Geology and Mineral Resources of Paraguay A Reconnaissance

12 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYgroup here may be related to the basaltic lava flows,hence of Triassic or Jurassic age; for some, however,there is strong geomorphic and other evidence that theyare no older than late Tertiary.The geologic structure of Paraguay appears to berelatively simple in its grosser aspects, but there ismuch to learn of even the larger features. In theeastern half of the country the beds dip gently eastwardtoward the great Parana basin of southeasternBrazil. This easterly dip appears to reverse along theaxis of a low anticline that trends north-south only afew kilometers east of the Rio Paraguay and exposesthe older sediments and the Precambrian basementrocks at the surface in many places. West of this axisthe beds either dip steeply westward, or are downfaultedto the west, into the depths of the Gran Chacobasin. The rocks in this basin are, in places, at least10,000 feet deeper than they are in eastern Paraguay;west of the basin, in eastern Bolivia they rise to thesurface in a series of anticlines and syncliiies that formthe foothills of the Andes.IGNEOUS AND METAMOBPHIC BOCKSBy CHARLES MILTON and EDWIN B. ECKELIn the course of a 6-month field study of the geologyof Paraguay, Eckel collected about a hundred specimensof rocks in 1952. Earlier, geologists had made suchcollections, notably Pohlmaim (1886), Lindner (Milch,1895), and Cariiier (1911c, 1913); these collections havebeen described respectively by Pohlmann (1886), Milch(1895 and 1905), and Goldschlag (1913a). Harringtonmade extensive collections in 1946 which, unfortunately,were lost, so that his otherwise excellent memoir onthe geology of Paraguay (1950) contains only field noteson the petrology. Milton has studied the collectionmade by Eckel and to a considerable extent compiledthe notes here presented.The igneous and metamorphic rocks may be classifiedin three main groups metamorphic and granitic, basalticand diabasic, and alkalic. Geographically, thereare a southern and a central area, both examined byEckel, and a northern area, examined by Carnier andothers. The sedimentary rocks collected by Eckel arebriefly described in connection with the stratigraphy;one peculiar sedimentary rock that has columnar jointingis described here in the section on diabase as' 'pseudotrachy te.''PRECAMBRIAN METAMORPHIC AND GRANITIC ROCKSIn eastern Paraguay there are two extensive areas,and several smaller ones, of Precambrian granitic andmetamorphic rocks. One of the larger areas is theregion around Centurion near the Rio Apa; it extendsnorthward into Brazil. The other is in southern Paraguay.This report discusses only briefly rocks of theRio Apa region, which have been investigated byCarnier (1911c), Goldschlag (1913a), and Boettner(1947); it deals more especially with the rocks fromthe southern and central region.The basal complex, a southern extension of the Brazilianshield, is overlain unconformably by Paleozoicand Mesozoic sedimentary rocks as in many other partsof the world. It underlies, at no great depth, all easternParaguay and the northern part of the Gran Chacowest of the Rio Paraguay, where there are a few smalloutcrops of these rocks. In the southern and westernparts of the Gran Chaco, however, the basement rocksare deeply buried beneath sedimentary rocks.The surface of the exposed Precambrian rocks in theRio Apa region is comparatively rugged, with relief ofmore than 200 meters, whereas in the southern regionthe surface is smoother, with relief of less than 100meters. Reasons for this difference in topographicexpression are not apparent.The metamorphic rocks, which are undoubtedly olderthan the igneous rocks, are made up largely of schistand gneiss. A great variety of other rocks, whose distributionis not known precisely, is associated with theserocks. They include ultrabasic igneous rocks as wellas argillite, quartzite, marble, and other kinds.Most of the igneous rocks are of intrusive origin, butsome are probably extrusive. Medium- to coarsegrainedgranite and quartz porphyry predominate, butthere are many bodies of aplite and pegmatite associatedwith them. In at least one place, near Caapucii, fragmentalpyrophyllitized volcanic rocks are associatedwith porphyritic rocks that show flow structure, indicatingthat they probably originated as surface flows.Probably the earliest rocks were gneisses and schiststhat were overlain by shale and sandstone beds, andthis whole sequence was then intruded by the graniticrocks, accompanied by extensive metamorphism and aconsiderable amount of folding and faulting. The Precambrianrocks were later eroded to a relatively smoothsurface before the deposition of the limestone andargillite of the Itapucumi series, of possible Cambrianand Ordovician age. This interpretation agrees withthat of Carnier (1913) and of Harrington (1950).Without more knowledge than is available at presentit is impossible to assign definite ages to any of theserocks. Harrington (1950) suggests correlation of themetamorphic rocks with the Cuiba series of MatoGrosso which has been assigned to the AJgonkian byOliveira and Leonardos (1943, p. 217), Evans (1894),and others. Possibly both Archean and Algonkian erasare represented, just as they are believed to be innortheastern Bolivia (Ahlfeld, 1946, p. 19-25). Forconvenience, the metamorphic rocks are grouped here