The 1:5,000,000 map of Paraguay gives a fairly acceptable picture of the distribution of the systems of rocks in the region east and south of Asunción. It is, however, unsatisfactory in the northern half of eastern Paraguay. Figure 1 shows the correct generalized distribution of the outcrops according to our present knowledge. In addition, a tiny exposure of fossiliferous Lower Devonian rocks should be plotted in the Chaco plains near Fortín Aroma (Lat. 20°12’ S., Long. 60°28’ W.).
The following account of the geology of Paraguay is mainly a summary of the writer’s paper (1950) on the eastern half of the country, with the addition of some information on the Chaco region, made available through the courtesy of the Union Oil Company of California.
The geologic literature on Paraguay is very meager. The most important papers are Milch’s (1894) and Goldschlag’s (1913) descriptions of some Mesozoic eruptive rocks, Beder’s (1923) discovery of Mesosaurus-bearing beds near Villarrica, Beder and Windhausen’s (1918) report on the discovery of Lower Devonian fossils, and Boettner’s (1947) brief account of the Precambrian and Lower Paleozoic rocks exposed south of the Rio Apa.
Two main areas of Precambrian rocks are known in Paraguay, one in the northern and the other in the southern part of the country. The northern exposures are mainly gneiss, micaschist, and phyllite intruded by biotite granite and crossed by granite pegmatite dikes many of which bear tourmaline and beryllium. These rocks were briefly described by Boettner (1949) between Puerto Fonciere and Toldo-cué, south of the Rio Apa. The metasedimentites appear to be a southern extension of the Cuibá group of Mato Grosso (Brazil) and are best regarded as middle Pre-cambrian.
Metasedimentites are scarce in the southern exposures. They crop out in small patches between Villa Florida and Paso Pindó on the Rio Tebicuary and consist of foliated gneiss, micaschist, and chlorite quartzite with occasional thin intercalations of marble. These rocks are intruded by biotite granite and granite porphyry. Extensive exposures of rhyolite and keratophyre seem related to the granite. They are little weathered and almost devoid of metamorphism and may be tentatively correlated with the late Precambrian Aiguá extrusives of Uruguay and the Castro and Maricá “porphyries” of southern Brazil.
Two small outcrops of pink biotite granite are known a little east and southeast of Asunción, one at San Bernardino on the east shore of Lake Ypacaraí (Fig. 2) and the other near Itá.