The Avanavero Suite
THE Roraima Group is the third distinct geological unit of the Northern Province of Guyana. The other two are the flat coastal plains and the granite-greenstone belt. The rocks of the Roraima Group form the high plateaux and hills of the Pakaraima Mountains in the central west of Guyana, south of the Cuyuni River. The Roraima Group is the location of Mountain Roraima; of the mysterious table-top mountains called Tepuis; of Kaieteur Falls; and of many other spectacular geological features of Guyana. The rocks of the Roraima Group, mainly the Pakaraima Mountains, cover an area of 25,000 square kilometers, and lie within Region Eight (Potaro/Siparuni). The Pakaraima Mountains extend into Brazil and Venezuela for over five hundred miles. Communities include Kato, Orinduik and Paramakatoi. The Roraima Group comprises thick sedimentary sequences of sandstone and conglomerates deposited on the northern part of the Guiana Shield, following the Trans-Amazonian orogeny of between 1.9 and 2.1 billion years ago.
Geological evidence indicates that the Roraima Group represents sedimentary fill in a shallow oceanic basin, with material that was derived from erosion of the mountains to the north and north-east, formed during the Trans-Amazonian orogeny of 1.98 billion years ago.
The group of rocks initially comprised unmetamorphosed sedimentary rocks, such as sandstone and conglomerates. These were, however, later intruded by igneous intrusive rocks to which geologists refer as the Avanavero Suite (or set) of dikes and sills emplaced in the mid-Proterozoic period, between 1.65 and 1.85 billion years ago.
In this process, magma was pushed up from deep within the earth into any cracks or spaces it could find, sometimes pushing existing rocks out of the way. The process of magma formation and intrusion and formation of dikes and sills within the sandstone occurred over a long period of time.
Dikes, by definition, are hard, intrusive rocks with formations of a vertical nature. Dikes are extremely resistant to weathering and erosion, and have given rise to impressive waterfalls and rapids, where they cut across the rivers and streams not only in the Pakaraimas, but throughout Guyana. They vary in thickness from intrusions of magma ranging in size not more than three feet across — which are exposed in the courses of the rivers only during very dry seasons — to hills and mountain ranges. Sills are originally dikes that had encountered resistance while rising to the surface, and consequently spread laterally just underneath a harder rock cap. The sill got its name from its flat, horizontal nature. Some sills form as deep as 3.2 kilometres below the surface, spread over a large area, and raise large tracts of the overlying land by thousands of feet. So, according to the literature on geomorphology, in western and central Guyana in the mid-Proterozoic period, voluminous amounts of upwelling magma intruded the sandstones and conglomerates of the Roraima Group. The large volume of magma, attributed to an aborted attempt at rifting of the Guiana Shield by divergent tectonic plate movements, formed dikes in some areas, and horizontal sills under the surface of the earth in others. The intrusions took place over an area of 73,000 square kilometers, some of it within Guyana, and the remainder in Venezuela and Brazil. Over time, the sandstone and conglomerates of the Group overlying the sills eroded, exposing these hard, flat rocks to the surface. Surrounding sedimentary rocks not protected by the hard cap also eroded, leaving isolated flat-top mountains. The mountains of the Pakaraimas are also called Tepuis, an Indian word meaning “House of the Gods.” Mount Roraima, at 2,810 metres (9,220 ft) above sea level on the Guyana, Venezuela and Brazil border, surrounded by cliffs 300 metres (980 ft) high, is the highest and most famous Tepui of the Pakaraimas. Mount Ayanganna is a heavily-forested sandstone Tepui 87 kilometres east of Mount Roraima, with a height of 2,041 metres (6,696 feet), and is the highest Tepui fully within Guyanese territory. Ever since Guyana became a Republic on February 23, 1970, the national flag is hoisted ceremoniously each year on Mount Ayanganna, on the eve of the country’s Republic Anniversary, by members of the Guyana Defence Force (GDF). The GDF has named their Georgetown base after this Tepui.
(Earth Matters is made possible through the ongoing support of local geologists, who are knowledgeable about the land forms of Guyana and the processes that shaped them.)

Next week: The Northern Provinces cont’d…