UNIVERSIDADE DE sAo PAULO INSTITUTO DE GEOCI~NCIAS ...

ivABSTRACT The La Colmena sheet, eastern Paraguay, is located some80 Km to the SE of the capital city of Asunci6n. It is limited byparallels 25 0 45'and 26 0 00' S and meridians 56 0 45' and 57°00'W,covering about 700 Km2.The sheet lies in the central, E-W trending segment ofthe Asunc i6n rift, which to the W changes into the westernsegment, with a NW-SE direction; the poorly known eastern segmentalso shows this direction. The rift is the site of emplacement ofmost of the Mesozoic and Tertiary alkaline occurrences of the"Central alkaline Province" of eastern Paraguay.The sheet can be divided into several structural blocks,limited by normal faults. The Potrero Alto block lies to the S,separated from the depressed Valle de Acahay block by the Acahayfault, an important structure that constitutes the southern limitof the rift's central segmento The San José fault, cutting theAcahay fault, occurs to the SW and limits the San José block. Thesouthwestern part of the Valle de Acahay block is taken up by theminor Chauria block, bound to the N by the Medina (E-W direction)and Chauria (N53W direction) faults.Rhyo lites and granites of the Caapucú Group constitutethe Neoproterozoic to Eocambrian basement in the area. The marinesediments of the pre-Llandoverian Caacupé Group, lying on top ofan erosional nonconformity, are formed by the Paraguariconglomerate (as discontinuous outcrops) and the sandstones ofthe Cerro Jhú (250 m maximum thickness) and Tobati (200 m minimumthickness) formations. The sediments cover the Potrero Alto, SanJosé and Chauria blocks; stratigraphic markers show a faultdisplacement of up to 200 m between the Potrero Alto and theother blocks.The Valle de Acahay block is mainly covered by threeunits informally called U1(basal, pelitic-psammitic, thicknessup to 50 m), U2 (psammitic, maximum thickness 100 m) and U3 (atthe top, with conglomerate showing a medium-to coarse-grainedsandstone matrix, minimum thickness 165 m); silicified (treefern?) trunks with poorly preserved structures are found in U1.