STRATIGRAPHY 747 



erosion, in which regard they are in strong contrast to the strata com- 

 posing the Tacuru formation and Bermejo series. Sandstones are poorly 

 cemented, soft, and crumbly; shales are for the most part easily friable 

 and indistinctly laminated; many of the beds are in reality unconsoli- 

 dated clay, marl, or sand. Occasionally there are beds of harder rock, 

 well cemented sandstones or compact shales, but these are rare. Typically 

 the Tatarenda beds are light chocolate brown, but in places the sandstones 

 are white and sugary in appearance. Elsewhere the sands may be maroon, 

 carmen, or other shades of red. Likewise the shales at many places depart 

 from the common brown tint and display brilliant hues of red, purple, 

 or green. 



It is not possible to state the maximum thickness of the Tatarenda 

 formation with accuracy because its upper beds are everywhere concealed 

 beneath the veneer of Quaternary deposits which lap up against the lower 

 slopes of the mountains and are widespread over the surface of the low- 

 lands. Doubtless, too, there is considerable variation in thickness from 

 place to place, for it is evidently a deposit made in large part by streams 

 debouching into lakes or shallow seas or spreading their debris across low- 

 lying lands. Along the eastern front of the Sierra de Charagua this 

 formation has a minimum thickness of slightly over 3,000 feet. 



Beds of this formation aggregating more than 3,000 feet in thickness 

 outcrop along the channel of Rio Saipuru west of the Sierra de Charagua 

 and are traversed by that stream before it passes through the portal be- 

 tween the hogbacks formed by the resistant top beds of the Tacuru forma- 

 tion. In that locality the lower 2,000 feet of the Tatarenda formation 

 consist of light chocolate brown and maroon clay shales and cross-bedded 

 sandstones, irregularly alternating with each other. These beds are for 

 the most part very poorly consolidated, soft and crumbly. The sands 

 contain numerous partings of clay which separate beds of varying thick- 

 ness, some of which are as much as 20 feet thick. Irregular masses of 

 clay also occur as inclusions within the sandy beds. About 2,000 feet 

 above the base of this formation there is a member consisting of thin- 

 bedded micaceous sandstone and shale, rather Avell cemented into firm 

 rock by calcareous cement and aggregating 200 feet in thickness. Where 

 Rio Saipuru crosses this member its beds are steeply inclined toward the 

 west and flexed into strongly pitching anticlines and synclines, as shown 

 in figure 17. Certain of the calcareous beds are crowded with the shells 

 of a species of pelecypod and are so thickly strewn with the tiny valves of 

 an ostracod as to appear oolitic. Above these marine fossiliferous strata 

 the beds resemble closely those in the lower part of the formation. The 

 materials composing them are poorly sorted, some of the strata contain- 



