Geology and Mineral Resources of Paraguay A Reconnaissance

SEDIMENTARY ROCKS 53SILURIAN SYSTEMCAACUPE SERIESThe Caacupe series, at least 700 meters (2,300 feet)thick, consists largely of arkosic sandstone with a basalconglomerate and with some beds of clay and shale.Fossil evidence shows it to be of Early Silurian age.The probable correlatives of these beds in the GranChaco basin, as determined by deep exploratory wellsand as described in another section in this report, arelargely black marine shale with comparatively littlesandstone.As shown on plate 1, the Caacupe series occupiesmuch of the heavily populated west central part of thecountry extending southward from the banks of theRio Paraguay above Asuncion to Quiindy and Mbuyapeyand from Ypacarai eastward to the longitude ofEusebio Ayala.The Caacupe series can be divided into four lithologicunits, as follows:ApproximatethicknessTop.1. Sandstone, red to brown, massive, fossiliferous; highlymicaceous in part; underlain by soft light-coloredshale or clay with thin beds of sandstone. _________ 2002. Sandstone, white, saccharoidaL _ __________________ 1003. Sandstone, brown to red-brown, fine- to coarse-grained,arkosic; several f ossilif erous clayey horizons nearbase._________________________________________ 4004. Basal Paraguari conglomerate. _____________________ 1-50These units, which are described below, cannot beshown separately on plate 1, but more detailed geologicstudies will almost certainly prove them to be easilymappable units that may ultimately deserve the statusof formations or members. It is also possible that theupper unit may be subdivisible into a lower shaly unitand an upper one of sandstone.Paraguari conglomerate. The lowest unit of theseries is a basal conglomerate that grades both laterallyand upward to conglomeratic sandstone. It wasnamed the Paraguari conglomerate by Harrington(1950) for the excellent exposures just northeast of thattown. There it is 40 to 50 meters thick, but elsewhereit appears to be thinner and is probably no more than2 or 3 meters thick in most places. The conglomeraterests unconformably on the weathered surfaces of thePrecambrian rocks and is overlain gradationally bycoarse-grained crossbedded sandstone. According toHarrington, it is exposed along the entire length of theYpacarai depression, from San Bernardino to Paraguari,at the foot of the Cordillera de los Altos thatforms its eastern side. Harrington, who thinks that aconglomerate at Pirayu, on the west side of the depression,may belong to this unit, also mentions theexposures farther south that are described below.The typical conglomeratic rock consists of a greatnumber of pebbles in a coarse-grained sandy matrix.The pebbles are smooth, ellipsoidal to well-rounded,and 1 to 30 centimeters in diameter. They consistexclusively of siliceous materials vein quartz, quartzite,and various forms of chert and jasper. There areno igneous and metamorphic rocks except quartzite.The most easily studied exposure of the conglomerateis probably that at the north edge of San Bernardino.a resort town on the east shore of Lago Ypacarai.Even here the actual contact with underlying rocks ispoorly exposed. The lower part of a small hill at SanBernardino is made up of coarse Precambrian granite,fresh near the lake level, but strongly weathered andcrumbly upward. The top of the hill is made up ofcoarse-grained brown to red-brown sandstone, at least7 meters thick. It is strongly crossbedded and containsmany thin lenses of conglomerate with pebbles1 to 2 centimeters in diameter. Between the weatheredgranite and the conglomeratic sandstone there is acovered interval less than 6 meters thick. The soiland slope wash in this interval, as well as on the uppermostpart of the granite slopes, contain an abundance olpebbles, 2 to 10 centimeters in diameter, of quartz,chert, and quartzite. These indicate that at least apart of the interval is made up of a true basal conglomerate,though it seems probable that even here there isa considerable amount of sandstone matrix.The basal conglomerate is well exposed in manyplaces along the highway between Quiindy and Caapucu,at Quyquy6, south of Acahay, and elsewhere.At all of these places it marks the contact betweenPrecambrian igneous and metamorphic rocks anderosional remnants of the lower part of the Caacupeseries. Nowhere in this southerly area does theconglomeratic horizon appear to be much more thanone meter thick. It grades upward to typical brownor red-brown sandstone of the Caacupe, and itsweathered surfaces are liberally strewn with roundedto semirounded pebbles of quartz, quartzite, andvaricolored chert. As elsewhere, pebbles of graniteand quartz porphyry, that form the surface on whichthe conglomerate was deposited are conspicuouslyabsent in the conglomerate.Arkosic sandstone. The arkosic sandstone, whichmakes up more than half of the Caacupe series, constitutesthe bulk of the rocks that are called the Piribebuysandstone by Harrington. It grades downwardto the basal conglomerate of the series and is overlain,with little or no gradation, by the thick white saccharoidalsandstone unit. The unit consists almost