80 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



amphiboles, pyroxenes, leucoxene, and hematite 

 (Goldstein 1942, p. 81). 



Most of the continental shelf west of the penin- 

 sula of Florida is hard rock, chiefly limestone, but 

 a thin veneer of detrital sediment is present in 

 local areas and fills some of the shelf depressions. 

 Stetson (1951, p. 1993) obtained two specimens of 

 hard limestone and a specimen of soft, chalky 

 limestone from this shelf by using a steel rock 

 dredge after core tubes were damaged by the 

 hard rock. 



The Florida Keys include a 200-mile chain of 

 islands curving southwestward along the edge of 

 the Florida Straits from Biscajme Key to Key 

 West and the Dry Tortugas. The northeastern 

 keys are old coral reefs, but the ones to the south- 

 west are remnants of a former island. Vaughan 

 (1910, p. 119) stated that silica, as sand, is abun- 

 dant in Biscayne Bay but decreases to the south- 

 west as calcium carbonate becomes more abundant 

 near the living coral reefs. The calcium carbonate 

 occurs "as a flocculent sediment or ooze over 

 practically the entire region from the lower portion 

 of Biscayne Bay to the gulf end of Florida Bay." 

 However, Trask (1932, pp. 166-172) found that 

 the basins in Florida Bay have coarser sediments 

 than the compact marl rims. The basin sediments 

 are "shell breccia embedded in a matrix of marl." 



The recent work of Lowman (1951, pp. 234-235) 

 provided the basis of division of the limestone 

 banks west of Florida. He found that the white 

 sands of the Pensacola beaches extended seaward 

 to the depth of 20 fathoms and that the sands were 

 free of mud and were highly fossiliferous, with 

 Mollusca and Foraminifera being the most 

 common forms. 



A second zone, extending out to 40 fathoms, was 

 found to contain many algae, forams, pelecypods, 

 brachiopods, bryozoans, and cup corals. The 

 Foraminifera showed a definite faunal break at 

 about 75 fathoms which Lowman (idem., p. 235) 

 suggested may be the result of changes in turbidity 

 and light penetration in the clear water. In the 

 more turbid waters west of the Mississippi Delta 

 a faunal break was noted at 45 to 50 fathoms. 



Bush (1951, pp. 102, 106) reported on a rock 

 specimen obtained by dredging in the Straits of 

 Florida, south of the American Shoals, at a depth 

 of 375 fathoms. This rock, apparently broken 

 from the ocean floor, was very fossiliferous and 

 was correlated with the Chipola formation (lower 



Miocene) of northern Florida. This suggests "the 

 dip and continuance of the lower Miocene strata 

 from the Florida Peninsula under the Straits of 

 Florida toward Cuba" (idem., p. 106). 



Between the Florida Straits and Cuba and also 

 west of the continental shelf the bottom sediments 

 are calcareous muds, and westward they grade 

 into blue mud and Globigerina ooze. 



MISSISSIPPI DELTA 



Most of the coarse sediment of the Mississippi 

 River is deposited near its mouth, but Trowbridge 

 (1930, p. 892) noted that outside the Southwest 

 Pass of the river, coarser sediment occurred oa 

 knolls in 30 fathoms of water. This coarser sedi- 

 ment apparently was not derived from the present 

 Mississippi River under present conditions. The 

 concentration of coarse sediments may have 

 resulted from the removal of the finer sediments by 

 winnowing due to stronger currents over the knolls. 



Shaw (1916, p. 107) stated that fine sand, sUt, 

 and clay were accumulating on the Gulf of Mexico 

 floor immediately beyond the mouth of the Mis- 

 sissippi River very near where they were dropped 

 by the river. He contrasted this with conditions 

 on the west Gulf coast where the sediments 

 brought to the Gulf by streams were being re- 

 worked by waves and currents yet not carried far 

 from the mouths of the streams. 



Mud and sand are recorded on many maps on 

 either side and adjacent to the Mississippi River, 

 but sampling by the writer shows silt and "mud" 

 to be greatly in excess of sand. Westward from 

 the delta there is a clay-silt zone with some sand 

 and shells. Dark gray to black "mud" is present 

 in most of the lagoons. 



Kellogg (1905, p. 34) and many others, including 

 the writer, have observed the hard crust that 

 develops during the winter. This crust is only 

 an inch or two thick and is underlain by soft silt 

 and "mud." The clay and finest particles have 

 probably been removed by winnowing during the 

 winter when the Mississippi River is in a low stage 

 and therefore carrying a minimum sediment load. 



The very high ratios of organic matter to 

 chlorophyll which occur near the mouth of the 

 Mississippi River "indicate large quantities of 

 organic detritus. The ratios fall so rapidly as 

 one proceeds out in the Gulf that it seems likely 

 that practically all the organic detritus of fresh 

 water origin is removed from the surface water 



