A seismic study of the shallow sub-bottom materials of the eastern Caribbean
Lawhorn, Thomas Warren
Officer, Charles B.
Master of Arts
Investigations were undertaken in the Caribbean-Antilles region in an attempt to describe the broad structural and compositional attributes associated with this striking geographic feature, The topographic feature of the Puerto Rico trench suggests an accumulation site for large thicknesses of sedimentary material, Hie terrestrial association between mountain ranges and thick stratigraphic sequences suggests at once that such a trench might at some future time become a mountain range. This and many other lines of evidence has led some geologists and geophysicists to propose a theory of geosynclinal origin for mountain ranges. If the existence of a broad crustal downwarp could be shown below the Antilles island arc - Puerto Rico trough then overwhelming evidence would be given that the association was a geosyncline and a zone of crustal weakness which, under stress, might be sheared and thrusted to form the great thrust sheet-fold Alpine type mountains. The seismic refraction technique was used since it measures velocity discontinuities and allows the calculation of the depth to the discontinuity (eg,, the Mohorovicic discontinuity) and any slope it might have as well as the seismic velocity in the various materials. This method also extends these determinations to large depths. It was also desired to obtain data on the sedimentary material using the reflection technique. Work was begun by H. Ewing, C. B. Officer et al in this area in 1951 when the ATLANTIS - 172 and the CARYN - 22 passed through the Western Venezuelan Basin. Some sound transmission studies had been carried out in the islands previously. Several cruises had been made before to study the structure of the Atlantic basin and Nares basin. Most of this work was done with the Lamont Geological Observatory and the Woods Hole Oceanographic Institution by C. B. Officer, J. B. Hersey, H. R, Johnson, M. Ewing, P. C. Wuenschel, G. H. Sutton, and others. T. F. Gaskell and J. C. Swallow, and I, Tolstoy have also done some work in this area. The chief intention of the 1951 cruise was to gather refraction and sound transmission data although tape recordings were made. These later proved to be of poor quality and no study was made. Research was begun in earnest in the Caribbean-Antilles area by C. B. Officer, J. Ewing, H. R. Johnson, and R. S. Edwards^ in the spring of 1955 aboard the ATLANTIS and CARYN of the Woods Hole Oceanographic Institution. This cruise covered the entire eastern Caribbean area in both deep and shallow water. The work was continued in the summer of 1956 to obtain better control and verification of the results obtained in 1955. This work was done on board the research vessels BEAR and ATLANTIS of the Woods Hole Oceanographic Institution with the joint cooperation of Lamont Geological Observatory and The Rice Institute. It was directed by C. B. Officer with the assistance of senior scientists J. Ewing, J. Hennion, and R. S. Edwards. Hie remainder of the scientific party was composed of professors, graduate, and undergraduate students from Rice, H.I.T., Harvard, Princeton, Utah, Wisconsin, and Cornell. Data taken on tiie cruises of 1955 and 1956 was extensive, especially that of the recent cruise. The completeness was greatly aided by improved and more versatile instruments giving the most complete data per shot obtained to the present. The data obtained is useable for refraction-crustal study, reflection-sediment study, and sound transmission study. Some modestly extensive gravity work has been done in this area. Vening-Meinesz (1926, 1937), Hess arid Ewing (1937), and Shurbet and Worzel (1953-6) have been major contributors to this research. This brief summary of previous work is given because the purpose of the two refraction cruises was to gather more data and correlate all the known phenomena observed from different approaches if possible into a theory for the history and future of this area.