Slip-Planes and Breccia Zones in the Tertiary Rocks of Peru

Abstract

The Peruvian oilfields lie on the coast about 4° south of the Equator, where South America juts out farthest west into the Pacific. At Ancon, in the Santa Elena peninsula of Ecuador, is another oilfield with rocks of the same age. The Tertiary rocks on the coast are cut off by the Amotape mountains on the south-east, about 20 to 25 miles inland. These mountains are a complex of Carboniferous slates and quartzites metamorphosed by intruded granites, with small outcrops of unmetamorphosed Cretaceous rocks. The range runs north-east and south-west, and most of the main tectonic features are parallel to this axis. The oil-bearing strata are Middle and Upper Eocene at Negritos, Talara, Lobitos, and Cabo Blanco in Peru, Upper Oligocene or Lower Miocene at Zorritos, 70 miles farther north, and Middle and Upper Eocene again at Ancon in Ecuador. A north-east and south-west section through Cabo Blanco shows the following features (Fig. 1) : a geofault five miles off-shore, where the sea-bottom drops to 2000 fathoms; a flat coastal shelf; a narrow belt of deeply dissected Tertiary rocks; the Mancora Tablazo (a flat tableland of Quaternary deposits 15 miles wide and 1000 feet above sea-level); foothills of Tertiary rocks; and the Amotape mountains of metamorphic rocks. The geology has been well described by Bosworth and again by Olsson and Iddings. The most striking feature, the slip-planes and breccia zones, have been described only in my private geological reports during the last twelve years. I have to thank the directors