AT THE WESTERN TIP OF THE GUIANA SHIELD Francesco Sauro The Caquetá region represents the western tip of the immense cratonic basin of the Guiana Shield. However, there are some important geological differences compared to the eastern tepuis of Venezuela and Guyana Esequiba. Here too, the geological history of the Serranía de Yarí and Chiribiquete began between 2000 and 1800 million years ago, during the Precambrian period, when the crystalline rocks of the basement were formed. The Guiana basement in Colombia is characterised by two regions or geological provinces: the first is the Mitú Complex formed by migmatic gneiss, granitic gneiss and granitoids, locally associated with some mica schists, quartzites and amphibolites. The second geological province is the so-called Parguaza Rapakivi granite, which is considered an intrusion of the Mitú Complex and emerges in the north-east of the Vichada Department in the eastern plains and on the Venezuelan border, in the form of bare rounded hills. Subsequently, a process of erosion and debris sedimentation occurred above this shield zone in an internal basin. This erosive event is indicated by a non-conformity that brings Precambrian crystalline rocks into contact with Ordovician quartzite sediments: a leap in time from 1.8 billion years, when life still only consisted of unicellular organisms, to approximately 485 million years, with the proliferation of the first complex living organisms, such as trilobites, gastropods and cephalopods. This is a significant difference compared to the quartzite rocks of the Venezuelan tepuis, which instead are much older, dating back to around 1.6 billion years ago.
The Ordovician quartz arenites of Caquetá are referred to as the “Araracuara Formation”, precisely because they emerge along the walls of the large canyon of the same name. Subsequently, the rocks of the Colombian Amazon basin were cut open by horst-type faults in the late Palaeozoic, forming the primordial structure of the long Serranía de Chiribiquete. However, it was only in the Neogene (25–5 million years ago) that this entire area was lifted up along with other Amazonian massifs such as the Lindosa, Tunahi, Naquen and Tigre, in association with the Andean orogeny. Further studies will be needed to establish whether the surface denudation and erosion rates of these mountains are similar to those of Venezuelan tepuis. However, the caves and collapses that can be observed in the plateaus are certainly of a considerable age to have developed into such pure and ancient quartz arenites. Bibliography Memorie inedite di Agostino Codazzi, Mario Longhena (Ed.) – edizioni Alpes – Milan 1930 German Castro Caycedo, Mi alma se la dejo al diablo – Editorial Planeta Colombiana – Bogotà 1986 Galvis J., “Estudio Geológico de La Sierra de Chiribiquete Y Zonas Aledañas (Parque Nacional - Natural Chiribiquete)” in Revista Academia Colombiana de Ciencias 119 (73): 275– 86, 1994. Mapa de una parte de los rios de los engaños ó Commiari – 1789 - https://www.loc.gov/resource/g5292y.ct 000309/?r=-0.309,0.038,0.678,0.261,0
The River Caquetá carved the deep Araracuara canyon in the formation of Ordovician quartz arenites of the same name
