Geology

The Rupununi sits on some of the oldest rock on Earth. The Guiana Shield is the geological foundation beneath Guyana, Venezuela, Suriname, and northern Brazil. It is a Precambrian craton that formed over two billion years ago. The Rupununi’s extraordinary biodiversity is, at its deepest level, a consequence of this ancient geology.

The Guiana Shield

The northern rim of the Takutu Basin is formed by the Pakaraima Mountains. These mountains include some of the most dramatic landscapes in South America. Tepuis dominate these mountains. They are spectacular flat-topped sandstone table mountains. Tepuis are formed from the Roraima Supergroup, a sequence of hard quartz sandstones deposited between 1.7 and 1.4 billion years ago. The tepuis rise abruptly from the surrounding savannas and rainforest, their vertical sandstone walls dropping hundreds of meters.

Mount Roraima, at the junction of Guyana, Venezuela, and Brazil, is the most famous tepui. Its summit plateau is permanently shrouded in cloud. This environment hosts plants and animals found nowhere else on Earth. These species are isolated by millions of years of erosion. Im Thurn described this landscape in 1884. Conan Doyle immortalized it as the plateau where dinosaurs survived.

The Takutu Graben – The Rift Valley

The Rupununi itself sits in the Takutu Graben — a fault-bounded rift basin filled with Mesozoic sediments and basaltic lavas. The Takutu Graben is one of several failed rifts. These rifts are linked to the opening of the South Atlantic Ocean about 130 million years ago. When South America and Africa separated, the rifting did not propagate uniformly. The Takutu started as a rift but never completed. This left a sunken valley that filled with sediments over millions of years.

This sunken valley is why the Rupununi savannas are so flat, and why they flood so dramatically every wet season. The flat basin floor holds water. It transforms the savannas into an inland sea. This inland sea connects habitats and supports the region’s great fisheries. The seasonal rise and fall of water is about 6 to 15 meters. It serves as the engine of the entire ecosystem.

The Kanuku Mountains

The southern rim of the Takutu Basin is formed by the Kanuku Mountains. They are composed of ancient high-grade gneisses. These are metamorphic rocks formed under great heat and pressure deep in the Earth’s crust. The Kanukus are geologically distinct from the tepui sandstones of the north. They are formed from the deep basement rocks of the Guiana Shield. Their forested slopes harbor the highest biodiversity in Guyana. They sustain over half the country’s bird species. They also sustain at least 70% of Guyana’s mammal species.

The Kanuku Mountains Protected Area was formally designated in 2011, making it one of Guyana’s most important conservation designations. Its management plan for 2024–2028 includes community governance by Makushi and Wapishana villages on the mountains’ boundaries.

Water and the Flood Cycle

The annual flood cycle occurs due to the Guiana Shield’s impermeable geology. It is driven by the Rupununi’s flat basin floor. This cycle is the engine of the ecosystem. Wet-season rains can raise water levels by up to fifteen meters. These rains connect rivers, savannas, and forests into a single aquatic system. Fish move from rivers onto flooded savannas to feed and spawn. Giant river turtles lay eggs on sandy riverbanks. Black caiman patrol the shallows. When the waters recede, concentrations of fish in shrinking pools support both human communities and the region’s predators.