Continuation of Underwater Geologic Studies in Lameshur 

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

 Part I. Bioturbation 



H. Edward Clifton and Ralph E. Hunter 

 U. S. Geological Survey 

 Menlo Park, California and Corpus Christi, Texas 



ABSTRACT 



Bioturbation, the mixing of sediment by organisms, is 

 one of the most important continuous sedimentary pro- 

 cesses in the semiprotected coral reef environment. 

 Experiments conducted during Tektite II documented 

 specific effects of the organisms on the sediment and 

 established rates and styles of bioturbation in dif- 

 ferent environmental settings near a coral reef. 

 Organisms affect the sediment in various ways: they 

 rotate empty pelecypod valves to a concave-up position, 

 control the distribution of coarse clasts along the 

 reef front, bury pebbles and cobbles on the sea floor, 

 destroy sedimentary structures on and beneath the sea 

 floor, and create new structures. 



INTRODUCTION 



Nearly all sedimentary rocks bear the imprint of processes that 

 were active at the time the sediment was deposited. Such imprints 

 are manifested by primary structures in particular — features formed 

 in the sediment in response to different processes within the de- 

 positional environment. Among such processes is bioturbation, the 

 mixing of the sediment by organisms living on or within it. Bio- 

 turbation may produce distinctive structures, such as specific 

 burrow forms or it may obliterate all internal structures. Little 

 is known about the rate at which bioturbation reorganizes the 

 sediment, either internally or at the surface and few data exist 

 regarding the relation between the style of bioturbation and the 

 environmental setting. 



The Tektite experiments provided an excellent opportunity to study 

 bioturbation the dominant everyday geological process involving 

 sediments in the area of the habitat. At no time during the 80 days' 

 undersea habitation of Tektite I and II were waves or currents ob- 

 served to disturb the carbonate sand in water deeper than 6 m in 

 eastern Lameshur Bay. Preliminary studies conducted by Clifton and 

 others (1970) during Tektite I indicated that bioturbation rates in 

 this environment were remarkably high and that certain previously 

 undociomented effects, such as bioturbational burial of pebbles and 

 cobbles and the effects of organisms on pelecypod shell orientation, 

 are of considerable geologic importance in the reef environment of 

 the West Indies. During Tektite II, specific aspects of bioturba- 

 tion were examined in detail before, during, and after the period 



VI-22 



