Geology and Mineral Resources of Paraguay A Reconnaissance

76 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYcontinue in Paraguay, roughly parallel to the RioParaguay. If so, they must be somewhat west of thepresent stream and buried by the Chaco alluvialsediments. Only further subsurface data, geologic orgeophysical, will solve the problem.Ther£ is some evidence that the anticline has been inexistence, though intermittently, since early in Paleozoictime. This would account for the sandy nature, as wellas the relative thinness of most of the Paleozoic andMesozoic sedimentary units along its axis as comparedwith their apparent correlatives in the Gran Chacobasin and as they are described in the Parana basin ofBrazil and in the Andes of Bolivia (Oliveira and Leonardos,1943, and Ahlfeld, 1946).The Rio Apa and San Juan Bautista anticline isdoubtless far more complex than described in the lastparagraphs, as indicated by the several faults and thediversity in attitudes of the Precambrian and Itapucumirocks near the Rio Apa and by the absence of olderrocks at the surface between Asuncion and Concepcidn,suggesting a sag in the structure in that area. The complexityis also indicated by the interesting but incompletelyunderstood structural and topographic featurecalled the Ypacarai depression. As described by Harringtonand shown on his map it is a rectilinear valley,65 kilometers long and 6 to 8 kilometers wide, thattrends northwestward from the vicinity of Paraguaripast the northern end of Lago Ypacarai. It is characterizedby a nearly flat floor, covered almost everywhereby swampy alluvium or by the shallow waters ofthe lake. It is flanked on both sides by relatively highabrupt hills whose faces suggest fault scarps or faultlinescarps in many places.Harrington believes that the depression is a long,very narrow graben bounded along its sides by majorlongitudinal faults and probably by a strong transversefault at its southeast end. As evidence of the grabenstructure, Harrington cites the presence, 2 kilometersnortheast of Paraguari, of varved clay and of a graniteerratic boulder seemingly belonging to the Tubaraobeds of Pennsylvanian age but in juxtaposition to bedsof the Caacupe series of Silurian age. He also givesevidence of strong faulting along the western side ofthe depression, marked by fault breccia, locally as muchas 300 meters wide, at a number of places from Paraguarito some distance northwest of the Vargas Penaclay pit near Itaugua. In. addition, he saw strong fracturesalong the eastern side of the depression.In short, Harrington believes that the Ypacaraidepression represents a downfaulted block that hasdropped the rocks several hundred meters, thus bringingthe Tubarao beds within it down to the level of thelower Caacupe1 beds along its sides. There is littleevidence of a transverse fault to close the southern endof the depression but there may be one. As mentionedby Harrington (1950, p. 51) such a fault, extendingfrom Caballero northwestward past Paraguari towardthe Rio Paraguay near Villeta, would explain the abruptdisappearance of the Misiones sandstone south ofa line between Villeta and Yaguaron.GRAN CHACO BASINStudies of the outcropping rocks of eastern Paraguayand of the records of the deep exploratory wells in theGran Chaco indicate the presence of a very deep basinthat centers in southeastern Paraguay, not far northof the Pirizal well. From this center the rocks risenorthward toward the outcrops of Devonian sandstoneand quartzite near the northern edge of the country.They also rise eastward, for some of the same rocks, orthen- close correlatives, appear at the surface alongthe Rio Paraguay and east thereof. Within a few tensof kilometers west of the Paraguayan border, in Bolivia,the rocks rise in a series of anticlines, some of themoil producing, along the eastern flank of the Andes.Available evidence is far too scanty to determine theage of the Gran Chaco basin or how much of it is dueto folding and how much to erosion and subsequentfilling. Nor are there any available facts as to the presenceor absence of older structures within the Paleozoicbeds of the basin as distinct from the broad downfoldindicated on section A-A', plate 1, which, as drawn,involves the Mesozoic beds as well as the Paleozoic ones.It seems likely that the major folding took place intwo stages once after the Devonian marine sedimentshad been laid down but before the red beds of Pennsylvanianthrough Triassic age were formed, and onceduring the Tertiary. The latter folding, associated withthe major uplift of the Andes, involved the continentalred beds as well as the older rocks.GEOLOGIC HISTORYGENERALIZED ACCOUNTWith the exception of the paragraphs below on theGran Chaco plains and on the Alto Parana valley,most of the following summary of the geologic historyof Paraguay is generalized from that given by Harring­ton (1950). During Precambrian time a series ofclastic sediments of unknown thickness was foldedand metamorphosed before being invaded by graniteand other acid rocks. Most of the acidic rocks weredeep-seated intrusives, but at some time during thisperiod there were extensive surface flows and nearsurfaceintrusions of porphyry. Between the formationof the granitic rocks and Late Cambrian or Ordoviciantime there must have been strong uplift anderosion followed by subsidence, permitting depositionof the Itapucumi limestone beds under marine con-