46 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Figure 14. — Regional geo-oceanographic classification, shorelines and coasts, Gulf of Mexico: 1, alluvial coasts; 2, drowned 

 limestone plateaus; 3, young orogenic coasts; 4, biogenous (organic) development on various coasts. Sub-sectors: 

 1.1, deltaic coasts, with 1.11, unentrenched simple deltaic plain, and 1.12, entrenched and embayed compound deltaic 

 plain. 1.2, terraced deltaic coastal plain; 2.1, unsimplified to little simplified drowned karst ; 2.2, limestone karst with 

 beaches; 3.1, erosional, and 3.2, depositional, orogenic coasts; 4.1, broad shelf; 4.2 shelf absent to narrow; 4.3 lesser 

 biogenous development (more extensive than shown). The two southerly Mexican 3.1 Sectors are volcanic salients. 



been shown to restrict coastal sedimentation. 

 This is true here, in that the shelf is wide off the 

 several sedimentary salients, but narrow in front 

 of the coastal mountain salients. 



ALLUVIAL COASTS 



Where the closest mountains, usually old 

 mountains, are located far or moderately far in- 

 land (Umbgrove 1947, pi. 5), the runoff and sedi- 

 ment load from the lands has been large and long 

 continued, interior plains are succeeded by broad 

 coastal plains and continental shelves, and the 



coast is of the deltaic (Fleming and Elliott 1950) 

 or alluvial coastal plain type. On such a coast, 

 after sufficiently long stillstand, shelf bottoms 

 are smooth except toward their outer margins, 

 organic reefs are inconspicuous, few or absent, 

 and shorelines are smooth or irregularly deltaic 

 (fig. 13, and No. 1 Sectors, fig. 14). .Sediments here 

 ai-e generally of even distribution to somewhat 

 spotty (Lynch, fig. 16). Sands extend from shore 

 out to about 5 or 10 fathoms, followed by silt or 

 sand and mud (charts), with mud further out to 

 the edge of the continental shelf. Mud or silt 



