Geological and geophysical evidence for a rift valley in the Guiana Shield

Abstract

Geological and geophysical evidence points to the existence in the Guiana Shield between latitudes 3°N and 4°N of a steep-walled graben structure which is filled with a thick packet of sediments of which the uppermost is the Takutu Formation showing rare outcrops beneath a blanket of laterite and Quaternary alluvium. The Takutu Formation has been dated on palynological evidence (van der Hamm en and Burger, 1966) as Jurassic to Cretaceous. A gravity reconnaissance was carried out over the western portion of the structure and revealed anomalies which indicate a depth to basement of several km, The strike of the geological structure appears to be ENE-WSW over 160 km, with a width of 50 km, and it is suggested that it is in the nature of a rift valley: it has been considered to constitute a major structural break which divides the folded Precambrian formations of Guyana into northern and southern geological provinces.