Geology and Mineral Resources of Paraguay A Reconnaissance

78 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYlate Tertiary; certainly it is continuing today. Thefull answer must await further detailed studies of theGran Chaco alluvial sediments themselves and of thegeomorphologic history of the eastern Andes.To the author's knowledge, Johnston (1876, p. 499)was the first to suggest that the Rio Paraguay is inprocess of being shifted toward the east as a result ofthe uplift of the Andes. There is considerable evidenceto support his belief that the river once flowedsome distance west of its present course and it is beingpushed eastward by the growing alluvial plains of theGran Chaco. This evidence lies chiefly in the relationsof the river to the outcrops of Itapucumi limestonefrom, the Rio Apa southward to San Salvador. On theeast bank of the river the limestone forms almost continuouscliffs, 3 to 30 meters high, along this entirestretch; in addition there are a number of small butsignificant hills made up of Itapucumi limestone alongthe Gran Chaco side of the river from near PuertoCasado northward (pi. 1). As seen in the field in thevicinity of Vallemi, as well as by aerial observation andstudy of aerial photographs farther south, the riverfollows almost exactly the curves in the strike of theItapucumi beds for the entire distance. The limestonecliffs along the river are almost everywhere steep; theirsurfaces are rough and pitted (see fig. 43).A few of the stronger joints in the rock are markedby narrow, steep to vertical-walled ravines, most ofthem choked by large joint blocks that have slid intothem. Most of the steeply dipping joints, however,are tight and almost imnoticeable on upper parts of thecliff faces. Yet within a few tens of centimeters of theriver level many of these joints have been wideneddownward by solution into open caves, shaped likeinverted Vs. This feature was also described andsketched by Carnier (1911c).In addition to the small tent-shaped caves along jointsthere is a striking series of longitudinal notchlike caves,that extend back beneath the cliffs from 60 centimetersto as much as 15 meters. As seen in April 1952, whenthe river was at a fairly high stage, these notches occurredat intervals from the water surface to a levelabout 5 meters above it; all were parallel to the watersurface. The upper limit marks the highest river levelin recent history. The solution notches are continuousacross different beds, though they tend to cut deeperinto the purer limestone beds than they do in thedolomitic ones.The relationship of the river to the general structureof the rocks, as well as to the solution features, seems tomean that the river is pushing against the limestone outcropslaterally, rather than that it has cut its way downthrough them in more orthodox fashion. Had it cutdownward,the limestone, which is extremely soluble inrelatively lime-free, carbon dioxide-rich water such asthat of this tropical stream, should have been dissolvedalmost completely. Outcrops, had they survived at all,should have been low and rounded, and joints shouldhave been widened into broad valleys. It seems morereasonable to suppose that until comparatively recentlythe Rio Paraguay flowed across the alluvium of theGran Chaco, west of its present course, and that theItapucumi rocks were subject only to subaerial erosion.Then, as the alluvium encroached eastward, it pushedthe river sidewise against the higher lands of what isnow eastern Paraguay, leaving "islands" of limestoneprotruding through the alluvium farther west, and allowingthe river to begin to gnaw away at the mainoutcrops.The only other explanation that occurs to the authoris that the Gran Chaco plains were once higher andmore extensive than they are at present and that theGran Chaco basin was then dropped relative to the rocksof eastern Paraguay. This would, it is true, expose thelimestone suddenly to solution by the river, but it failsto explain the outcrops of limestone on the Gran Chacoside of the river nor does it account for the sinuouscurves of the river that conform to the structure of thelimestone.MINERAL RESOURCESSUMMARYParaguay possesses large quantities of certain nonmetallicmineral resources, notably clays suitable forbrick, tile, and pottery; limestone and other raw materialsfor portland cement and for lime; common andornamental building stones; glass sand; talc; and mineralpigments. Supplies of all these materials appear tobe more than sufficient to meet all conceivable domesticrequirements.Except for iron ore, of which there are many small butrich deposits, the country appears to be very poorlyendowed in most other mineral resources, metallic andnonmetallic. It has a little manganese, copper, mica,and beryl, but unless larger and richer deposits arefound in the future than have been in the past, none ofthem seem to offer much promise. There is some slightchance that rumors of the existence of gold, tin, tungsten,lead and mercury, and gem stones may yet besubstantiated by actual discoveries or rediscoveries, butthe possibility of finding large or rich deposits of any ofthem is rather remote. There are, however, goodgeologic reasons for hoping that worthwhile deposits ofsalt, gypsum, and bauxite may yet be uncovered.Even if rich mineral resources were to be discovered,the general lack of fuels and power would preclude thedevelopment of manufacturing industries on a largescale. Aside from wood and water power that is both