Geology and Mineral Resources of Paraguay A Reconnaissance

SEDIMENTARY ROCKS 51and, in view of differences and fairly frequent changesin the opinions of geologists as to the ages of theserocks and of their supposed equivalents in Brazil andBolivia, no closer age assignment than that ofCambrian (?) and Ordovician(?) seems justified.The distribution of the series and the attitudesymbols, as shown on plate 1, are based on groundand air observations by the author along the RioParaguay, particularly in the vicinity of the Vallemicement plant, on his interpretations of aerial photographs,and on various published observations byCarnier (1911c), Kanter (1936), Boettner (1947) andHarrington (1950). Several relatively small outliersof the Itapucumi series rise from the alluvium alongand near the Gran Chaco side of the Rio Paraguay.Most of them form low rounded hills scarcely morethan 5 to 10 meters high, but the largest one, CerroGalban, just southwest of Puerto Casado, rises 140meters above the swampy plain and is about onekilometer long. It is sketched and described by Krieg(1931) as made up almost entirely of limestone, witha small hillock of red sandstone near its eastern end.The limestone from the main hill, which is quarried forvarious uses at Puerto Casado, is dark blue gray andcrystalline.The series is almost continuously exposed along theeast bank of the Rio Paraguay from near San Salvadorto the mouth of the Rio Apa (pi. 1). Over much ofthis distance there are conspicuous cliffs 15 to 20meters high (figs. 21 and 23), backed by rounded andheavily forested hills that are 100 to 200 meters high.Eastward from the river the limestone is more or lesscontinuously exposed as far as the Apa highland, in thevicinities of Cerro Paiba and Loma Pora. There theywere seen by Carnier (191 Ic) to rest on or against olderPrecambrian rocks, unconformably in some places andin fault contact in others. The belt trends southwardaround the end of the Precambrian body but is apparentlynot well exposed there for it is not describedby any investigators. It reappears along the eastside of the Precambrian rocks, where it is describedand sketched by Boettner (1947) between ArroyoYtaguy and Arroyo Hermosa.The Itapucumi series is thought by Harrington(1950) to be about 300 to 400 meters thick. This isonly an approximation, however, since no detailedmapping has been done and the general pattern offolding, indicated on plate 1 by scattered observationsof attitude, has yet to be worked out.Figure 43 shows the character of the limestone anda description of the rocks exposed in the Vallemiquarry is given in the section on cement. In general,the series is made up of light- to dark-gray limestonein well-defined, relatively thin beds. Locally the bedsare thick and massive, alternating with shaly limestoneand marly shale. All the rocks are hard and dense andmost of them are fine grained, but some are partly orentirely recrystallized and cut by veins of calcite ordolomite. Oolitic and lithographic limestone, as wellas various kinds of green- and pink-toned marble havebeen reported from a number of places.The rocks range from nearly pure limestone with 95percent or more calcium carbonate to nearly puredolomite. The analyses below are believed to befairly representative. As noted on page 80, the purerlimestone at Vallemi contains approximately 95percent calcium carbonate, whereas the varieties thatare streaked green by chloritic material contain 80 to85 percent calcium carbonate.Analyses of typical limestones of Itapucumi seriesCaO ___ ____ __ _____ ______MgO_-_ - _- _-(Fe, A1) 2O3_-- _ __ _______H 2O___ __ _____ ___ ___ ___ _CO2_______ ___ ____ __ ________[M. Goldschlag (1913c), analyst]13.3450.900.932.971.7540.98100. 9523.9052.691.9741.300.69100.5531.1252.853.030.3642.2099.6643.3830.9117.942.541.8543.79100.41NOTE. Explanation of localities of samples:1. Limestone, Cordillera de las Quince Puntas, 10 kilometers west-northwest ofCenturion.2. Oolitic limestone, 35 kilometers north-northwest of Puerto Max on RioParaguay (near Puerto Fonciere).3. Limestone, Cerro Paiba, 10 kilometers southwest of San Carlos.4. Dolomitic limestone, left bank of Rio La Paz, east of San Luis de la Sierra.The limestone series rests uncomformably on the Precambrianbasement rocks that make up the PotreroSaty. They are shown on plate 1 as everywhere overlaindirectly by the Tubarao series of Pennsylvanian orPermian age but only in the extreme northeastern partof their belt of outcrop, near Santa Luisa, is this contactactually known. Not far from the town of Santa Luisa,and along Arroyo Hermosa, Boettner (1947) found tilliteof the Tubarao beds, with striated pebbles, resting onthe Itapucumi limestone.It appears certain that the Itapucumi beds do not extendfar into southern Paraguay, because Silurian andyounger beds rest directly on the Precambrian basementin many places farther south. According to availableknowledge, none of the wells drilled by the Union OilCompany were deep enough to have reached these beds,so it is not known whether the Itapucumi rocks extendbeneath the Gran Chaco. Quite possibly they do not,for no rocks of similar character seem to be known in theeastern Andes, where the deeper beds of the Gran Chacobasin reappear at the surface (Ahlfeld, 1946, geologicmap).Boettner (1947) notes that marble was once quarriedat Puerto Max for ornamental stone and DuGraty