The Proto-Berbice: A former world
THE entire North Rupununi Savannahs are part of a Mesozoic (200 to 150 million years old) graben called the Takutu Basin.
As indicated before, that 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, centered in Lethem, Region Nine (Upper Takutu/Upper Essequibo). The Takutu Basin is also central to the hydrologic evolution of the Guiana Shield since the Cretaceous, a geologic period from circa 145.5 to 65.5 million (Ma) years ago.

Early rifting of the Takutu Graben resulted in volcanism in the Late Triassic (205Ma) to Early Jurassic (109 Ma), but the depression received freshwater sediments since the Middle to Late Jurassic (170 to 150 Ma).

Lake Maracanata, an endorheic lake (a closed drainage basin) approximately 75 to 100m deep, occupied the Takutu Graben/Northern Rupununi Savannahs until the Early Cretaceous (140 Ma) (Crawford et al. 1985).

This ancient lake, though becoming shallower as time progressed, and fluctuating in depth during dry periods, received predecessors of the modern Ireng, Cotinga, Takutu and Uraricoera Rivers (Brazil); and the Rupununi, Rewa, and Essequibo Rivers in Guyana (McConnell 1959; Sinha 1968, Berrangé 1975; Crawford et al. 1985).

Between the Late Cretaceous (70 Ma) to the Paleogene (60 Ma), Lake Maracanata, within the Takutu Graben, transitioned to a fluvial (depositional) environment with a  trunk stream, basically a large stream into which tributaries carry water and sediments, the proto-Berbice.

One of the largest drainages of the central Guiana Shield during much of the Cenozoic Era (covering the period from 65.5 Ma to the present) was the proto-Berbice (early Berbice), a northeast-flowing river draining portions of the Roraima State in Brazil, most of Guyana, and parts of southern and eastern Venezuela and western Suriname (Sinha 1968; C. Schaefer and do Vale 1997).

In its heyday, between 30 and 25 Ma, the proto-Berbice River emptied the Branco, Essequibo, Demerara and Berbice drainage basins. (Mc Connell 1959).
The proto-Berbice River flowed northeasterly through the North Savannas Gap, and exited to the Atlantic, between what are now the modern towns of New Amsterdam, Guyana; and Nickerie, in Suriname.

But by the end of the Pliocene (5 Ma), and during the Pleistocene (3 Ma), a process of river capture (natural diversion of the headwaters of one river into the channel of another stream having greater erosive activity and flowing at a lower level) marked the decline of the proto-Berbice as the main stream of the Guiana Shield.

Geologists have identified the tilting of the underlying basement, both in the North Rupununi Savannas and across the Guiana Shield, as the most frequent driver of head cutting (formation of new channels) and river capture.

Tilting is usually the result of tectonic plate movements. Other factors include terrain subsidence or uplift, also attributed to tectonic plate movements.
Tilting of the terrain is said to have occurred as recently as the Holocene, a geological epoch which began at the end of the Pleistocene (around 12,000 years ago) and continues to the present (Gibbs and Barron 1993).

Tilting to the west in the South Rupununi Savannas, for example, led to rejuvenation and steepening of east-bank tributaries of rivers, and aggradation (increase in land elevation due to the deposition of sediments) and sluggishness in west-bank tributaries of the north-flowing Takutu, Rupununi, and, in part, Kwitaro Rivers (Gibbs and Baron 1993).

Head-cutting by the Branco River in Brazil into the western end of what had been Lake Maracanata robbed the proto-Berbice of its Cotinga and Uraricoera tributaries at the end of the Pliocene(5Ma), then the Ireng and Takutu tributaries in the Pleistocene(3 Ma).

Evidence of a shift away from the lower Berbice River as the more important trunk stream can be observed in a so-called “elbow of capture” near Massara, at the eastern edge of the former Maracanata Lake, now the Takutu Basin.

This is the point at which the modern upper Essequibo shifts abruptly westward, away from a nearby north-flowing Berbice tributary, which has aggraded in response by raising the level of its stream bed (Gibbs and Baron 1993).
Through these acts of capture, facilitated by tilting, subsidence or uplift of the crust, the once mighty Berbice River has shrunk to its modern size and is now subordinate to its former tributaries of 25 Ma, the Essequibo and Corentyne Rivers.

It has been said that it seems likely that the Mazaruni and Cuyuni rivers were also only recently linked with the Essequibo, and that they historically exited to the Atlantic via their own mouths, separate from that of the proto-Berbice River system.