STRATIGRAPHY 745 



The lower strata of the Tacuru, as observed in that section, are red and 

 white sandstones which alternate irregularly in the stratigraphic succes- 

 sion. Above these the sandstones are cross-bedded and are brown or gray. 

 Near the top of the formation the sands are soft, crumbly, cross-bedded, 

 and chocolate brown or maroon. The intervening shales are not well ex- 

 posed, but are in general brown or some shade of red; they are, of course, 

 non-fissile. The uppermost member of the formation is a very resistant 

 gray-white sandstone which forms the western line of hogbacks along the 

 margin of the cuestas. 



The Tacuru formation underlies the gently sloping hogbacks in the 

 western half of the Mandiyuti range. About 2,000 feet of strata belong- 

 ing to this formation are there exposed and its top was not seen in that 

 locality. As elsewhere, there are cross-bedded sandstones with consider- 

 able amounts of interstratifLed shale. Yellow and white are the predomi- 

 nating colors of these rocks, but in some places the weathered surfaces 

 are a brilliant vermilion or some other shade of red. Immediately east 

 of the Mandiyuti fault, which crosses Eio Cuebo 3 miles west of the vil- 

 lage of Cuebo, at the eastern front of the Mandiyuti range, there are 

 several beds of light gray sandstone which outcrop along the channel of 

 the river. These are separated from the Bermejo beds immediately up- 

 stream by the major fault, to be described in a subsequent paragraph, and 

 are probably the uppermost beds of the Tacuru formation. 



Farther south in the Sierra de Aguarague the Tacuru formation ap- 

 pears to be thinner than in the more northern localities, as measurements 

 made in that range indicate its thickness to be between 1,800 and 2,200 

 feet. The amount of shale in the stratigraphic sequence is also very much 

 reduced. Thus the formation in this mountain range is chiefly sand- 

 stone, light gray or white, occurring in massive resistant beds, some of 

 which are as much as a hundred feet thick. Between some of these sand- 

 stone beds there are small amounts of soft crumbly sands and clays. 



In the Quebrada de Los Monos an unusual and very significant feature 

 was noted near the base of this formation. Just before reaching the point 

 at which farther progress upstream on muleback is barred by huge boul- 

 ders entirely blocking the stream channel, there is an outcrop of a thirty- 

 foot conglomerate which is believed to represent the base of the Tacuru 

 formation at this locality in the Sierra de Aguarague. The matrix of 

 this conglomerate is fine red sand. The pebbles are of three kinds. Some 

 are small subangular fragments of quartz, quartzite, and chert a quarter 

 to a half inch in diameter. Others consist of subangular slabs and chunks 

 of red Bermejo sandstone and sandy shale 2 to 6 inches in length. These 

 are irregularly distributed throughout the conglomerate without any 

 XLIX — Bull. Geol. Soc. Am., Vol. 33, 1921 



