lo Bulletin 39 , 182 



age of these beds probably ranges from Lower to Upper 

 Oligocene. 



The Costa Rican Miocene 



An important part of the stratigraphic succession of north- 

 ern Costa Rica is composed of rocks of Miocene age, whose 

 thickness varies according to locality and completeness of sec- 

 tion, from a few hundred to several thousands of feet. They are 

 divisible into two stages or formations by an unconformity or 

 disconformity of greater or less degree. In their natural order 

 of sequence, these divisions of the Costa Rican Miocene are: 



Gatun Stage or Formation: Middle and Upper Miocene 



Uscari Stage or Formation: L^ower Miocene 



The; Uscari Stage 



The Uscari stage or formation derives its name from Uscari 

 creek in the Talamanca valley of eastern Costa Rica. It con- 

 sists principally of soft, dark-colored shales and because of their 

 slight resistance to denundation, their outcrop frequently forms 

 wide valleys and interior basins. These shales were deposited in 

 quiet and moderately deep water, and their fauna is largely 

 composed of the smaller types of foraminifera, such as Globig- 

 erina, Textularia, Lagena, Nodosaria and Miliola, but the 

 large Orbitoidal Lepidocydince, abundant in the underlying Oli- 

 gocene, are absent. Mollusks are very uncommon in this forma- 

 tion. The small fauna described by Gabb from Sapote, Costa 

 Rica, belongs to the lower part of the Uscari formation. 



The Gatun Stage 



The upper division of the Costa Rican Miocene is equivalent 

 in part to the Gatun formation of the Canal Zone. The typical 

 Gatun, so named from its well-known exposures at Gatun and in 

 which a part of the Panama Canal and the locks at Gatun were 

 excavated, lies in an independent sedimentary basin across the 



