of Grootpan Bay and Europa Bay. The ages, 1,800 + 200 years BP for 

 Grootpan Bay and 1,050 + 200 years BP for Europa Bay, indicate an 

 average submergence rate greater than a foot per hundred years. This 

 rate far exceeds the rate of eustatic sea rise as reported by Scholl 

 and others (1968) for Florida, by Emery (1967) and by Wigley and 

 others (1968) for the Atlantic continental margin. These dates, 

 however, must be considered tentative owing to possible introduction 

 of younger carbonate material by marine organisms . Further radiometric 

 age dating will be required to establish approximate ages of beachrock 

 cementation and corresponding rates of sea level change. 



If the dates are accurate, they indicate that local structural move- 

 ment has affected the rate of sea level advancement in the St, John 

 area. Post-Pleistocene faulting on the shelf southwest of St. Thomas 

 has produced prominent scarps 5-10 meters in height (Donnelly, 1965) ; 

 this suggests that recent structural movement resulted in the partial 

 submergence of St. John. 



CONCLUSIONS 



Changes in sea level caused by a combination of tectonic movement and 

 eustatic sea level rise have formed a complex record of sea level 

 history on the St. John shelf. Near the site of the Tektite experiment, 

 a stillstand of the sea seems to have occurred approximately 43 feet 

 below the present sea surface. Beachrock formed along the shoreline, 

 an erosional platform was cut into the bedrock slope, and constructional 

 platforms formed on the east side of St. John. 



The sea advanced to approximately 24 feet below the present sea level. 

 Beachrock formed in niimerous locations and erosional platforms were cut 

 into the bedrock along the south coast of St. John. After formation of 

 an erosional platform, at 18-30 feet, the sea level rose, accompanied 

 by continual formation of beachrock, to depths of at least 6 feet belov7 

 the present mean sea level. 



Tentative radio-carbon age dates suggest that the sea level rise is 

 controlled by both eustatic and tectonic factors. Further age dating 

 may establish the rate of tectonic subsidence of St. John in the 

 recent past. 



VI-79 



