74 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Upwellings of clay, locally known as mud- 

 lumps, occur near the mouths of the Mississippi 

 River passes and have never been reported from 

 any other delta. These mudlumps have been the 

 subject of written discussion for more than a 

 century, but only a few writers have attempted a 

 scientific explanation of them. The most recent 

 study has been made by Morgan (1951) in 

 conjunction with the Corps of Engineers at New 

 Orleans. 



Mudlumps and mudlump islands have at- 

 tracted much attention since they may have mud 

 cliiFs with a relief of up to 10 feet in an area where 

 the average relief is usually 2 feet or less. 



Most mudlumps have central cores of fine- 

 grained plastic clay surrounded and sometimes 

 capped by irregularly stratified layers of clay 

 and silt. The upwelling of the clay core usually 

 produces fissures and faults with vertical dis- 

 placements resulting in central grabens. The 

 stratified layers dip away from the islands, often 

 forming doubly plunging anticlinal structures. 

 Local cones along the faults and fissures are 

 formed by the discharge of mud, gas, and salt 

 water. 



Morgan (ibid.) believed that the "formation of 

 new lumps and rejuvenation of old lumps occurs 

 as a direct result of excessive sedimentation at the 

 river mouths" and "the deforming force which 

 caused mudlump uplift is the static pressure of the 

 sedimentary mass continually being dumped 

 beyond the mouths of the passes." 



NORTHERN GULF OF MEXICO 



The continental shelf off Louisiana and Texas 

 is somewhat uniform and has a gentle slope to 

 about the 50-fathom contour. From this point 

 the slope increases to the 70-fathom line where it 

 has an increase in gradient to the 100-fathom 

 depth. Some increase in slope is noted beyond the 

 100-fathom line, but the bottom becomes so 

 irregular that the true slope becomes obscure. 



Probably the chief characteristic of the con- 

 tinental slope of the northern Gulf is the hum- 

 mocky topography. Shepard (1937, p. 1350) 

 found 26 topographic features off the coast of 

 Louisiana some of which had a relief of several 

 hundred feet. Charts revealed that the belt 

 of domes can be traced definitely for 180 miles 

 west and southwest of the Mississippi submarine 

 trough. More recent data show that some of the 



depressions are 2,000 feet deep, and some of the 

 hills have a relief of at least 2,500 feet. 



Carsey (1950, p. 376) found 164 topographic 

 features along the shelf off the coast of Louisiana 

 and Texas. An area of apparent concentration 

 of these features is shown in figure 16. However, 

 it is probable that there are many somewhat 

 similar features elsewhere on the continental shelf 

 and slope. They seem to be most prevalent in 

 the area between the 100- and 750-fathom con- 

 tours. 



It is particularly interesting to note that no 

 stream patterns have been found other than the 

 troughs on the margins of the slope off the Mis- 

 sissippi Delta and the Apalachicola River (Shepard 

 1948, p. 178). 



Price (1951, p. 32) observed that the "rugged 

 topography of the northwestern shelf-margia or 

 slope seems to contain dislocated segmeats of 

 submarine canyons" which differ in late history 

 from the canyons along the less rugged slope to 

 the east. This suggests that the front edge of 

 the shelf was faulted down in slices as it was 

 built out into the Gulf. 



Available maps of the topography of the Gulf 

 bottom vary widely in their representation of the 

 physiographic features. The amount of time as 

 well as the number of soundings available influence 

 the choice of the contour interval. Thus, the 

 Treadwell (1949) map of the continental slope of 

 the northwestern Gulf of Mexico, contour interval 

 of 50 fathoms, shows a great number of closed 

 basins and knobs between 91° and 95° W. Long, 

 and 27° to 28° N. Lat. Also, there are suggestions 

 of drainage patterns that are not evident in the 

 map by Shepard (1948, p. 178, fig. 64) with a con- 

 tour interval of 100 fathoms. Some of these 

 differences may be due to the contour interval, 

 but some may also be the result of additional 

 data and the choice of the cartographer when 

 more than one interpretation of the data exists. 



All available maps of the continental slope of 

 this region show the same general characteristics 

 of the Gulf bottom: a very irregular, hummocky, 

 knob and basin topography. 



Minor near-shore features of ridge and trough 

 were noted by Kindle (1936, pp. 866-867) along 

 the Louisiana coast. He waded across a 1,500- 

 foot traverse and found ridges whose crests were 

 10 feet wide and separated by troughs from 60 to 

 90 feet wide. The same traverse was repeated 



