Geology and Mineral Resources of Paraguay A Reconnaissance

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54 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYentirely of a monotonous succession of arkosic sandstone,mostly light brown to light reddish brown butmarked by thick dark brown to dark-red soils in manyplaces. It is coarse grained locally but mostly fine tomedium grained, is usually poorly cemented, and consistsof well rounded grains of quartz and feldspar, withconsiderable quantities of mica. It occurs in thinbeds, most of them strongly crossbedded.At several places along the western side of theYpacarai depression the sandstone beds contain intercalatedlayers of light-colored fossiliferous clay severalmeters thick, probably the partly weathered productof soft gray or black shale. According to the author'sinterpretation of the structure, all of these are in thelower part of the unit.The arkosic sandstone unit is exposed along theentire western front of the Cordillera de los Altos; itsupper part is most accessible and most easily studiedin the cuts along the main Asuncion-Caacupe highwaywhere it traverses this front and rises more than 200meters between the lake level and the crest of the hill.The unit also makes up the western part of the Cordillerade los Altos, where it extends a little east of Caacupe,as well as most of the area between Asuncion andYpacarai. Except for one hill just southeast ofCarapegua, which is capped by the white saccharoidalsandstone unit, all the southern body of Caacupe rocksis made up of the arkosic sandstone unit, underlain bythe basal conglomerate. (See fig. 44.)The following more detailed descriptions of specificlocalities give some picture of the nature of the intercalatedclay layers mentioned above and of the enclosingsandstone.One clay deposit that was examined is on the westside of tbe railroad, 1 to 2 kilometers southeast ofAregua. There, clay underlies a low-level bench, 100meters or so wide, that parallels the shore of LagoYpacarai. Clay is exposed in a large number of pits todepths of 3 to 4 meters. It is very micaceous, white tolight gray and mottled near the surface with brown andred; it is used for making bricks of excellent qualityand rich-red-brown color. In the dense woods at thebase of a slope just above the broad bench, hence justabove the clay stratigraphically, there is a small building-stonequarry in buff to brown, fine-grained micaceous sandstone in beds 1 to 2 meters thick. Thissandstone, which yielded a few poorly preservedfossils and which strikes N. 65° W. and dips 32° SW., isoverlain by thin-bedded, very shaly sandstone that isalso micaceous but redder in color than the lower beds.Rocks that form the upper parts of the hill are poorlyexposed, but fragments in the soil and slope wash makeit evident that there is a series of buff-colored sandstonebeds, at least 60 meters thick, fine to medium grained,quartzitic in places, and locally rich in iron oxides. Anintermediate bench suggests that there is at least onemore thick bed of clay like the one near lake level, butit is not exposed.The Vargas Pefia clay pit, 2 kilometers northwest ofthe town of Ypacarai, which is described by Harrington(1950, p. 21-22, 48) and is one of the key fossil localitiesnow known in Paraguay, provides another good exampleof the relations of clay beds and sandstone.The beds strike N. 55° W. and dip 12° SW. Clay isexposed in two pits, about 25 meters apart vertically.The clay is white, very micaceous, containsHUttle or noFIGURE 44. Sandstone hills of the Caacupe' series southeast of Paraguari. The dark hill at the left is basal conglomeratic sandstone of the series.
