The Eastern Venezuela Basin is a depression in the igneous-metamorphic rock basement filled with Mesozoic and Cenozoic sediments to a thickness of as much as 40,000 feet. The basin has a general east-west trend and is some 800 kilometers long and 250 kilometers broad. It is asymmetrical in cross section with a long gentle southern limb flanking the continental nucleus of the Guayana Shield and an abruptly rising northern limb formed by the folded and faulted Caribbean Ranges. The basin has been the site of geosynclinal deposition through Mesozoic and Cenozoic time with the axis of deposition shifting progressively southward and nearer to the continental shield with successive orogenic episodes in the northern borderland.
The region may be divided into physiographic provinces which in general reflect the areal distribution of formations and the regional geologic structure. These are the Northeastern Mountain, Northwestern Mountain, Foothills, Coastal Plains, Delta, Mesa (and Dissected Mesa), and Rolling Plains provinces.
The basin is bordered on the south by igneous and metamorphic rocks of the Guayana Shield and on the north by the mildly metamorphosed sediments of the peninsulas of Paria and Araya. No igneous or metamorphic rocks are known within the interior of the basin except where deep wells have penetrated the basement.
The oldest sediments are the Hato Viejo and Carrizal formations known only through wells in the southern portion of the basin. These formations are poorly dated but are tentatively considered of early Mesozoic age. They are unconformably overlain by neritic and nonmarine clastics of the Cretaceous Temblador formation.
At the northern edge of the basin the oldest sediments are the formations of the Lower Cretaceous Sucre group (5000–6000 feet) which consists of nonmarine clastics at the base grading upward into reef limestones and other shallow-water marine sediments. The Sucre group is overlain conformably by the Upper Cretaceous Guayuta group (3000–4000 feet) of carbonaceous-bituminous limestones, shales, and cherts representing a deeper-water environment with poor bottom circulation. These in turn give way to the Santa Anita group (3000 feet) of Late Cretaceous-Tertiary age consisting largely of shallow-water marine sandstones, calcareous-dolomitic siltstones, and glauconite beds.
Epeirogenic movements in early Eocene time resulted in widespread emergence. The Upper Eocene-Oligocene Merecure group of sandstones, orbitoid milestones, and coal-basin sediments (6000–7000 feet) marks the beginning of a new cycle of deposits, unconformable on the Cretaceous sediments except in the deepest portion of the geo-syncline. The Merecure group is known to extend to both flanks of the basin. It is followed by the even more widespread marine and brackish-water deposits of the Oligocene-Miocene Santa Inés group which locally is 20,000–30,000 feet thick. The succeeding Miocene-Pliocene Sacacual group (as much as 5000 feet) marks a return to brackishwater or nonmarine deposition. At the northern edge of the basin it is markedly unconformable on the Santa Inés. The Pleistocene Mesa formation forms a blanket of several hundred feet of continental deposits, laid down following the last major uplift of the northern mountains and extending over almost the whole area of the basin.
Each of the numerous individual formations comprising the above-mentioned groups is described briefly with respect to name, type locality, areal distribution, lithology, thickness, surface expression, stratigraphic relations, environment of deposition, paleontology, age, and correlation, using both outcrop and well data available throughout the basin. The geologic history of the basin is outlined.