Ch. 2— Resource Assessments and Expectations • 55 



have been calculated for two of these areas, the 

 Cabo Rojo area off the west end of the south coast 

 of Puerto Rico and the Escollo de Arenas area north 

 of the west end of Vieques Island (near the east 

 coast of Puerto Rico). The total volume of sand in 

 these deposits is estimated at 220 million cubic 

 yards, which could supply Puerto Rico's construc- 

 tion needs for over 20 years. ^^ 



In the U.S. Virgin Islands, several sand bodies 

 contain an estimated total of 60 million cubic yards. 

 Some of the more promising are located off the 

 southwest coast of St. Thomas, near Buck Island, 

 and on the southern shelf of St. Croix. 



Placer Deposits 



Heavy mineral studies along the north coast of 

 Puerto Rico found a strong seaward sorting with 

 relatively heavy minerals such as monazite and 

 magnetite enriched on the inner shelf relative to 

 pyroxenes and amphiboles. The high degree of 

 nearshore sorting may indicate a likelihood of the 

 occurrence of placers, particularly in the inner shelf 

 zone.^^ Gold has been mined in the drainage ba- 

 sin of the Rio de La Plata which discharges to the 

 north coast of Puerto Rico, although no gold placers 

 as yet have been found on the coast. 



^'R.W. Rodriguez, "Submerged Sand Resources of Puerto Rico 

 in USGS Highlights in Marine Research," USGS Circular 938, 1984, 

 pp. 57-63. 



^'O.H. Pilkey and R. Lincoln, "Insular Shelf Heavy Mineral Par- 

 titioning Northern Puerto Rico," Marine Mining, vol. 4, No. 4, 1984, 

 pp. 403-414. 



GULF OF MEXICO REGION 



The Gulf of Mexico is a small ocean basin whose 

 continental margins are structurally complex and, 

 in some cases, rather unique. The major structural 

 feature of the U.S. EEZ in the northern Gulf of 

 Mexico is the vast amount of sediment that accu- 

 mulated while the region was subsiding. The struc- 

 tural complexity of the northern Gulf margin was 

 enhanced by the mobility of underlying salt beds 

 that were deposited when the region was a shallow 

 sea. In general, the sedimentary beds dip and 

 thicken southward and are greatly disrupted by di- 

 apiric structures and by flexures and faults of re- 

 gional extent. 



Sulfur and salt are both recovered from bedded 

 evaporite deposits and salt domes in the Gulf re- 

 gion. Sulfur is generally extracted by the Frasch 

 hot water process, which is easily adaptable to oper- 

 ation from an offshore platform. Sulfur has been 

 recovered from offshore Louisiana and could be 

 more widely recovered from offshore deposits if the 

 market were favorable. 



Sand and Gravel 



The sand and gravel resources of the Gulf of 

 Mexico are even more poorly characterized than 

 the Atlantic EEZ. Most of the shallow sedimentary 

 and geomorphological features of the Gulf were 

 simOarly developed as a result of the sea-level fluc- 



tuations during the Quaternary. The Mississippi 

 River dominates the sediment discharge into the 

 northern Gulf of Mexico. Over time, the Missis- 

 sippi River has shifted its discharge point, leaving 

 ancestral channels and a complex delta system. As 

 channels shift, abandoned deltas and associated bar- 

 rier islands are reworked and eroded, forming 

 blanket-type sand deposits and linear shoals.^"* A 

 number of these shoals having a relief of 15 to 30 

 feet are found off Louisiana. Relict channels and 

 beaches are also good prospects for sand deposits. 

 Relict channels and deltas have been identified off 

 Galveston, containing over 78 million cubic yards 

 of fine grained sand which may have uses for beach 

 replenishment or glass sand. Sand and gravel re- 

 source estimates for the U.S. EEZ are given in ta- 

 ble 2-4. Based on an average thickness of 16 feet, 

 these are projected to be around 350 billion cubic 

 yards of sand for the Gulf EEZ. No gravel resources 

 are identified on the Gulf shelf although offshore 

 shell deposits are common and have been mined 

 as a source of lime. Until more surveys aimed at 

 evaluating, specific sand and gravel deposits are con- 

 ducted, resource estimates are little more than an 

 educated guess. In any event, the resource base is 

 large, although meeting coarser size specifications 

 may be a limiting factor in some areas. 



"Williams, "Sand and Gravel Deposits Within the United States 

 Exclusive Economic Zone," p. 381. 



