Continuation of Underwater Geologic Studies in Lameshur 



/ 



Bay Area, St. John U. S. Virgin Islands—'' 

 Part 2. Sea Level History 



By R. Lawrence Phillips 

 . S. Geological Survey, Menlo Park, Calif, 



and Denny W. Bowman 

 University of Texas, Galveston, Texas 



ABSTRACT 



Relict shoreline features which represent former 

 stillstands of the sea occur at approximate depths 

 of 24 feet and 43 feet below mean sea level on the 

 south coast of St. John near the Tektite experiment. 

 These features include submerged beachrock conglom- 

 erate that parallels the present shoreline, unce- 

 mented gravel composed of rounded bedrock cobbles 

 and boulders on the carbonate sand flats, and eroded 

 wave-cut platforms at depths of 18 to 30 feet and 45 

 to 55 feet. The changes in sea level are believed 

 to be caused by a combination of tectonic movement 

 and eustatic sea level rise. 



INTRODUCTION 



Part of the U. S. Geological Survey research program during 

 Tektite I and II was directed toward determining the submerged 

 record of the sea level history of St. John. This report 

 describes two submerged beachrock conglomerates and other 

 evidence indicative of two stillstands of the sea late in the 

 geologic history of the island. 



The southern coast of St. John is composed of water-laid 

 volcanic rocks consisting of keratophyre flows, flow breccias, 

 tuffs, spilite flows, and radiolarites of the Water Island 

 Formation of Donnelly (1966) . The general structure of the 

 island is a northward dipping homocline. 



The rugged shoreline of St. John suggests a submergent coast. 

 High cliffs, many of which are not notched at sea level, drop 

 off to depths of 20 to 35 feet. Beaches are absent or steep and 

 narrow. All the beaches but two are composed of cobbles and 

 boulders; these two are sand beaches. Meyerhoff (1926) defined 

 the St. John coast as an embayed mountain shoreline. 



Fluctuations of sea level have formed a complex record of sea 

 level history on St. John and the adjoining shelf. Vaughn (1916) 

 postulated that three stages of recent submergence of the 

 Virgin Islands produced marine terraces at depths of 156-180 feet, 

 84-120 feet, and 36-60 feet below the present sea level. A 

 depth-restricted coral reef is associated with each of the three 



Publication authorized by the Director, U.S. Geological Survey 



VI-70 



