I love it when the dramatic and violent history of the Earth is laid bare for anyone to see. The fault, folds, and crenulations in this marble are the unmistakable evidence of a traumatic past for this tiny patch of the world.
Located in the Jazida do Urubu marble quarry, this outcrop was exposed during mining operations.
Here’s how to see what geologists found so fascinating the photographed this outcrop for the United States Geological Survey in 1957: The dolomitic marble outcrop is home to reverse faults, drag folds, and crenulations, all of which offer insight into the regional history near Minas Gerais in Brazil.
Folds are large-scale bends in the rock, places where the rock ductilely deformed under compressive stress. A single large fold dominates the outcrop, a sideways S-curve disrupted by later processes. Less than a meter above the geologist’s head (always a good move to include a human-for-scale in photographs like this!) are a series of tiny crenulations, little scribbles of wonkiness. The outcrop is just a tiny sample of the forces that have torn the region—the entire zone has north-south trending folded from large-scale thrusting with up to 300 meters of displacement. The region is scattered with superpositions of smaller southeast-trending folds like those in the marble quarry walls.
Faults occur when the stress on the rock is too much, and it brittlely fails. The result is a sharp offset, with the direction of motion naming it a normal or reverse fault. From the basic stratigraphic principles that guide geological interpretation, we know that a thing needs to exist before it can be altered. From that, we can tell that although some faults formed early on and were partially recrystallized in the subsequent chaos, the steep reverse faults formed late after all the folds. Drag folds along the steep reverse fault are distinct near the base of the exposure, getting more distorted and unrecognizable farther up.
With roughly 1,500 to 4,000 tons of crushed marble extracted between 1954 and 1961, geologists had plenty of opportunities to ogle exposed geologic structure to uncover hints about the Earth’s history in this little corner of Brazil. The outcrops were quarried into blocks of marble with a wire saw, creating 1 to 2 meter thick slabs between 10 to 30 meters long and 5 to 15 meters high. For ease, the slabs typically followed the steep bedding and cleavage planes, detaching ad falling until a prepared surface. At a massive 3 metric tons per cubic meter, the slabs had to be cut down even further once detached, getting cut into smaller 2 to 10 ton blocks for transport.
This outcrop was included in U.S. Geological Survey Professional paper 341-F published in 1965.