Geology and Mineral Resources of Paraguay A Reconnaissance

66possible Cretaceous age. Baker (1923) also mentionssedimentary rocks younger than the lava in the samearea. Consideration of the structure, as well as observationsby others, suggest that the rocks constitute awindow of Triassic red beds as provisionally shown onplate 1. DeMersay (1860) and other writers describethe remarkable masonry churches and other early missionbuildings as having been built of resistant finegrainedred sandstone that is abundant in the environsof Jesus and Trinidad. DeMersay also notes the presenceof a white tough sandstone, suitable for whetstones,between the two missions and along the Argentinebank of the Rio Parana. These observations aresubstantiated by those of Robert M. Miller, who reportedorally that the rocks in and near these coloniesare largely red beds, capped by basalt on the higherridges.As described by Harrington (1950), and substantiatedby the author's few direct observations, the Misionessandstone is probably between 150 and 200 metersthick and is dark red, uniformly colored where fresh,but weathered to pink or gray in places. It is mediumto coarse grained and made up almost entirely of quartzgrains that are well rounded to spherical and havepitted or frosted surfaces. Generally the sand grainsare poorly cemented by hematitic clay, but locally therocks are silicified to form tough quartzite. Most of thebeds are massive and thick, but here and there, as in thevicinity of San Juan Bautista, they are thin bedded orcrossbedded. For example, at a point 9 kilometerswest of San Juan Bautista and 1 kilometer north of theroad, there are some small quarries that produce ex-cellent red flagstone, almost quartzitic in character,from the lower beds of the Misiones sandstone (seefig. 51).Occasional small pebbles of quartz and quartzite,ranging from well rounded to subelliptical, are scatteredthrough some of the more massive beds. Though notabundant, there are some true conglomerate beds in afew places. At a point 3 kilometers west of San JuanBautista, for example, a coarse conglomerate withrounded pebbles of quartzite and chert is exposed inroad cuts.Harrington describes a similar conglomerate, 1 meterthick, near San Juan Bautista, with a few pebbles aslarge as 15 centimeters in diameter, with fully 15 percentof the smaller pebbles of banded agate. He also describesthe sandstone on the hill 7 kilometers east ofSanta Rosa as containing abundant nodules of epigeneticagate. These nodules, as much as 20 centimetersin diameter, are very irregular in shape and show blue,light yellow and white in concentric bands. Their relationto the surrounding sandstone is such that they arecertainly concretionary in nature and probably relatedto hydrothermal activity associated with the SerraGeral lavas.Though clay or shale are not described in the literatureas an important constituent of the Misiones section,there must be considerable quantities of such material,for a large part of the outcrop area betweenCarmen del Parana and Yhu appears from the air to beunderlain by red clay or shale. In the immediatevicinity of Yhu, moreover, workable deposits of whitepottery clay and red clay suitable for brick manufacture,are well known (see fig. 52).Because of its stratigraphic position between theIndependencia series and the Serra Geral lavas, Harringtonbelieves that the Misiones sandstone is of lateTriassic age and is correlated directly with the Botucatusandstone of southeastern Brazil.TERTIARY AND QUATERNARY SYSTEMSFIGTJKE 51. Typical flagstone quarry, 3 kilometers west of San Juan Bautista. Therock is well-cemented coarse red sandstone of the Misiones series of Triassic age.A very large part of eastern Paraguay is covered byalluvial deposits of clay, silt, and fine sand of Tertiaryand Quaternary age. The deposits are probably asmuch as several hundred meters thick in places, as inthe vicinity of Pilar in the extreme southwestern cornerof eastern Paraguay where they are known to containpeat deposits, at least locally. Elsewhere their thicknessprobably ranges from 1 to 20 meters in mostplaces. Virtually every swale in the country containssome alluvium but the larger deposits are, of course,along the broad, nearly level valleys of the majorstreams. There they are almost invariably marked bypoor drainage conditions or by permanent or intermittentswamps. The boundaries of the alluvial materials