Geology and Mineral Resources of Paraguay A Reconnaissance

SEDIMENTARY ROCKS 69west bank of the Rio Paraguay. Some belong to thePrecambrian basement complex that doubtless underliesthe entire Gran Chaco basin; others belong to theItapucumi series of probable Cambrian and Ordovicianage. Those near Villa Hayes are reportedly of redsandstone and are mapped as belonging to the Misionessandstone of Triassic age. The outcrop between PuertoGuarani and P6rto Murtinho is an extension of thesupposedly late Tertiary syenitic body of the Paod'Agucar on the Brazilian side of the river.There are several outcrops of sandstone and possiblyof quartzite in the extreme northern part of the GranChaco, west of the Rio Paraguay. None of them wasseen by the author except in very distant air views andthere are no known geologic maps that show them withany exactitude.The largest outcrops make up Cerro Leon and CerroCristian (pi. 1). In addition to the several smallerbodies sketched on the map, there are a number ofsmall hills on both sides of the Bolivian border that arereportedly made up of the same rock. Some of thesehills have been used as triangulation points in boundarysurveys.In written communication to the author, Harringtonreports that the rock of Cerro Leon is a medium-grained,reddish-brown ferruginous sandstone. From this rockthe geologist for Union Oil Company collected a fewwell-preserved fossils that were identified by Dr. Har­rington as Leptocoelia flabellites. This identificationleaves no question as to the lower Devonian age of therocks, which would appear to be about equivalent tothe Itacurubi series; they are so mapped on plate 1.The quartzite and sandstone beds of eastern Boliviaand extreme northern Paraguay are briefly describedby F. Prieser (in Ahlfeld, 1946, p. 32-36). They arenearly horizontal in most places and form vast tablelandssome distance north of the international border.Some of the hills in and near Paraguay maybe erosionalremnants of these tablelands, but Prieser thinks someof them may represent uplifted fault blocks, or horsts.In Bolivia, the sandstone beds are slightly ferruginousand yellow or orange to white. They are mediumgrained, crossbedded, and cemented with secondaryquartz. Their total thickness is about 200 meters.Several thin conglomerate beds are known. The rocksare provisionally assigned to the Ordovician period byPrieser, but on admittedly thin evidence; Dr. Harringtonbelieves that they, like the rocks of Cerro Leon inParaguay, are probably of Early Devonian age.The sandstone and quartzite deposits are overlain inBolivia by shale and sandstone of possible Permian andCretaceous age locally, but by the Chaco formation ofTertiary and Quaternary age in most places. In northernParaguay these rocks are everywhere overlain orsurrounded by the Chaco sediments.SUBSURFACE ROCKSWELL RECORDSThe well records that follow are translations andcondensations of the logs furnished officially by theUnion Oil Company to the Government of Paraguay atthe close of the exploratory campaign for petroleumin the Gran Chaco; they were made available to theauthor by the Paraguayan Ministerio de Obras Publicasfor use in this report. They have doubtless beenchanged in condensation, as well as in a series of translationsfrom English to Spanish and again to English.Nevertheless, they contain some hitherto unavailablefacts as to the subsurface character of the Gran Chaco.The logs are given below; the author's attempt at theirinterpretation and correlation is sketched in figure 53.Records of wells drilled by Union Oil Company in the Gran Chaco,western Paraguay, 1944 to 1949, inclusive[Translated and condensed by Eicardo Mazo U. from logs transmitted by thecompany to the Government of Paraguay.]_. ., -Picuiba well B-l:Thickness(feet)Depth(feet)Sand, yellow, fine- to medium-grained,grades downward to darker sand withlaminae of clay and pebbles cemented byred or gray clay_____.---------------- 1,008 1,008Sand and sandstone, dark-red, with claycementedangular to rounded pebbles ofquartz and other materials--___________ 87 1, 095Sand, light-green, uniform grain size withsome pinkish-brown grains_____________ 125 1,220Sand and siliceous siltstone, bluish-gray,very fine grained, some incrustations ofpyrite at 1,400 ft_-_----------_------- 300 1,520Quartz sand, fine- to medium-grained, withmuchpyrite------------------------- 260 1,780Sand, pale-greenish-gray, medium-grained;contains pyrite and one 1-ft bed of wellstratifiedsandstone- __________________ 487 2,267Sand, gray, medium-grained, with abundantsmall rounded pebbles.________________ 10 2,277Sand and sandstone, pale-greenish-gray,with some pebbles and siltstone________ 163 2,440Siltstone, olive-green, with streaks of sandand of pyrite ----------------------.- 70 2,510Sand and sandstone, greenish-gray, fine- tomedium-grained, with some pebbles and alittle shale__.------------------------ 363 2,873Sand, siltstone, and shale, brick-red, withpale-green nodules of shale_____________ 227 3, 100Sandstone, fine-grained, silty, pale-red andgreen, with some grains of coarse sand_ _ _ 44 3, 144Shale, lustrous, dark-purple-red, with partingsof fine sandy shale____-_-_-----__- 316 3,460Sand, gray, fine- to medium-grained_____ 15 3, 475