The Sedimentary Successions of Itararé Group in Mirador, SC — Southern Brazil

Records of a glaciation long gone

Along the roads nearby the city of Mirador, in Santa Catarina (Southern Brazil), there are many outcrops of successions of sedimentary rocks of various types. Usually, these successions are composed by layers with a few centimetres to a few decimetres of thickness, with approximately tabular shape and lateral extensions of hundreds of metres, if not more. Two main types of rocks can be identified: i) bright grey to bright yellow mudstones, with planar lamination and, eventually, low-angle tabular cross bedding; and ii) bright yellow fine do medium sandstones, with a series of sedimentary structures typical of river transport. Amongst the structures found in the sandstone layers, there are tabular cross-bedding, trough-shaped cross-bedding, planar lamination and ripple marks, the last usually found at the top of the layers. In addition to that, load casts and flame structures can be found along some of the interfaces between the sandstones and the mudstones. Occasional dropstones can also be found. Some of these features are illustrated bellow.

The outcrops around Mirador, SC — Southern Brazil, are fascinanting. Here we have: a) an outcrop showing the sequence of sandstones and mudstones; b) a zoom-in of the outcrop; and c) a sketch of the main features observed. Notice the load cast and flame structures, and the tabular aspect of the sedimentary layers.

The lamination in the mudstones is showcased by the intercalation of brighter and darker laminae. Although not diagnostic, this feature allows the inference of possible seasonal variations during its formation. It is possible that the darker laminae are deposits richer in organic matter, possible due to the high mortality caused of microbial fauna during winter. That, in addition to the occasional dropstones, indicates the possibility of glacial influence during the formation of the sedimentary successions; the mudstones, or at least some of the bedsets, could, therefore, be diamictites, a rock typically formed in glacial environments. The structures on the sandstones, on the other hand, indicate flux-driven processes, allowing one to assume that these rocks were formed by rivers. If the hypothesis of the glacial influence is correct, the sandstones could have been formed by rivers associated with glaciers. It is possible that the successions seen formed along the shores of sea or lake, in moderate to close proximity to a glacier. The system would have been far away enough to preserve a strong continental/river-like aspect, but still close enough to show signs of direct glacial influence.

May different sets of sedimentary rocks and structures can be found in the Mirador region. Here we have examples of: a) sandstones with convoluted folds and load cast structures; b) heterolite with convoluted folds and strong internal lamination; and c) intercalation between shales and ritmites.

According to Milani et al. (2007), the rocks exposed in the region belong to the Itararé and Guatá Groups, parts of the Gondwana I Sequence of the Paraná Basin, deposited between the Early Carboniferous and the Late Triassic. They probably belong to the Lagoa Azul, Campo Mourão and Taciba Formations, which are the records of the Early Palaeozoic glaciation that took place in the Paraná Basin (Vesely & Assine 2004). According to França & Potter (1988), the rocks of these formations compose finning-upwards bedsets associated with sea level and weather variations, including gradations between sandstones and mudstones. In addition to these, diamictites (both laminated and massive) and ritmites have been identified (Rocha Campos1967, apud Milani et al. 2007), pointing to a complex mix of diverse glacial environments.

References:

França, A. B. & Potter, P. E. 1988. Estratigrafia, ambiente deposicional e análise de reservatório do Grupo Itararé (Permocarbonífero), Bacia do Paraná (Parte 1). Boletim de Geociências da Petrobras, Rio de Janeiro, v. 2, n.24, pg. 147–191.

Milani, E.J.; Melo, J.H.G. & Souza, P.A. et al. 2007. Bacia do Paraná. Boletim de Geociências da Petrobras, Rio de Janeiro, 15 (2), pg. 265–287.

Vesely , F. F. & Assine, M. L. 2004. Sequências e tratos de sistemas deposicionais do Grupo Itararé, norte do estado do Paraná. Revista Brasileira de Geociências. v. 34 (2). pg. 219–230.