Geology of Paraguay Basin. These tholeiites were erupted in the Early Cretaceous in association with the initial opening of the South Atlantic. The Cretaceous volcanism did also created minor outcrops of potassic alkaline volcanics in a graben structure developed between Asunción and Villarrica and in Amambay Department. Cretaceous volcanism did also left small units of sodic alkaline rocks in Misiones Department in southern Paraguay. Geology of Paraguay The country of Paraguay lies geologically at the borderzone between several cratons. Due to thick Cenozoic sediment cover and regolith development few outcrops are available in Paraguay. East of Paraguay River that Precambrian and Early Paleozoic
Geography of Paraguay Paraná; the mountain chain has an average height of . Separated from the Eastern region by the Paraguay River, the Chaco region is a vast plain with elevations reaching no higher than and averaging . Covering more than 60 percent of Paraguay's total land area, the Chaco plain slopes gently eastward to the Río Paraguay. The Paraguayan Chaco is subdivided into two parts. The "Alto Chaco" (Upper Chaco), also called "Chaco Seco" (Dry Chaco) is the western three-quarters of the region, bordering on Bolivia, while the "Bajo Chaco" (Lower Chaco) or "Chaco Húmedo" (Humid Chaco) borders on the Paraguay River.