The North Rupununi is situated on the Guiana Shield. The Guiana Shield is Precambrian and has a complex geology that includes plutonic, volcanic, metamorphic and sedimentary rocks and various rifting, uplifting, sedimentation and erosion events. Understanding the geology of the North Rupununi is critical because it fundamentally defines the topography, soils, hydrology and ultimately the economy of the area.
The North Rupununi is part of a Mesozoic graben – the Takutu Basin. The basin is 280 km long and 40 km wide, is over 7 km deep, and covers more than 11,200 km2 in Guyana and Brazil.
The basin is bounded on either side by parallel faults, today seen as the Pakaraima Mountains to the north and the Kanuku Mountains to the south. The basin is intra-cratonic and lies entirely within the Pre-Cambrian Guiana Shield and was formed 200 to 150 million years ago.
The geological history of the North Rupununi includes a major volcanic phase and three depositional phases. The rift began to form in the late Triassic to early Jurassic with the extrusion of the Apoteri flow basalts which underlie the basin in a layer 1,700 m thick; these volcanic basalts outcrop along the southern and eastern margins of the basin and include Toucan Hills which is a 135 mya lava flow. Since formation, the basin has been filled with sediments to a depth of 5,400 m. These sediments include lower Jurassic non-marine clastics of the Manari formation, which are overlain by lower to middle Jurassic non-marine shales of the Pirara formation, and in turn overlain by non-marine clastics of the middle to upper Jurassic Takutu formation which were deposited in lake a delta environments. Some sediments in the Takutu Basin were deposited by the Proto-Berbice which between 30 and 25 mya emptied the Branco, Essequibo, Demerara and Berbice drainages into the Atlantic . There are minor rifts within the Takutu Basin and laterite beds that formed about 35 mya in the Karanambu-Nappi areas.
The areas surrounding the Takutu Basin contain much older rocks. The Iwokrama, Maipa and Watamung Mountains are parts of the andesitic tuffs and lavas that flowed in the region between 1,900 and 1,200 mya and have underlying sub-volcanic granites. The Pakatau Hills, Turtle Mountain, and Kurupukari Falls are made up of dolerites that formed as dikes and sills within the conglomerates of the Pakaraima Mountains about 1,700 mya and were later exposed. Makarapan Mountain is a granite massif formed between 2,400 and 1,900 mya and is one of the oldest above ground formations in the Guiana Shield. The Pakaraima Mountains are part of the Roraima sedimentary table lands – a huge sheet of sedimentary conglomerates and sandstones that has been substantially eroded. There are also granites and silica rich rocks that were formed about 1,500 mya along the Siparuni and Essequibo Rivers – for example near Rappu Falls. The Kanuku Mountains are a massif that is part of the Kanuku Complex that includes metamorphic rocks mixed with igneous granite and dolerite intrusions – the Kanuku Mountains appeared as a result of block faults lowering the Takutu Basin; the South Savannahs are part of the Kanuku Complex and are therefore geologically very distinct from the North Savannahs.