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South America and Southern Africa
The Devonian System in Colombia
Devonian rocks in Colombia occur mainly in the Cordillera Oriental and the Sierra de Perija. Thicknesses of complete sections vary from 200 to 800 m; lithologies are typically shale and siltstone with subordinate sandstone. The base of the Devonian section is an angular unconformity above igneous and metamorphic rocks (including Ordovician metasediments). The top of the Devonian appears to be conformable with Carboniferous rocks.
Typical sections are sparsely fossiliferous. Bryozoans, brachiopods, corals, trilobites, and gastropods are the most common fossils. These marine faunas occur mainly in the lower part of the Devonian. Brachiopods indicate a late Emsian to Eifelian age for the fossiliferous zones. The upper part of the Devonian section in Colombia may be nonmarine, but diagnostic fossils are rare, and therefore the top of the Devonian is not well dated. Devonian sediments apparently formed in a north-south epicontinental basin.
The Emsian and Eifelian strata lie unconformably on a terrain that includes metamorphosed Ordovician rocks with K-Ar dates indicating uplift and cooling in the Silurian. The lack of known Silurian sedimentary rocks (except to the northeast in the Merida Andes of Venezuela), coupled with the radiometric dates, indicates a post-Ordovician and pre-Emsian tectonic and thermal event of unknown origin along the continental margin as far south as Ecuador.
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