Geology and Mineral Resources of Paraguay A Reconnaissance

GEOLOGY AND MINERAL RESOURCES OF PARAGUAYare that it assembles in one place most of the hithertowidely scattered published information on the geologyof Paraguay; it contains a considerable amount ofhitherto unpublished information collected from varioussources; it presents new facts, based on modernpetrographic and chemical methods, on a few of theigneous rocks; and it contains an objective summaryof the country's mineral resources and its potentialitiesas a mineral producer. This summary is not as optimisticas many would hope, but it does point out afew real possibilities and narrows the areas that mostdeserve further study.The report is, then, a compilation and interpretationof existing information on the geology and mineralresources of Paraguay based on a 6-month reconnaissancestudy, but if it were confined to recording personalobservations during that period it would be a"thin" report indeed. Travel conditions in Paraguayare difficult at best, and were made somewhat more soby greater than average rainfall during the winter of1952. These and other tangible and intangible factorspermitted field work for considerably less than halfthe total time available for the project. The author'spersonal observations were limited to a comparativelysmall part of central Paraguay, plus spot studies atthe termini of a few airplane trips; these flights permittedexcellent aerial views of representative crosssections of the country. These personal observationswere greatly supplemented by the use of aerial photographs,by the study of specimens in various museumsand private collections, and by data supplied by severalgeologists and others who had observed and recordedthe geology at particular spots.The facts gathered would, by themselves, have beenbeen utterly insufficient as a basis for a report of thescope attempted here. Personal observations, supportedby studies of the small collections of rocks andfossils have, however, given a clearer understandingof the existing literature en the subject. They havealso given a modicum of confidence in this attempt togather published information on the geology of Paraguayand to integrate it with facts observed by workersin neighboring countries, where geologic knowledge ismore advanced than in Paraguay.Throughout this report an effort has been made todistinguish between fact and fancy as well as betweenthe author's own contributions and those of others.Since facts that are absorbed by the mind, regardlessof their source, have a tendency to emerge as newdiscoveries, these efforts have doubtless been onlypartially successful. The reader would, therefore, dowell to remember that the present author is a compilerof scattered information, rather than a recorder ofhis own observations. Naturally, he must bear responsibilityfor all the interpretations, good or bad, thathave been drawn from the basic data.PREVIOUS GEOLOGIC INVESTIGATIONSA considerable body of knowledge on the geology andmineral resources of Paraguay has been accumulatedthrough the years. Most of this information, however,is widely scattered through the world literature ofgeology, where it appears in several languages, orexists only in unpublished files or in the memories ofindividuals who have made observations of particularspots or restricted areas. It should be remembered,too, that until 1938 more than half of the Gran Chacobelonged to Bolivia, hence descriptions of this areain the older literature are referenced under the nameof that country. The pertinent literature that hasbeen found is listed in the annotated bibliography,which contains a number of items that are not referredto specifically in the text. Such items haveserved a purpose by providing needed background forother facts and all will be of interest to future seriousstudents of the subject.The most complete descriptions of the geology ofParaguay that have appeared in print are the recentpapers by Harrington (1950, 1956). Unfortunately,his work is limited to the eastern part of the countryand is based on only a few weeks fieldwork. Even withthese limitations, he was able to present a clear andaccurate picture of the stratigraphy and geologic history,one on which the present author has leaned veryheavily in preparing this compilation. The only othercomprehensive study of the geology known to theauthor is that made by the Union Oil Company duringits exploration for petroleum in 1944-49. In additionto the drilling of five deep wells, this company is understoodto have done reconnaissance geologic mapping ofthe entire country, plus a very large amount of geophysicalexplorations in the Gran Chaco. Of theinformation accumulated, only the records of the deepwells and fossil identifications made by Dr. Horacio J.Harrington are available for publication.Among the older descriptions of the geology of easternParaguay, those of Carnier (1911, 1913), supplementedby Goldschlag's (1913) chemical and petrographicdescriptions of Carnier's rocks, are by far the mostuseful; much dependence has been placed on themhere and in the compilation of plate 1. Kanter (1936)gives the most complete description extant of the geologyof the Gran Chaco region. DeMersay (1860) andDuGraty (1865) give far more complete accounts ofthe country's mineral resources than any more modernworkers and both give, in addition, many notes on thegeology. M. S. Bertoni (1921), A. deW. Bertoni (1939),G. T. Bertoni (1940), Conradi (1935), and Boettne