERPRETING THE SOURCE ROCKS

Resistates (= residual sediments consisting chiefly of minerals chemically resistant to weathering processes; _resistatos_ in Portuguese) can hold incredibly amounts of geological and paleontological data. Fossil-rich resistates can be found in deeply eroded regions considered of low potential for paleontological prospection given the nature and origin of the local geological formations. In this study, based on fossil-rich resistates from the Triângulo Mineiro region, Minas Gerais State, Brazil, we show that these relict particles can offer significant paleontological/stratigraphic information. The resistates were discovery by one of us (AAG) in distinct sites of the Ituiutaba (Morro do Corpo Seco, Morro Três Cruzes, Renascer rural settlement) and Monte Alegre de Minas (Vale do Babilonia) counties. Locally, the landscape is marked by small plateaus and mesas sculpted on Cretaceous sandstones/conglomerates (Bauru Group) in the Tijuco river watershed. The resistates are composed of highly silicified shell-rich pebbles found in lag gravel deposits (= cascalheiras) and in stone lines in soil profiles. Over a hundred of fossil-bearing resistates were collected, and detailed laboratory work have revealed the presence of well-preserved bivalves. Hence, individual shells were numbered, photographed, stored, and studied, and small polish slabs revealed the original biofabric of the shell beds. At least two groups of Permian bivalves were precisely identified: _Guiratingia mendesi_ and _Runnegariella fragilis_. Therefore, the resistates are sedimentary particles made of silicified Permian limestones (shell-beds) of the Tatuí and Teresina formations, in which the original stratigraphic position of those species is precisely well-constrained. Both formations are not mapped in the study area, where Cretaceous siliciclastic rocks of the Bauru Group and underlying basalts of the Serra Geral Formation are cropping out. Probably, the residual sediments covering the plateaus and mesas were locally originated from weathering of conglomerates and sandstones of the Marilia Formation, but the presence of shell-rich pebbles clearly indicates previous erosion of silicified Permian source rocks and long-distance transport of fossil-bearing clasts. Considering this case study, we emphasize that geologists and paleontologists should pay attention to resistates, because relatively small samples found in gullies, stream banks, and weathering profiles may yield significant fossils of key importance to regional paleogeographic reconstructions.