Quantitative bedrock geology of Brazil

Abstract

We quantitatively analyze the area-age distribution of sedimentary, igneous, metamorphic, and ultramafic bedrock on the basis of data from the digital geologic map of Brazil, published as a GIS map by the Brazilian Geological Survey. Bedrock units exclusively encompassing sedimentary rocks, igneous rocks, or metamorphic rocks cover 40.4%, 31.5%, and 17.7%, respectively, of the total bedrock area. These numbers have to be considered minimum estimates of the areal abundance of sedimentary, igneous, and metamorphic bedrock because polygons defined by mixed lithologies cover ~8.5–9.5% of the total bedrock area. These mixed units are sedimentary rocks with igneous and/or metamorphic contributions (1.4%), metamorphic rocks with sedimentary contributions (1.2%), metamorphic rocks with igneous contributions (1.5%), igneous rocks with sedimentary and/or metamorphic contributions (4.4%), and ultramafic units with sedimentary, igneous, and/or metamorphic contributions (~1–2%). The average ages of major lithologic units, weighted according to bedrock area, are as follows: sedimentary rocks (average stratigraphic age of 248 ± 5 [1σ] Myr; median stratigraphic age of 87.5 Myr), igneous rocks (1153 ± 13 [1σ] Myr), metamorphic rocks (1678 ± 30 [1σ] Myr), and ultramafic rocks (~1227 ± 25 [1σ] Myr). The average bedrock age of Brazil is 946 ± 7 [1σ] Myr. The range in lithologic composition and age structure of the various bedrock units reflects the complex tectonic makeup of Brazil that ranges from Neogene sedimentary cover in the Amazon Basin to Precambrian cratons (Guyana and Brazilian shields) and Transamazonian greenstone belts. The average spatial resolution of the data is 232 km2 polygon−1 and is sufficient to perform area-age analyses of individual river drainage basins larger than ~5,000 km2.