GULF OF MEXICO 



45 



Young orogenic coasts have their shorelines 

 dominantly parallel (concordant, Suess 1888) 

 with the structural trends (folds and faults) 

 of the mountains. The Gulf provides no ex- 

 amples of coasts where the shoreline is more 

 than very locally discordant with the structural 

 trends on land.' This accounts to a large extent 

 for the almost complete lack of islands in the 

 Gulf other than sandy barriers close to shore, 

 karst islets of Florida, some lava-rock islets in 

 Sector 3 in Mexico, and coral and detrital reefs 

 on shoals. From the meager data of the charts 

 we conclude that, because the Mexican mountains 

 are mostly not younger than Miocene, coastal 

 sediments have built out around or otherwise 

 protected most of their outpost hard rock folds 

 from the Gulf. However, a large mountain range 

 projects eastward under water some 50 miles off 

 Tampico and two parallel mountain ridges trend 

 northwestwardly from the edge of the continental 

 shelf off the Rio Grande delta. 



The Tertiary mountains of Cuba (Palmer 

 1945) rise from a short distance back of the coast. 

 The folded rocks come down to the coast or are 

 overlain there by a thin cover of younger deposits. 

 The island may be divided into several areas of 

 different tectonic structure, but overthrust folds 

 rising up to the south dominate some sectors, as 

 in the extreme west. The Gulf bottom off the 

 north coast descends at angles of 4° to 6° or more, 

 a slope which conforms fairly well to some of the 

 folds. A narrow shelf occurs only where fringing 

 reefs have grown up with a rising sea level to form 

 barrier reefs, so that the lagoon has been filled to 

 a shallow depth with sediment and organic growths 

 (3.2 Sectors, fig. 14). 



The drainage of northwestern Cuba is largely 

 southward, so that only small streams enter the 

 Gulf and the coralline lagoons. The sedimenta- 

 tion along the northwest shore has, therefore, 

 been negligible except where coral reefs and 

 mangrove growth have trapped marine and land- 

 derived materials. An erosional sector occurs 

 between the barrier reefs east and west of Havana. 



The Sierra Madre Oriental, the eastern Cordil- 

 lera of Mexico, slants southeastward toward the 

 coast, one of the outpost folds in limestone rock 

 making a minor protuberance at Punta Jerez 



(fig 13).'° The coastal plain becomes gradually 

 narrower southward from the delta of Rio Grande. 

 It is, however, as much as 60 or more miles wide 

 in places. 



The Southern Volcanic Range of Mexico 

 (Sierra Neo-Volcanica, Tamayo 1949), a zone 

 of Tertiary-to-Recent volcanic peaks, runs from 

 the Pacific coast due east through Mexico City 

 to form the broadly protuberant Jalapa Salient 

 north of Veracruz at 20° N. Lat. A similar 

 salient south of the city, that of San Martin 

 Tuxtla, separated from the range, may be geo- 

 logically associated with it. The range includes 

 some of the greatest peaks of Mexico, including at 

 the east, in sight of the Gulf, Orizaba and Cofre 

 de Perote, reaching elevations of 18,696 and 14,048 

 feet, respectively, above sea. Between and on 

 each side of these salients are sedimentary embay- 

 ments (fig. 14, 3.2) with fairly broad coastal plains. 

 Only a narrow belt of low shoreline deposits 

 seems to be present along the fronts of the vol- 

 canic salients. These salients are composed of 

 confluent and overlapping flows of volcanic rocks, 

 some of which make small jutting points at the 

 shoreline. Of these, Roca Partida and Punta 

 Delgada have cliff ed faces reported to be 1,000 

 feet high, with islets of lava rock. 



There are several volcanic peaks in the San 

 Martin salient, including San Martin Tuxtla, 

 which has been active in historic time. On air 

 photographs of this sector, the writer counted 

 some 20 small cinder cones aligned in a zone about 

 10 miles wide and 40 miles long parallel with the 

 coast. One of the cones stands in the 

 intermountain Lake Catemaco with its crater 

 invaded by the water. 



The continental shelf off the orogenic coast of 

 Mexico is poorly mapped. It is narrow and, 

 where mapped, the gradient is convex, becoming 

 steep, like that near the outer edge of the shelf 

 of Texas and Louisiana. The grain sizes of the 

 sediments, so far as is revealed by the data on 

 the charts, decrease more regularly outward than 

 on some better-known orogenic coasts, as that 

 of California where there are separate offshore 

 sedimentary basins both on and off the shelf, 

 each with its own sedimentary distributional 

 pattern. The small size of the sub-aerial drainage 

 basins where mountains stand near the coast has 



' Discordant coasts are found today chiefly where old mountain areas, as 

 from New England to Newfoundland, have been drowned by sinking of 

 coasts under load of Pleistocene ice sheets. 



'0 The convexity here is exaggerated on O. O. Chart 2056 as compared 

 with the later W. A. C. 689, made from a photographic base. 



