730 K. F. MATHER FRONT RANGES OF THE ANDES 



Totora series ; Devonian ; including in the northern part of the area the 

 Espejos formation and in the southern part the Los Monos shale. 



Los Monos shale ; blue gray to black, fissile shale with interbedded 



micaceous sandstones ; base not exposed ; more than 1,000 feet thick. 



Espejos formation ; dark bluish or greenish black carbonaceous fissile 



shale with intercalations of dark, fine-grained, brittle limestone in 



beds two or three inches thick; total thickness unknown. 



ESPEJOS FORMATION 



The Espejos formation was first described by Heald and Mather in a 

 report upon the geology of the eastern Andes a short distance north of 

 the area now under consideration. 6 The type locality is in the Espejos 

 quebrada, in the Sierra de Santa Cruz where the dark carbonaceous 

 shales and thin limestone strata carry trilobites and brachiopods of De- 

 vonian age. 



In the Front Eanges of the Andes south of the Sierra de Santa Cruz, 

 only one small outcrop of this formation was observed. That outcrop is 

 in the quebrada of Rio Tacuru, in the northern part of the Sierra de 

 Charagua. It is probable that other exposures of the Espejos beds occur 

 just beyond the western margin of the area traversed during the recon- 

 naissance studies on which this report is based. It is also possible that 

 the Espejos formation is at the surface within the gorge through 

 which Rio Parapiti traverses the Sierra de Charagua, for it is likely that 

 much lower beds are exposed within that gorge than along the floor of 

 the parallel canyon of Rio Charagua. It is doubtful, however, whether 

 there are other exposures than these anywhere else within the region 

 under discussion. 



The beds exposed in the Tacuru quebrada are dark bluish or greenish 

 black, richly carbonaceous, fissile shale with intercalations of dark, fine- 

 grained, brittle limestone in beds 2 or 3 inches thick. In all respects they 

 are identical with the Espejos formation at the type locality in the Espejos 

 quebrada and elsewhere in the Sierra de Santa Cruz, north of the eigh- 

 teenth parallel; the lithology is so distinct that there can be no question 

 of the correctness of the correlation, although no fossils were found in the 

 Tacuru exposure. 



In the Tacuru quebrada the Espejos beds are faulted against much 

 younger strata, and the nature of the normal contact between them and 

 the next younger formation has not been ascertained. In the Sierra de 

 Santa Cruz, however, this contact is displayed as a great unconformity. 

 In the region traversed by the trail from Cochabamba to Santa Cruz 



K. C. Ileald and K. F. Mather, 1922. 



