Geology and Mineral Resources of Paraguay A Reconnaissance

28 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYswirled flow structures that resemble pillow-structures.All the flows seen are dark brown to black and aredense glass with sparse small dark phenocrysts. Amygdaloidalfillings and irregular discontinuous veinletsconsist of dull- to bright-green chlorite and variouskinds of silica chalcedony, agate, carnelian, opal, andclear to pale amethystine quartz crystals. Otherauthors have also recorded calcite, zeolites, native copper,and copper carbonate minerals in the amygdules.There is little doubt that copper minerals do occur inthe basalts in places, but it also seems probable thatmost of the recurrent reports of discoveries of richcopper deposits in the lava-covered parts of southeasternParaguay are based on misidentification of the greenchlorite that characterizes many flows.Structure sections drawn across the lava-coveredarea, such as sections A-B and C-D, plate 1, suggestthat the total thickness of lava in southeastern Paraguayis not much more than 200 meters. This figure is basedon assumptions that the lava dips very gently towardthe southeast and that comparatively little lava hasbeen removed by erosion. In the valley of the Paranaat Foz de Iguacu, where neither top nor bottom isexposed, there is a little less than 100 meters of lava.Farther east, in Brazil, the entire series is generally atleast 400 meters thick and locally as much as 800 meters(Gordon, 1947, p. 15).The lavas rest on red beds of the Misiones sandstone.Conradi (1935) gives an excellent description of theirregularity of the contact between lava and sandstonein the vicinity of Pedro Juan Caballero and of themetamorphism of the sandstone there. Several authorsincluding Washburne (1930) and Gordon (1947) mentionthe fact that beds of sandstone are interlayeredwith the lava flows, and are more numerous towardthe base of the lava series than near its top. Such interlayeringwas not observed by Eckel, nor by Baker(1923), who examined the lavas along the entire lengthof the Parana valley; Baker believes that interlayeringmay be confined to the edges of the Parana basin butWashburne describes and pictures some of the red bedsat Salto del Guaira. Whether the interlayering is dueto intrusion of sills among the red beds, or to alternatingdeposition of red beds and surface flows is not known.Unless the small body of sedimentary rock along theParana valley and northeast of Encarnacion is ofCretaceous age, an unlikely possibility that is discussedin the section on the Misiones sandstone, the SerraGeral lavas are not covered by younger rocks anywherein Paraguay. They are, however, overlain by continentalsediments of the Caina formation age in adjacentparts of Brazil. The Caina, which is very similar tothe underlying Botucatu (Misiones) sandstone, hasbeen shown by Scorza (1952) to be of latest Triassicor earliest Jurassic age. It is possible that similarsediments once overlay the lavas in Paraguay, thoughBaker (1923) presents fairly convincing evidence thatover most of the Parana basin the lavas have beenexposed to erosion ever since their deposition.Since the Serra Geral lavas rest on beds of Triassicage, and are overlain in Brazil by Late Triassic orEarly Jurassic age, the lavas themselves are quiteprobably also Triassic. Washburne (1939), Oliveiraand Leonardos (1943, p. 500), and Harrington (1950)incline toward this view, but Baker (1923), Gordon(1947) and others consider them to be possibly or probablyJurassic. Even though the work of all these writersantedated that of Scorza (1952), who determined theage of the overlying Caina, it seems safest here to assignthe lavas to the Upper Triassic or Jurassic.The only good exposures of the Serra Geral lavasin Paraguay, known to the author, are along the RioAlto Parana and the lower reaches of some of its tributaries.Elsewhere the lava is weathered to depths of3 to 8 or more meters to a chocolate-red clayey butcrumbly soil; close to the surface even the chloriticand chalcedonic amygdule fillings have disappeared.The resultant deep soils support luxuriant vegetationeverywhere and form, incidentally, the best coffeegrowinglands in both Paraguay and southern Brazil.The valleys of the Parana and its tributaries tend todevelop steplike profiles, both in longitudinal and transversedirection, due to differences in erodability of theflows. Except at major waterfalls, however, such asGuaira, Iguacu, and the smaller ones on the Nacundayand Monday rivers, the steplike pattern is not generallydiscernible on casual inspection.The 30-meter zone between high and low water onthe Parana is a good field for study of erosion of resistantvolcanic rock by slow-moving, relatively clearwater. The Parana carries remarkably few tools forerosion (silt, sand, or larger size pebbles), no doubtbecause the dense vegetation prevents active erosionof the deep soils. Yet the river has, since Jurassictime, carved a gorge several kilometers wide and 100to 200 meters deep into the lavas. The dominantmeans of downcutting appear to be constant removalof chemically weathered rock by running water andabrasion by the little sand and silt carried by the stream.There are pot holes and undercut notches in someplaces (fig. 20), but they are rare. Angular blocks ofrock tend to remain on the river bed close to theirsource, and there are almost no boulders, cobbles, orother large cutting tools. In many places the rocksurfaces are beautifully etched, as if by a wire brush,and are evidently abraded by fine sand.