SEDIMENTARY ROCKS 55sand, and is richly fossiliferous. In places it is stainedred, yellow and brown and contains an abundance oflayers and veinlets of siderite, which is largely alteredto limonite. Massive, crossbedded micaceous lightbrownto red sandstone crops out between the upperand lower clay pits. Similar sandstone appears tooverlie the clay in the upper pit. The relations are notclear, but it is believed that the lower sandstone liesbetween two distinct layers of clay, each severalmeters thick. Possibly, however, the clay representsa single bed of altered shale that has been repeated bydropping along a northwestward-trending fault.The two series of sandstone and clay just describedprobably belong in the lower part of the arkosic sandstonemember of the Caacupe series not far above itsbase. The one near Aregua is probably a few meterslower in the section than that near Ypacarai, but theymay be actually or very nearly equivalent.White saccharoidal sandstone. The white saccharoidalsandstone is one of the most striking and mosteasily recognized of all the sedimentary rocks in Paraguay.About 100 meters thick, it rests on the arkosicsandstone unit and is overlain by a unit of alternatingclay and sandstone. Its main outcrop is a broad, southward-trendingbelt on the eastward flank or dip-slope ofthe Cordillera de los Altos, and along the west side ofthe Rio Piribebuy valley. Thus it extends through thetowns of TobatI and Piribebuy and thence southwestwardto make up the prominent hills east of Paraguari.The southeastward-trending hill just south of Carapeguais capped by an outlier of the same unit.The sandstone is so friable, at least near the surface,that it fails to crop out in many places, but on some ofthe hills east of Caacupe and in the general vicinity ofPiribebuy, it forms extensive outcrops. Probably thebest exposures are near TobatI (see figs. 45 and 46).There the road from Tobati to Caacupe crosses the baseof the unit about 3 kilometers north of Caacupe. Thecontact itself is obscured, but it is marked by an abruptchange from the red sandy soil derived from the arkosicsandstone unit to white or light-gray sandy soil. Fromthis point to Tobati the road skirts prominent and impressivehills, roughly 50 meters high, that are made upof the white sandstone. Near the base the sandstoneis thinly bedded in layers 15 to 60 centimeters thick, butthe remainder of the unit is in massive beds, locallycrossbedded, and as much as 17 meters, thick. Thebeds dip at low angles toward the east and southeast,with occasional minor rolls. A series of vertical eastward-trendingjoints, 3 to 70 meters apart are so erodedFIGUBE 45. Typical weathering pattern, resembling mudcracks on case-hardened joint faces of white sandstone unit of Caacupfi series of Silurian age.Three kilometers west of Tobati.
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Geology and MineralResources of Par
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UNITED STATES DEPARTMENT OF THE INT
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IVCONTENTSMineral resources._______
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GEOLOGY AND MINERAL RESOURCES OF PA
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INTRODUCTION(1947, 1952) are the on
Page 12 and 13: GEOGRAPHYFIGURE 1. Index map showin
Page 14 and 15: GEOGRAPHYbisects the Republic of Pa
Page 16 and 17: GEOGRAPHY 9thorny semidesert plants
Page 18 and 19: GEOLOGY 11chiefly logs, rough lumbe
Page 20 and 21: IGNEOUS AND METAMORPHIC ROCKS 13and
Page 22 and 23: 15FIGURE 7. Arkosic sandstone (spec
Page 24 and 25: IGNEOUS AND METAMORPHIC ROCKS 17FIG
Page 26 and 27: IGNEOUS AND METAMOKPHIC ROCKS 19acc
Page 30 and 31: IGNEOUS AND METAMOEPHIC ROCKS 23TAB
Page 32 and 33: 25FIGURE 18. Ehyolite (specimen P-3
Page 36 and 37: IGNEOUS AND METAMORPHIC ROCKS 29PET
Page 38 and 39: IGNEOUS AND METAMORPHIC ROCKS 31DIA
Page 40 and 41: IGNEOUS AND METAMOEPHIC ROCKS 33a d
Page 42 and 43: GEOLOGICAL SURVEY PROFESSIONAL PAPE
Page 44 and 45: 36 GEOLOGY AND MINERAL RESOURCES OF
Page 50 and 51: 42dated by two kinds of porphyry. O
Page 52 and 53: 44FJGTJEE 34. Porphyry (specimen P-
Page 68 and 69: 60lower third of the series consist
Page 74 and 75: 66possible Cretaceous age. Baker (1
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Geology and Mineral Resources of Paraguay A Reconnaissance

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