Geology and Mineral Resources of Paraguay A Reconnaissance

SEDIMENTARY ROCKS 63kilometers north of Paraguari. The gap between thesmall hill and Cerro Jhu is traversed by the main roadbetween Paraguari and Piribebuy. The western footof Cerro Jhu is made up of Paraguari conglomerate,surmounted by arkosic sandstone typical of the Caacupeseries. These beds strike about N. 30° E. and dipabout 45° SE.; the varved shale of the smaller hill, onthe other hand, strikes about N. 70° W. and dips about45° SW. These relationships convince Harrington, aswell as the present author, that the Tubarao beds tothe west must have been downfaulted against theCaacupe beds to the east along a northeastward-trendingfault that traverses the gap between the two hills.The correlation of the tillite and related sandstoneand shale with the Tubarao series of Brazil, as made byHarrington, seems to be entirely justified on the basisof lithologic and paleontologic evidence available. Theage of these beds in South America, and of theirprobable or possible correlatives in South Africa andAustralia is still in dispute. Many geologists believethem to be of Permian age, whereas others assign themto the Pennsylvanian. This question is left open here.PERMIAN SYSTEMINDEPENDENCE SERIESThe Independencia series, named by Harrington(1950) for Independencia, 25 kilometers northeast ofVillarrica, is about 400 meters (1,310 feet) or less thickand made up very largely of sandstone. It containssparse fauna and flora of late Permian age.The distribution of the series as mapped is basedlargely on Harrington's map; the rocks have only beenstudied in the central region, in the general vicinityof Villarrica, and Harrington himself expresses somedoubts as to the extension of the belt of outcrop as farnorth as Bella Vista on the Rio Apa. As shown onplate 1, the series is about 400 meters (1,310 feet) thickin the mountainous area east of Villarrica but elsewhereit is less than half this thickness in almost all places.How much of this apparent difference is real and howmuch is due to gross errors on the topographic orgeologic maps is not known.The best exposures known are in Sierra Ybyturuzuand its vicinity, but Harrington maps a number ofknown outcrops in the mountains north of Caaguazu.The series is described by Harrington, from his studiesalong the road between Mbocayaty and Independencia,as consisting of two thick sandstone units, separatedby a thinner unit of alternating shale and sandstone.Only the lower sandstone was examined by the presentauthor, and this only in one place, at the west baseof Sierra Ybyturuzu, almost due east of Villarrica.There, the basal part of the sierra, which rises abruptlyfrom the plains of Tubarao tillite, is massive sandstone,60 to 90 meters (200-300 feet) thick, that forms steepto vertical cliffs (see figs. 49 and 50). The sandstonehas siliceous cement and is light reddish brown, mediumgrained and moderately hard. Individual layers areless than 1 centimeter thick, but the bedding planesare tight so that the rock appears from a little distanceto be made up of massive beds from 2 to 8 metersthick. Near the top of the unit the sandstone hascharacteristic deltaic crossbedding, with topset andforeset beds ranging from 1.5 to 5 meters thick.Above the rock are about 60 meters (200 feet) ofsandstone that is similar in most respects but somewhatredder in color and softer so that it forms gentler slopesand supports heavier vegetation. The rocks above thissecond unit are concealed, at least in that part of themountain that was examined, by soils and dense forestgrowth. The slopes are moderate to steep and mostof the upper part of the section is probably made upof sandstone similar to that lower down.Harrington's description of the lower sandstoneunit, which he found exposed 10 kilometers east ofMbocayaty, is substantially like that reported aboveexcept that he found the rock to be somewhat coarsergrained, less well cemented and more arkosic thanthat seen by the author.FIGURE 49. Coarse-grained arkosic, crossbedded sandstone of the Independenciaseries of Permian age, 13 kilometers east of Villarrica. The dark slotlike ravine isin a deeply weathered dike of basic alkalic rock that contains much biotite. Seefigure 28.