Geology and Mineral Resources of Paraguay A Reconnaissance

100 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYsupply problems of the Gran Chaco are not necessarilyinsurmountable. A vigorous but careful effort tosolve them might well result in development of sufficientwater to make the Gran Chaco outstanding in meatproduction and possibly agricultural food products.The ideas presented here are nebulous in the extreme,because of lack of specific local knowledge. They aresupported, however, by many facts from the ArgentinePampa, which is the southerly extension of the ChacoBoreal of Paraguay and where the geologic conditionsare similar. Stappenbeck (1926) gives many detailsof the geologic and ground water conditions in themuch more highly developed and more thickly populatedPampa. His work includes many well logs andchemical analyses, as well as a small-scale map showingthe distribution of artesian conditions not far south ofthe Paraguayan Gran Chaco.In any attempt to develop water supplies in the GranChaco, ground water must be used rather than themore obvious surface supplies. Even though the surfacewaters are abundant and of good chemical qualityduring wet seasons, the topography is such that nolarge or deep reservoirs are feasible. It thus seemsimpracticable to count on storage of fresh flood watersfor use during droughts when the streams dry up orbecome too salty even for cattle. Instead, it wouldseem that the principal problem offered by the surfacewaters is one of drainage and of flood-protection. Suchworks, desirable though they may appear, are doubtlessbeyond the ability of the Paraguayan economy in theforseeable future.Too little is known of the geology of the Gran Chacoto permit pinpointing of the area or areas that mightyield usable supplies of ground water or formulation ofany detailed suggestions as to the best methods fordeveloping it. Two possible geologic sources of waterare apparent, however, and both include very largegeographic areas.One source is the great sedimentary basin thatoccupies all but the northern part and the easternmostfringe of the Gran Chaco. The subsurface geology ofthis basin is known in a most general way from therecords of the few deep exploratory wells put down bythe Union Oil Company. No records are available tothe author as to the quantity or chemical quality ofany water that may have been in these wells. Anysuch information would be of doubtful value, for wellsdrilled in search of petroleum are notably prone to missevidence that bears on water supplies.It is known that the Gran Chaco basin contains athick series of unconsolidated sediments and sedimentaryrocks, that many of the rocks must be permeableto water, and that some crop out at the surfaceand, hence, provide catchment areas in Bolivia andnorthern Paraguay. Owing to the basinal structure,artesian conditions probably are widespread, thoughwhether flowing wells can be obtained even on lowground is not known. Both the quantity and quality ofthe water in these sediments are of course conjectural,but by analogy with other sedimentary basins throughoutthe world it is reasonable to suppose that they containfresh water in places, which might be recoverableby pumping or artesian flow from deep wells. Anymore definite knowledge as to the availability of waterat depth must await much more detailed knowledge ofthe subsurface geology of the Gran Chaco than ispresently available. Considering the number of aquifersthat must be present, it is entirely possible thatwildcat wells might strike fresh water. The knowledgegained by means of such wells could be made the basisof a deep-well drilling program, backed by thoroughgeologic and geophysical studies.The chances of developing fresh-water supplies fromshallow aquifers in the Gran Chaco are somewhat morepromising and easier of accomplishment than are thechances from deeper parts of the basin.In a few parts of the Gran Chaco, the permanentwater table is relatively near the surface and yieldsfresh water readily to pumped wells. In some placesthe lower part of the fresh-water zone grades downwardwithin a few tens of centimeters to water that isso highly mineralized as to be unfit for use; elsewherethe fresh water is separated from underlying mineral­ized water by impermeable layers of clay. The fewavailable analyses of "salt" waters in the Gran Chacoindicate that the principal constituents are of sulfatesand chlorides of sodium and magnesium. No informationis available as to whether any fresh-waterzones exist beneath the "salt" waters.The near-surface stratigraphy of the Gran Chaco is,of course, no better known than is the stratigraphy ofthe deeper and older beds. Because of the continentalalluvialconditions under which the younger beds weredeposited, it is likely that most if not all of the youngerbeds are decidedly discontinuous, and, hence, thatconditions found in any one well could not be extrapolatedmore than a few kilometers at most from thepoint of discovery.If any serious attempt is made to develop nearsurfacesupplies of fresh ground water in the GranChaco, the first necessary step would be to make an inventoryof available information on all existing wells.On the basis of the data so derived, a series of exploratorywells 30 to 100 meters deep could then be drilled,with provision for offsetting any successful wells so asto delimit the areas that yield fresh water at reasonabledepths. Any such program should be under the directsupervision of a geologist experienced in ground-water