i8i Costa Rica Miocene— Olsson 



General Stratigraphy op Northern Costa Rica 



The northern or Atlantic slope of Costa Rica, the so-called 

 "Tierra Caliente" is largel}^ composed of Tertiary sedimentary 

 rocks, which except for some folding, dip away from the slopes 

 of the Cordillera Central towards the sea. To the east, this 

 sedimentary belt may be traced into the Province of Bocas del 

 Toro of western Panama, where its upper and highly fossilifer- 

 ous beds are well developed and exposed on the islands of the 

 Chiriqiii lyagoon. This Tertiary sedimentary belt ends some 35 

 to 40 miles east of the city of Bocas del Toro. The Miocene 

 sedimentary rocks of Batun and of the northern Canal Zone 

 form a small independent unit, which begins about 30 miles 

 west of Colon and ends close to the eastern limits of the Canal 

 Zone. 



In a general way, the stratigraphic succession in northern 

 Costa Rica is as follows: 



Pleistocene Slightly elevated coastal swamps, witli deposits of cross-bedded 

 sands, clay-marls etc. 



Pliocene Certain reef-limestones composed of coral and hydrozoa re- 

 mains; also marls, blue lignitiferous clays, blue coarse sand- 

 stones, and a predominating hard conglomerate, composed of 

 large boulders of igneous rocks ( the so-called boulder-clays of 

 several writers on Costa Rican Geology). 



Pliocene The Gatun Stage or Formation. Coral-reef limestones, (near 



the coast), marls, more generally a blue or blue-black sand- 

 stone, dark colored shales, sandy and impure limestones, ligni- 

 tes and conglomerates. Generally very fossiliferous. 



The Uscari Stage or Formation. Usually dark-colored shales 

 but with some sandstones, limestones and conglomerates. The 

 smaller types of foraminfera are usually quite abundant, and 

 are associated with fish-scales, and small echinoid spines, with 

 mollusks generally rare. 



Oligocene Sandstones, shales and a complex of hard, often semicrystal- 

 line limestones which pass through all possible gradations to 

 i. coarse arkosic sandstones. The most abundant and characteris- 

 tic fossils are the Orbitoidal foraminifera {Lepidocydina) . The 



