Geology and Mineral Resources of Paraguay A Reconnaissance

STRUCTURAL GEOLOGY 7555 meters along the Rio Paraguay to a maximum ofonly about 400 meters along the western border of thecountry,is covered by unconsolidated alluvial materialshere called the Chaco sediments. These materials,called the Chaquena formation in Bolivia (Ahlfeld,1946, p. 52, and Oliveira and Leonardos, 1943, p. 780)and the 'Pampa formation in Argentina (Stappenbeck,1926, p. 68-170) are little known even at the surface.The Chaco sediments, that is, those above the Gondwanaor Santa Catarina sediments, are probably similarto those of northern Argentina, described in considerabledetail by Stappenbeck (1926, p. 68-170). There, theyrange from Miocene to Recent, are largely unconsolidatedclay with some sand, and include both marine andcontinental deposits. The amount of material of eolianorigin included in these beds is unknown. Sand dunes,some stabilized and some active, are known at the surfacein the extreme northwestern corner of Paraguay.Kanter (1936) failed to confirm the presence of trueloess in his travels through the Paraguayan GranChaco, but as Ahlfeld (1946, p. 53) points out it ispossible that some of the material at the surface, as wellas in the deeper beds, is loess, deposited by the strongwinds that blow northward from the Argentine pampasat present and probably also in the past.Four known occurrences of fossil mammals are re­ported from the Gran Chaco by Vellard (1934). Oneof these is near Reventon, on the lower Rio Pilcomayo;the other three are along the upper Pilcomayo nearthe forts of General Bruguez, General Delgado, andSalto Palmares. Not all of these localities are shownon plate 1. Vellard also notes unconfirmed reports ofsimilar fossils on the banks of the Rio Negro.None of these localities have been explored; the findshave been made by soldiers assigned to the militaryposts. The fossils are said to be exposed in the bedsof rivers or along their banks; new exposures are madeat frequent intervals by the constantly changing streamchannels. Vellard says that bones from Reventonhave been identified by A. de W. Bertoni as those ofGlyptodon clavipes, Mastodon andium, and Machrancheniaboliviensis.STRUCTURAL GEOLOGYGENERAL RELATIONSParaguay occupies parts of two great sedimentary andstructural basins the Parand basin of southern Brazilto the east and the Gran Chaco basin of Paraguay,Bolivia, and Argentina to the west. In its grossaspects, the geologic structure amounts to a deepsyncline beneath the Gran Chaco whose western limbrises gradually toward the Andes Mountains and whoseeastern limb rises to a gentle northward-trending anticlinethat is parallel to and only a short distance eastof the Rio Paraguay. The easterly limb of this anticlinedips at very low angles eastward toward theBrazilian center of the Parana basin.The major structural features shown on the geologicalcross sections that accompany plate 1 are based moreon the distribution of the major rock units and theirrelationship to the topography than on detailed observationsor other knowledge of structural features them­selves. In many places there are, however, bits ofdetailed evidence as to local geologic structures. Theyare all shown on the map and many of them are mentionedin the text. Thus, the local attitudes of bothmetamorphic and sedimentary rocks, as observed bythe author or recorded in the literature, are known atmany widely scattered localities throughout easternParaguay. A few faults are also shown on the map,though most of them are mapped on very scanty evi­dence. There is good evidence of minor thrust faultingin the Itacurubi beds south of Eusebio Ayala (seefig. 48); the importance of thrusting in the structuralhistory of the region is quite unknown, however.ANTICLINE BETWEEN RIO APA AND SAN JUAN BAUTISTAThe anticlinal flexure that separates the two basinsexposes the older Precambrian and Early Paleozoicrocks from the Rio Apa as far south as San Juan Bautista,where it appears to die out or to plunge southward,for the southward-dipping Triassic rocks coverthe older rocks south of that point. The sedimentaryrocks and the overlying Serra Geral lavas on the eastlimb of the anticline dip generally at very low anglestoward the east, but there are reversals of dip locally.Most of the rock units probably also thicken eastwardfor although there is little known local evidence of this,the entire sedimentary-volcanic section is thicker insoutheastern Brazil than it is in eastern Paraguay.Available data indicate that the western limb of theanticline is much steeper than the eastern one, for thebeds that occur at the surface along the anticlinalaxis are more than 3,000 meters (10,000 feet) deepbeneath the central part of the Gran Chaco basin.It is entirely possible that this drop, or part of it, isdue to a major fault or faults that parallel the generalcourse of the Rio Paraguay, evidence being maskedby the Chaco alluvial sediments. It seems moreprobable that folding has played a more importantrole than faulting in development of the structure butthis statement is based on little evidence. Strongfaults with downthrow to the west are known along theRio Parana in Argentina, south of Paraguay (Harrington,oral communication). Somewhat similar faultsystems are also known north of Paraguay in thegeneral vicinity of Corumbd, Brazil (Almeida, 1945,and Dorr, 1945). Possibly these fault systems also