GULF OF MEXICO 



81 



before it gets more than ten or fifteen miles from 

 the mouth of the river" (Riley 1937, p. 91). 



It is noted in figure 16 that the blue mud province 

 extends northward to near the mouths of the 

 Mississippi River. Since the front of the delta 

 overlaps the continental shelf nearly to its outer 

 edge, the sediments of the deeper Gulf approach 

 the tip of the delta. Likewise, the Globigerina 

 zone lies close to the land at the delta. 



LOUISIANA SHELF 



The numerous submerged hills rising above the 

 sea floor near the outer edge of the continental 

 shelf materially influence the local sediments. 

 Trask, PUeger, and Stetson (1947, p. 461) noted 

 that the slopes of these hills are covered with 

 "silty, calcareous sand, and the tops by round 

 Lithothamnium balls and little or no sandy material 

 . . . while the adjacent flat continental shelf is 

 underlain by sandy silt." The Lithothamnium 

 balls, diameters up to 10 cm., must have been 

 moved by the water since they seemed to be alive 

 on all sides. Corals, similar to those common in 

 the West Indies, were dredged with the Litho- 

 thamnium balls. These areas are included in 

 figure 16 in the patches of coral lying along 28° N. 

 lat. between 91° and 95° W. long. 



The dominant sediment on the continental 

 shelf along the Louisiana coast west of the Mis- 

 issippi Delta is mud and sand. Locally, near 

 shore, sand predominates to form a sand beach 

 and shore zone. The common, heavy minerals 

 of these sediments are amphiboles, epidote, 

 dolomite, pyroxene, ilmenite, and biotite. 



Near the outer edge of the shelf and particularly 

 on the continental slope there are many topo- 

 graphic features of considerable relief. Carsey 

 (1950, pp. 377-379) noted 164 such topographic 

 features along the Louisiana-Texas slope and 

 made a study of their density distribution accord- 

 ing to their degree of relief. This study showed 

 that two-thirds of these features have a relief of 

 less than 300 feet, while some rise 600 feet above 

 the floor of the Gulf. 



The sediments on the tops and flanks of topo- 

 graphic features, having a relief in hundreds of 

 feet, may be greatly different from those on the 

 ocean floor only a short horizontal distance away. 

 Corals have been dredged from the tops of a few 

 of these knobs or domes, but little is known con- 

 cerning the deposits on the flanks. The finer 



sediments may have been washed from the tops 

 of these knobs to settle on the Gulf floor around 

 the base. More detailed sounding and dredging 

 in this area are needed to adequately study the 

 sedimentology of the area. 



Over a 50-year period numerous "oil spots" or 

 "seeps" have been reported as having been ob- 

 served in the northwest Gulf of Mexico. The 

 locations of these seeps are noted on the map 

 (fig. 16), and it is seen that they are concentrated 

 between 91°-93° W. and 26°30'-27°30' N. Since 

 several of these "oil spots" were said to be several 

 scores of miles long, their origin, although un- 

 known, is of interest. 



WESTERN GULF 



The rivers of Texas are not heavily laden with 

 sediment, except during flood stages, and for this 

 reason it can be assumed that the Recent alluvial 

 deposits found on the continental shelf will not be 

 of great thickness. Also, these streams have 

 little velocity as they cross the wide coastal plain, 

 and only fine-grained mechanical sediments are 

 carried to the Gulf. This has been shown by 

 Storm (1945, p. 1313) in a series of samples col- 

 lected in the Gulf out from Corpus Christi, Texas. 

 Beyond the near-shore fine material sands with 

 0.21 millimeter average diameter occurred in a 

 narrow belt about 12 miles from shore. Twenty 

 miles from shore the grain size had decreased to 

 an average of 0.03 millimeter, while 30 miles from 

 shore it had increased to an average of 0.18 milli- 

 meter. From 30 to 40 miles off shore the grain 

 size remained about the same, but beyond 40 

 miles it decreased again. These variations seem 

 to be closely associated with the currents. 



In 1948 Mattison (p. 77-78) found a string of 

 coral heads off the Brazos River mouth about 8 

 miles off shore. They occur in 6 to 8 fathoms of 

 water and have a relief of 2 to 3 fathoms. They 

 have been seen by fishermen who describe them as 

 having the appearance of sunken icebergs but 

 having sea fans and other marine growth forming 

 solid coral or white limestone in an area of black 

 mud. Coral heads occur approximately along 

 the 40-fathom line in front of Corpus Christi, 

 Texas, and Smith (1948, p. 82) noted that six of 

 these heads were reached within a foot or two of 

 31 fathoms of water. 



Along most of the east coast of Mexico from 

 Texas to the Gulf of Campeche the charts show 



