Thegeology of Uruguay combines areas ofPrecambrian-agedshieldunits with a region ofvolcanic rock erupted during theCretaceous and copioussedimentaryfacies the oldest of which date from theDevonian. Big events that have shaped the geology ofUruguay include theTransamazonian orogeny (2000million years ago (mya)), the breakup ofRodinia (700–500mya) and theopening of the South Atlantic (~145 mya).
ThePrecambrianshield in Uruguay includes part ofRío de la Plata craton, an extensive area of stablecrystallinebasement rock underlying Uruguay and large areas of easternArgentina and southernBrazil. The shield in Uruguay goes under the name of Rio Grande do Sul—Uruguay Shield and includes parts ofSouthern Brazil.[2] Though the Río de la Plata Craton underlies practically all of Uruguay, it only shows up in the south and east of the country since in other parts of the country, it is covered by younger volcanic rocks or sediments. The shield was formed during twoorogenic events; one 2000million years ago (mya) for the western part and another 700–500 mya for the eastern part.[3] The later orogenic event is the result of theaccretion of blocks andterranes in the aftermath of the breakup ofRodinia whenSão Francisco and Río de la Plata cratons formed one plate in the west of theAdamastor Ocean.[4] The eastern part of the Uruguayan shield region is crossed by two majorshear zones both running roughly in north-south direction thedextralSarandí del Yí-Piriápolis Shear Zone and thesinistralSierra Ballena Shear Zone.[3] West of Sarandí del Yí Shear Zone the Río de la Plata Craton is intruded by theLate PaleoproterozoicFlorida dyke swarm.[5]
The area between Sarandí del Yí and Sierra Ballenashear zones –that is the Nico Pérez Terrane– is made ofmetamorphic andgranitoid rocks which are the oldest rocks in Uruguay.[6] It has not been established to which degree this terrane belongs to or has affinity with the Río de la Plata Craton.[6] The rocks of Nico Pérez Terrane formed originally in theArchean eon andPaleoproterozoic era and much were later affected by theBrasiliano orogeny inNeoproterozoic era.[6]
After theassembly of the finalshield configuration the territory of Uruguay have been covered by several sedimentary formations ranging fromDevoniansandstones toQuaternaryloess. All of the sedimentary formations covers only patches of the country sincedeposition have not been uniform anderosion have cleaned surfaces, creeks and shores.
Sedimentary rocks ofLower Devonian age are found in the central part of Uruguay being exposed as a narrow east-west to northeast oriented band. The upper and better known part of thissequence is made up ofsandstone. The lower sandstones of this sequence have characteristics similar to theFurnas Formation ofSão Paulo inBrazil.[7] During theLate Paleozoic the territory of Uruguay was affected by theKaroo Glaciation and was subsequently covered by ice lobes of the greatice sheet that covered large parts ofGondwana.Glacial striae onshales andvarve-like sediments found in Uruguay have been associated with this glaciation.[8]
During the earlyrifting stages of the South Atlantic the area of southernParaná Basin suffered a gentle uplifting that deviated sediments into the Tacuarembó region of Uruguay. These changes led to the formation of the Itacuanbú and Tacuarembó formations during the MidJurassic to EarlyCretaceous. Parts of theTacuarembó formation came to be preserved thanks to a unit of the Paraná traps, the Arapeybasalts, that erupted 132 mya and covered the sediments.[9]
In the north of Uruguay, volcanic materials from theParaná continental flood basalt province form a majorlithological unit extending beyond Uruguay's borders into Argentina and Brazil, with parts of it now lying inNamibia on the other side of the Atlantic due toplate tectonics. This volcanic material erupted in theCretaceous period during the opening of theSouth Atlantic and has been linked to theTristan da Cunhaplume.[10] The bulk of this volcanic material isbasalt but there arerhyolites as well. Associated with this volcanism are alsosyenite and othergranitoidintrusions exposed asoutcrops in small areas of Southeastern Uruguay.[11] Although volcanic rocks of the Paraná volcanism underlies more than half of Uruguay, they have in many areas been covered by younger sediments so that the lavas only crop out as a geological province in the Uruguayan northwest.[12]