26 "^ vaughan: geologic history of coral reefs 



GEOLOGY. — Sketch of the geologic history of the Florida coral 

 reef tract and comparisons with other coral reef areas. ^ Thomas 

 Wayland Vaughan, U. S. Geological Survey. 



GEOLOGIC FORMATIONS OF SOUTHERN FLORIDA 



The southern end of the mainland of Florida is underlain by 

 two limestone formations, Viz : the somewhat sandy, rather soft, 

 light colored Miami oolite, and the more or less sandy, non-oolitic, 

 dark or light Lostmans River limestone, which may be friable 

 or indurated, and sometimes is crystalline. Four principal kinds 

 of material are represented in the Florida keys. The surface of 

 the keys north of Cape Florida is sandy. The most conspicuous 

 constituent of the main keys from Soldiers Key to the southern 

 end of Big Pine Key is the Key Largo limestone, an elevated 

 coral reef rock. The prevalent rock from the Pine Keys west- 

 ward to Boca Grande Key is the soft, non-arenaceous Key West 

 oolite. The surface material of the Marquesas is largely com- 

 posed of the detrital remains of calcareous algae probably over- 

 lying an oolitic foundation, while that of the Tortugas is largely 

 coral detritus underlain by more consolidated limestone. 



In order to indicate the relative importance of oolite and the 

 elevated coral reef rock in the formation of the keys and nearby 

 parts of the mainland, the following rough planimeter measure- 

 ments are given: Miami oolite, area 1300 square miles; Key 

 West oolite, area 370 square miles — oolite, total area 1670 square 

 miles. Key Largo limestone, elevated coral reef, area 66 square 

 miles. According to these measurements chemical precipitation 

 of calcium carbonate with subsequent transformation into oolite 

 has predominated over the constructive agency of corals in the 

 ratio of roughly 25: 1, but, as undoubtedly the figures for the 

 oolite are too small and those for the elevated coral reef are too 

 large, 100 : 1 'is probably nearer to the proper ratio. In this 

 connection it will be stated that on Andros Island, Bahamas, the 

 ratio of the constructive work of the present reef to that of the 

 agencies that previously resulted in the formation of the Pleisto- 



1 Published by permission of the Director of the U. S. Geological Survey and 

 of the President of the Carnegie Institution of Washington. 



