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The Guará Formation is a Late Jurassic sedimentary unit, outcropping on the southeastern edge of the Paraná Basin; in the western part of Rio Grande do Sul state (Brazil) and western Uruguay. It is described as fine to conglomeratic sandstones, deposited in arid to semiarid climate, composing fluvial channels and flood plains in the northern portion and aeolian dunes alternating with ephemeral fluvial systems in the south. The main fluvial paleocurrents are from NE to SW, marking a unique period in geological history of the Paraná Basin where its depocenter was shifted to the SW.

The Guará Formation is a Late Jurassic sedimentary unit, outcropping on the southeastern edge of the Paraná Basin; in the western part of Rio Grande do Sul state (Brazil) and western Uruguay. It is described as fine to conglomeratic sandstones, deposited in arid to semiarid climate, composing fluvial channels and flood plains in the northern portion and aeolian dunes alternating with ephemeral fluvial systems in the south. The main fluvial paleocurrents are from NE to SW, marking a unique period in geological history of the Paraná Basin where its depocenter was shifted to the SW.

[[file:Guara_01.jpg|left|Figure 1. Simplified geological map of Paraná Basin in Rio Grande do Sul state, Brazil. (modified from Scherer & Lavina, 2005)<ref name=Scherer_et_Lavina_2005 />]]

[[file:Guara_01.jpg|thumb|Figure 1. Simplified geological map of Paraná Basin in Rio Grande do Sul state, Brazil. (modified from Scherer & Lavina, 2005)<ref name=Scherer_et_Lavina_2005 />]]

==Definition==

==Definition==

It is exposed in western Rio Grande do Sul (Brazil), bounded on the northeast by a fault system NW-SE ([[:file:Guara_01.jpg|Figure 1]]); it extends to the north of Uruguay, where it is named as Batoví Member of Taquarembó Formation; and possibly to the east in Argentina, where is poorly known. In Rio Grande do Sul, it has discordant contacts with the overlying Botucatu Formation (Lower Cretaceous), and underlying Sanga do Cabral Formation (Lower Triassic) ([[:file:Guara_02.jpg|Figure 2]]).

It is exposed in western Rio Grande do Sul (Brazil), bounded on the northeast by a fault system NW-SE ([[:file:Guara_01.jpg|Figure 1]]); it extends to the north of Uruguay, where it is named as Batoví Member of Taquarembó Formation; and possibly to the east in Argentina, where is poorly known. In Rio Grande do Sul, it has discordant contacts with the overlying Botucatu Formation (Lower Cretaceous), and underlying Sanga do Cabral Formation (Lower Triassic) ([[:file:Guara_02.jpg|Figure 2]]).

==Age==

==Age==

The Guará Formation is described as fine to conglomeratic sandstones of whitish to reddish color, with trough cross stratification from small to large size and horizontal lamination. The facies associations characterize fluvial to fluvio-eolian deposits, with a distinctive change in facies throughout its exposure: the northern part consists in facies associations of braided fluvial systems (Fig. 3); in the southern part, there is a tendency to replace the predominance of fluvial facies to aeolian associations (Figs. 4 and 5).

The Guará Formation is described as fine to conglomeratic sandstones of whitish to reddish color, with trough cross stratification from small to large size and horizontal lamination. The facies associations characterize fluvial to fluvio-eolian deposits, with a distinctive change in facies throughout its exposure: the northern part consists in facies associations of braided fluvial systems (Fig. 3); in the southern part, there is a tendency to replace the predominance of fluvial facies to aeolian associations (Figs. 4 and 5).

[[file:Guara_04.jpg|left|thumb|Figure 4. Vertical log of Guará Formation near the Brazil-Uruguay frontier, showning facies examples for the fluvio-eolian system.]]

[[file:Guara_04.jpg|left|thumb|Figure 4. Vertical log of Guará Formation near the Brazil-Uruguay frontier, showning facies examples for the fluvio-eolian system.]]