Geology and Mineral Resources of Paraguay A Reconnaissance

SEDIMENTARY ROCKS 61fine-grained olive-brown sandstone probably in thehills just south of town. This rock, which containsmuch clay-sized material, splits into solid but easilyworkable blocks from 12 to 30 centimeters thick.At a point about 10 kilometers southeast of Itacurubide la Cordillera, the surface is covered by 1 to 2 metersof lateritic iron ore, on thin-bedded alternations ofmicaceous sandstone and clay. The sandstone ispurplish gray, very soft, and contains at least 50percent fine-grained muscovite mica. The clay, nodoubt a weathered shale, is reddish brown and is farless micaceous than the sandstone.Two kilometers farther southeast, or 3 kilometersfrom San Jos6, a shallow road cut exposes an extensivebody of white, fine-grained micaceous sandstone thatbreaks along joints and horizontal bedding planes intosmooth but relatively soft flagstones, from 2 to 6centimeters thick. The vertical joints in this rock areseamed with brown limonite and many blocks of sandstoneshow diffusion banding that has worked awayfrom the joints. Several poorly preserved fossils werefound at this place.The Itacurubi series rests on the Caacup6 series oflower Silurian age and is overlain by the Tubaraoseries of Pennsylvania!! age. Neither contact hasactually been observed, but both evidently are markedby unconformities. There is, indeed, some evidence ofa major erosional unconformity at the base of theDevonian beds. Harrington states that the Itacurubiseries rests directly on the Piribebuy sandstone thewhite saccharoidal member of the Caacup6 series andeven considers the possibility that this sandstoneshould be grouped with the Devonian rocks. Therelationship described by him is quite possibly true inthe area directly west of Itacurubi and near Valenzuela.Farther north, however, the fossiliferous beds in thevicinity of Arroyos y Esteros, which are definitely partof the Caacupe" series, are stratigraphically above thewhite sugary sandstone and beneath the Itacurubi beds.These relationships are discussed on pages 56-57.If the present author's concepts are correct, the relativelythick section of upper Caacup6 beds that ispresent between Tobati and Arroyos y Esteros mustbe missing in the latitude of Itacurubi. This stronglysuggests the presence of a major erosional unconformitybetween the Silurian and Devonian rocks.Harrington (1950) lists the following fossils from aquarry at Cariy Loma, a few kilometers east of Itacurubide la Cordillera. These fossils were found in sandstonenear the top of the series.Favosites sp. indet.Schellwienella inca (d'Orb.)Australostrophia conradii HarringtonChonetes falklandicus Morris and Sharpe465871 59 6Spiroaphe(?) sp. indet.Tentaculites crotalinus SalterPhacopina itacurubensis HarringtonCalymene sp. indet.A small collection of fossils was made by the authorand Ricardo Maz6 U. from micaceous sandstone nearthe base of the series, 8 kilometers southeast of EusebioAyala. A. J. Boucot reports as follows:This collection contains an unidentified plicated brachiopod,probably a rhynchonellid, whose brachial valve bears a medianseptum. It resembles the genus Camarotoechia. Another finelyplicated brachiopod is possibly an orthoid belonging to theWattsellidae, Silurian or Devonian.A. R. Palmer reports on trilobites from the samecollection:Hadrorachis(!) sp., cranidium; Homalonotus sp., thoracic segment.Similar forms have been described from the Devonianof adjacent South American countries; however, neither genusis necessarily definitive of a Devonian age. On the basis of thetrilobites, this collection cannot be more closely dated thanSilurian-Devonian.At the "L6pez" sulfur workings, in the middle shalymember of the series, the author found one fossil,identified by R. J. Ross as follows:A single partial specimen of an orthoconic nautiloid possessingthe ornamentation of the "form genus" Spyroceras. Similarforms are known in North America from mid-Ordovician toDevonian strata.Boettner (1945) describes a single specimen of Homalonotussp. from this same locality.The white flaggy sandstone mentioned above as 3kilometers west of San Jos6 yielded an unidentifiedsmooth pelecypod and fragments of Homalonotus sp.and Dalmanites(?) sp. as identified by A. R. Palmer.Fossils that were found in the Santa Rosa and Picuibaexploratory wells in the Gran Chaco, and that areascribed to the Devonian by Harrington are listed inthe tables on pages 73 and 74.These faunas, which contain species that are typicalof the classic Devonian localities of Brazil, Uruguay,and Argentina, indicate the Early Devonian age of theItacurubi series of Paraguay, as well as its generalcorrelation with the Devonian sedimentary rocks ofthe surrounding countries.GONDWANA (SANTA CATARINA) BEDSThe Paraguayan equivalent of the widely distributedGondwana beds consists of a thick series of continentaland epicontinental sedimentary rocks that overlie themarine Devonian beds unconformably (Du Toit, 1927;Oliveira and Leonardos, 1943; Harrington, 1950). TheGondwana of Paraguay is 1,000 meters (3,280 feet)or more thick, including its extensive cap of basalt.Because the name Gondwana is still widely used bygeologists in Brazil and other parts of the world it is