GULF OF MEXICO 



75 



2 days later, and while the ridges were free of mud, 

 the depressions were filled with several inches of 

 mud. Therefore, the whole character of the 

 local bottom was changed in 48 hours. This 

 shows the futility of making sweeping conclusions 

 from only a few data, especially in the shore zone. 



MEXICO 



Too few data are available on the topography 

 adjacent to Mexico to make a detailed study of 

 either the continental shelf or slope of this region. 

 However, some generalizations may be made 

 from the scanty sounding data and geological 

 maps of the adjacent land. 



Mountain ranges, trending northeast-southwest, 

 have been mapped 90 and 110 miles east of the 

 mouth of the Rio Grande. The range nearer 

 the coast has a known relief of 2,750 feet with a 

 summit reached at a depth of 540 fathoms and 

 the other range has a known relief of 3,810 feet 

 with a summit at a depth of 839 fathoms. 



Due east of Tampico a mountain i-ange, with a 

 bearing of N. 65°-70° E., extends some 40 miles 

 and has a relief of 5,800 feet with a summit rising 

 to within 33 feet of the surface. 



Along the extreme western edge of the Gulf of 

 Mexico, south of Tampico, the continental shelf 

 is narrow, and the adjacent coastal plain is also 

 narrow, being locally practically absent. Tertiary 

 and later igneous rocks occur in the Misantla- 

 Japala area, northwest of Veracruz, and in the 

 Alvarado-El Paso area, south of Veracruz. Some 

 of the highest peaks of Mexico occur just northwest 

 of Veracruz. Lava flows cover much of the 

 near-shore land area and locally form 1,000-foot 

 cliffs at or very near the shore. South of Vera- 

 cruz other smaller cones are very near the coast. 

 While local narrow beaches are formed and break 

 the surface continuity of igneous rocks, undoubt- 

 edly the offshore irregular topography is due to 

 underwater outcropping of these igneous rocks. 



Practically all of the Yucatan Peninsula forms 

 a broad coastal plain. This peninsula tilts north- 

 westward and passes under the Gulf to form a 

 continental shelf averaging over 125 miles in 

 width. The shelf terminates abruptly to the 

 west and north, and the topographic contours 

 along its edge are undoubtedly also structural 

 contours and represent faulting. 



MEXICAN BASIN 



There is within the Gulf of Mexico, but not 

 centrally situated, a large triangular area with 

 deeps exceeding 2,000 fathoms. It lies north- 

 west of the Campeche Banks approximately 

 between 22° and 25° N. Lat. and 89° and 95° W. 

 Long. Regarding this area, Hilgard is quoted by 

 Agassiz (1888, p. 101) as follows: "The large sub- 

 marine plateau below the depth of 12,000 feet has 

 received the name of the 'Sigsbee Deep', in 

 honour of its discoverer." Since the "depth of 

 the basin does not attain 3,000 fathoms, it is not 

 a 'deep' in the Murray sense, but it is an enclosed, 

 distinctive basin, for which Sigsbee's name may 

 appropriately be retained" (Vaughan 1940, p. 66). 



More recently, however, the name "Sigsbee 

 Deep" has been restricted to the deepest measure- 

 ment in the basin, and the name "Mexican 

 Basin" is used here for the broad, enclosed basin. 



The bottom of the Mexican Basin is very flat, 

 especially when contrasted with the continental 

 slope of the Gulf. The depths range from 2,000 

 to 2,070 fathoms over the deepest part of the 

 basin. The bottom rises rather uniformly to the 

 shore in the west in a distance of 180 miles, but 

 the northern slope is more gentle and apparently 

 more irregular in its distance of 300 miles. The 

 slopes toward Florida and the Yucatdn Peninsula 

 are broken by abrupt changes which undoubtedly 

 represent faults in the bottom. 



One of the most prominent mounds in the Gulf 

 is found in the northeast portion of the Mexican 

 Basin. It has a relief exceeding 890 fathoms, a 

 possible width of 60 miles, and its top is encoun- 

 tered at a depth of 916 fathoms. 



SEDIMENTS OF GULF OF MEXICO 

 SOURCE OF SEDIMENTS 



The near-shore sediments, at least, should be 

 expected to be closely related to the sediments of 

 the adjacent coastal plain except near the mouths 

 of major rivers. Much study has been given 

 samples obtained from wells and outcrops in the 

 area surrounding the Gulf of Mexico. Such 

 studies have shown that each formation varies 

 widely in its composition as it curves around the 

 Gulf from Florida to Mexico. 



