A fragment of a lost continent
Rising gently from the vast, flat plains of the Pampas, the Tandilia Hills are a quiet anomaly. This low mountain range, extending 350 kilometers, is not from recent geological upheaval. Instead, it is the deeply eroded remnant of a Paleoproterozoic orogeny, a mountain-building event that occurred over two billion years ago. The rocks here are the exposed foundation of the Río de la Plata Craton, one of the ancient stable blocks that form the core of the South American continent.
The basement complex consists of igneous and metamorphic rocks, including granitic gneisses, migmatites, and amphibolites, with ages clustering between 2.26 and 2.07 billion years. These rocks record a complex history of continental collision and formation, a part of Earth's early supercontinents long before the famous Pangaea. The entire system shows a time when the fundamental architecture of our planet's continents was being established. For hundreds of millions of years following their formation, these mountains eroded, creating a deep unconformity, a massive gap in the geological record, before younger sedimentary layers were deposited on top.
A record of atmospheric change
The age of the Tandilia basement rocks places them directly in the timeframe of one of the most significant events in planetary history: the Great Oxidation Event (GOE). This period, roughly 2.4 to 2.0 billion years ago, marks the first time that biologically produced oxygen, created by early photosynthetic cyanobacteria, began to accumulate in the atmosphere. While the basement rocks themselves do not contain the fossils, they form the stable platform upon which later Neoproterozoic sediments, like the Sierras Bayas Group, were deposited. These younger layers contain stromatolites—fossilized microbial mats, which are evidence of the very lifeforms that changed the world.
The GOE had dramatic consequences. Oxygen was toxic to much of the anaerobic life that existed at the time. It also reacted with dissolved iron in the oceans, causing it to precipitate and form massive banded iron formations, the source of most modern iron ore. By reacting with atmospheric methane, a potent greenhouse gas, the rise of oxygen also triggered the Huronian glaciation, a series of global ice ages. The ancient craton of Tandilia remained to this planetary transformation, preserving the geological context of Earth's first breath.