The oldest rocks in Venezuela formed during the
Precambrian and occupy the Guiana shield in the southern tier of the country near Guyana and Brazil, east of the El BaUl swell. In the western
Guiana Shield, within the Amazonas Territory, Precambrian Roraima Formation zircon grains have been dated with uranium-lead dating and rubidium-strontium dating. Metamorphism and intrusive activity formed
gneiss with sedimentary and igneous protoliths around 1.8 billion years ago. Plutons emplaced
granite and
tonalite after inferred collision and subduction tectonics until around 1.55 billion years ago. The formation's volcanic rocks were produced 1.74 billion years ago. Metamorphism was driven in places by the
Transamazonian orogeny. Geochemistry research on the Imataca Complex within metaigneous and metasedimentary rocks reaching
granulite grade on the sequence of
metamorphic facies suggests high temperature decompression, based on assemblages of
sillimanite,
kyanite,
garnet,
orthopyroxene,
plagioclase and
quartz.
Paleozoic (539-251 million years ago) The
Paleozoic period in Venezuela is primarily exposed in the Andes and the central-western part of the country. Research in the Merida Andes in the 1960s revealed unmetamorphosed sedimentary rocks from the
Ordovician,
Silurian,
Carboniferous and
Permian, as well as metamorphosed slate. During the period, North America and South America collided, producing quartz-feldspar gneiss bodies spanning
Paria Peninsula into Trinidad. In western Venezuela, geologists have recognized the Apure allochthon, a remnant Paleozoic mountain belt.
Mesozoic (251-66 million years ago) The Merida Arch, a remnant mountain range from the
Pennsylvanian controlled sedimentation in the Tachira, Barquisimeto and Machiques as well as the Maracaibo and Barinas basins. The basins are primarily filled with
Jurassic and
Cretaceous shallow water carbonates. Throughout the
Mesozoic, Venezuela was a
passive margin of the South American continent.
Cenozoic (66 million years ago-present) Pull-apart basins began to form, filling with sediments between the
Eocene,
Oligocene,
Miocene and
Pliocene. For the most part, Eocene and Oligocene sedimentary rocks survive only in fault-bounded basins. In the mid-
Cenozoic, fragments of basement rock were detached by collision of the South American and Caribbean plate. The El Mango Gneiss formed. More broadly schist and gneiss formed in the
accretionary wedge. During the
Miocene and
Pliocene, extensive sediments accumulated in the western Guarico Basin and eastern Maturin Basin. ==Oil and gas reserves==