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Geology and the Secret Stone

Preserved chapter of Earth's history

Located in the municipality of Caçapava do Sul, Serra do Segredo is composed of hills that rise sublime in front of the surrounding relief. Its formation dates back to remote periods in Earth's history, consolidating itself as the most complete and well preserved record of the Neoproterozoic / Paleozoic transition between the moving band of the Brasiliana orogeny and the stable phase of the South American Platform (Almeida, 1969). With about 300 meters thick, the continental sedimentary rocks constitute the youngest geological units of the Santa Bárbara Formation, linked to the Camaquã Basin.

Its association of facies indicates a paleoenvironment of dynamic formation, with a fluvial character dominated by wide intertwined rivers that flowed between adjacent mountains. The flow capacity of the fluvial system, combined with the contribution of alluvial fans from the mountains, enabled the deposition of sandy fractions to gravel sediments, which were later lithified in a relatively deep diagenesis process.


The Serra do Segredo is formed by conglomeratic sandstones and poorly selected conglomerates, which are arranged in lenticular and scoop-shaped layers with grooved cross stratifications (Borba & Misuzaki, 2003). In addition, there are lenticular and tabular layers of clusters supported by clasts. The lithological fragments are composed of volcanic, metamorphic and granitoid rocks from Caçapava do Sul, suggesting a direct interaction with the depositional system. They have a tight packaging, with calcite and iron oxides as the main cement, which suggest a continental oxidation environment.

In more recent periods after the intense process of lithification, tectonic processes played an important role in its uplift. Faults and fractures associated with the Permian and Triassic, with reactivations along the cretaceous (Borba et al., 2002), appear especially in the NW - SE direction. The presence of tafoni cavity structures, and the presence of alveolar alteration features found in humid cavity walls facing south, register climatic fluctuations of the last 500 thousand years that alternate between wet and dry climates (eg Suertegaray & Silva, 2009 ). Under ideal conditions, these cavities are essential for the structuring of a habitat, harmonizing geological elements with fauna and flora.

The long and complex geological evolution of Serra do Segredo reveals a still preserved history of high natural, cultural and scientific value. The result of its admirable resilience is realized in the formation of a singular geodiversity that allows us to preserve, understand and disseminate its value in the geological history of the Earth.

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