Stratigraphy of Northeastern Anzoategui, Venezuela

Abstract

A reference section for the 40,000 feet of Cretaceous and Tertiary sediments of northeastern Anzoategui (Venezuela) was established several years ago on Rio Querecual and in the vicinity of the town of Santa Ines. Subsequent studies on the west and east have given a more complete three-dimensional picture of the original formations and their facies changes and have filled those parts of the stratigraphic section poorly represented on Rio Querecual. During the course of this work numerous fossil collections have been made and these have been submitted to specialists for study with the result that more authoritative data on the ages of the several formations are now available. The oldest rocks of northeastern Venezuela are the schists, phyllites, quartzites, and metamorphosed limestones of the Caribbean series, probably early Mesozoic in age. These rocks are in fault contact with unmetamorphosed Lower Cretaceous sediments which are well represented in northeastern Anzoategui and include about 5,000 feet of laterally variable shallow-water sandstones, shales, and reef limestones belonging to the Barranquin, El Cantil, and Chimana formations. These are followed by a deeper-water facies of 3,000-4,000 feet of black carbonaceous-bituminous limestone and calcareous shale with local sandstone and chert developments in its upper part, constituting the Guayuta group (Querecual and San Antonio formations) of Upper Cretaceous age. Conformably overlying the Guayuta gr up is the Santa Anita formation consisting of a lower sandstone member (Upper Cretaceous), a middle shale member (Upper Cretaceous-Paleocene), and an upper member of calcareous and dolomitic siltstones, glauconitic sandstones, and shales (Paleocene-Eocene). The whole formation is about 2,400 feet thick. The Santa Anita formation is overlain by the Merecure formation. In the eastern part of the area the contact between the two appears transitional but in the vicinity of Barcelona there may be an erosional unconformity. The basal Merecure contains orbitoidal foraminifera indicative of late Eocene age. The upper part of the formation is formed by the thick Naricual coal-bearing member and is probably in part Oligocene in age. The total thickness of the Merecure formation is 5,500-7,500 feet. The non-marine sediments of the upper part of the Merecure formation grade upward without a break into the more marine Santa Ines formation consisting of about 24,000 feet of sandstones, grits, conglomerates, shales, and claystones of middle Oligocene to late Miocene age. The marine character of this formation increases from west to east, and in the eastern part of the area it includes a fairly deep-water, highly foraminiferal shale member (Carapita shale). The Santa Ines formation is unconformably overlain by the Sacacual group (Pliocene) which is in turn unconformably overlain by the Mesa formation (Pleistocene). The sedimentary sequence of northeastern Anzoategui indicates that in early Cretaceous time a marine transgression began in this area which reached its culmination with respect to depth of water in Turonian time (Querecual formation). Toward the close of the Cretaceous (San Antonio and Santa Anita formations) there was a tendency toward emergence and a shallowing of the sea although its extent was still great. Marine deposition appears to have continued into Paleocene time, however, and in the eastern part of the area appears to have persisted with little or no indication of a break until the late Eocene. In the Barcelona area there may have been emergence and erosion in the early End_Page 1------------------------------ or middle Eocene (Santa Anita-Merecure unconformity). In the late Eocene (lower Merecure) marine deposition was widespread but was soon replaced by a coal-basin environment which continued into Oligocene time. In the middle Oligocene another major marine transgression began, during which the sediments of the Santa Ines formation were deposited. The area was then folded in late Miocene time before the deposition of the Sacacual group (Pliocene) and probably again in Pliocene time before Mesa (Pleistocene) deposition. The depositional history of the area is related to the pressure of the northern borderland of Paria against the continental nucleus of the Guayana shield and the consequent persistent southward shifting of the axis of the intervening Eastern Venezuelan geosyncline. During Cretaceous and early Tertiary time northeastern Anzoategui lay on the stable southern side of the geosyncline where deposition was relatively continuous, and formations follow each other without angular relation. In Oligocene and Miocene time it occupied a nearly axial position and received the thick flysch-like deposits of the Santa Ines formation. Toward the close of the Miocene it actually became a part of the northern flank of the geosyncline and as such its sediments were folded with each subsequent movement of he mobile northern borderland.