EASTERN GULF AREA

The Florida Plateau includes not only the State of Florida but an equally great or greater area that lies submerged beneath water less than 50 fathoms deep and forms the Florida shelf (H. Gunter, 1929, p. 41). This plateau has been in existence since ancient time and is a part of the Gulf of Mexico "plate" of Suess and Schuchert. Its history includes submergence during Upper Cretaceous, part of Oligocene, and Upper Miocene. Since Miocene time uplift has continued, and erosion has removed much of the once continuous cover of Miocene sandy limestone. The Florida Peninsula now has very little relief. It has a wide continental shelf off its west coast, thus demonstrating the physiographic similarity between the coastal plain and the adjacent continental shelf.

The 1947 expedition of the United States Coast and Geodetic Survey ship Hydrographer in the waters on the continental slope southwest of the Apalachicola River, Florida, has been reported, in part, by Jordan (1951, pp. 1978-1993). Many new and interesting data have been secured in the 25,000-square-mile area of this report.

The greater part of the continental shelf west of the peninsula of Florida is covered by about 40 fathoms of water, and the slope out to the 100-fathom contour is for the most part gradual. The westward slope varies from 1° at the north to 5° at the south end of the shelf.

In the 25- to 80-fathom depths, domes, ridges, and troughs were discovered; escarpments and knobs with a relief of more than 300 feet were found in the 70- to 90-fathom depths. Most of these features occur along the shelf margin.

Within the 400- to 1,760-fathom zone the continental slope contains a deep escarpment, faults, and the terminus of the De Soto Canyon, as well as domes and depressed areas.

The continental slope escarpment is of special interest since it may materially aid in the ultimate solution of the origin of the Gulf of Mexico. Jordan (op. cit., p. 1991) noted a 35° gradient on a 4,000-foot drop, contrasting with 1° gradients or less above and below the escarpment. A ridge 30 miles long parallels the escarpment at 700 to 800 fathoms, and ridges and troughs with relief up to 600 feet occur along the bottom of the escarpment. The main escarpment undoubtedly

represents faulting, and some of the minor troughs and ridges may have a like origin.

There can be little doubt that the Florida. Plateau has been faulted along its western edge, but the faulting is difficult to date. Schuchert believed this faulting was due to the inbreaking of the Gulf of Mexico "plate" and that it probably began in the Upper Cretaceous. However, Weaver (1950, p. 359) believed "that the Gulf of Mexico as a deep sea is young" and therefore the faulting must have occurred at a much more recent date.

MISSISSIPPI DELTA AREA

The Mississippi River brings to its mouth a daily load of sediment in the order of 2 million tons. This material has permitted the Mississippi to build its delta out on the continental shelf with the overlapping delta reaching within some 10 miles of the landward edge of the continental slope. It might be expected that a deep trough would exist in the outer edge of the continental shelf in front of the Mississippi River, but such is not the case.

An ancient, deeply buried channel is found about 30 miles southwest of the passes of the Mississippi River. Shepard (1948, p. 213) stated that this trough, which has a depth of 1,800 feet, is the only major indentation in the shelf margin in the Gulf of Mexico and that the trough-head penetrates the shelf for nearly 30 miles. The sides are steep, and the flat floor is filled with loosely consolidated sediments. The canyon has been traced out on the continental slope to a depth of 900 fathoms before it becomes merged in the irregularities of the slope.

A second trough, called De Soto Canyon, has been discovered off the Apalachicola River of southwestern Florida. Shepard (1948, p. 179, fig. 65) reproduced a map of this trough or canyon as contoured by H. W. Murray of the United States Coast and Geodetic Survey. This map shows a series of depressions, some with relief exceeding 20 fathoms, along the bottom of the trough and a few depressions along the sides of the trough. This canyon is shown in Jordan's map (1951, p. 1982, fig. 2) of the continental slope. The canyon has a relief of about 600 feet, heads near the 240-fathom contour, and terminates near the 500-fathom contour. Stetson (1951, p. 1993) stated that cores of the steepest walls of the canyon showed sediment and no bed rock.

Upwellings of clay, locally known as mudlumps, 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 attracted much attention since they may have mud cliffs 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 finegrained 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 displacements 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 continental slope of the northern Gulf is the hummocky 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 contours.

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 Mississippi Delta and the Apalachicola River (Shepard 1948, p. 178).

Price (1951, p. 32) observed that the "rugged topography of the northwestern shelf-margin or slope seems to contain dislocated segments 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 contour 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 1,500foot 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

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 range, 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 MisantlaJapala rea, 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 Veracruz other smaller cones are very near the coast. While local narrow beaches are formed and break the surface continuity of igneous rocks, undoubtedly the offshore irregular topography is due to underwater outcropping of these igneous rocks.

Practically all of the Yucatán Peninsula forms a broad coastal plain. This peninsula tilts northwestward 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 northwest 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 submarine 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 measurement 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 Yucatán 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 encountered 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.

The Tertiary outcrops in the Gulf Coastal Plain include thick continental sandy and lignitic deposits and thinner marine sands and clays. Down-dip from the outcrops, drilling has shown that the Tertiary continental deposits pass into brackish water and near-shore marine deposits. According to Lowman (1949, p. 1941), rapid transgressions and slow regressions produced cyclical effects in the sediments with most of the sediments deposited during the regressive phases of the cycles. Farther down-dip or seaward the sediments change to a succession of offshore marine clays.

In general, the Gulf coastal area may be divided into intergrading depositional areas as follows: Rio Grande Embayment, East Texas Basin, Mississippi Embayment, the Gulf coastal region of Alabama, Georgia, and North Florida, and South Florida. The amount of rainfall on the land area surrounding the ancient Gulf may have been the chief factor in determining the contemporaneous deposition of many sedimentary deposits ranging from anhydrite and salt to shales and limestones. Rolshausen (1947, p. 5) suggested that during pre-Eagle Ford Cretaceous time, west of the Appalachian Mountains, rivers entering the Gulf from the north and northeast supplied the major load of sediments. East of the mountains the rivers entered the Gulf from the northwest and west. After Eagle Ford time, rivers entering the Gulf from the west, and probably draining the western part of the present Mississippi basin, were the chief source of sediments. The Rio Grande may have been the major source of sediments from the late Cretaceous through early Miocene time with the Mississippi River contributing little sediment during that time.

PLACE OF DEPOSITION

The sediments brought to the Gulf of Mexico are probably not carried far from shore. Parr (1935, p. 62) showed that at a point only 70 miles out in front of the mouth of the Mississippi River the water has "transparency practically equal to the clearest ocean water known." It is a generally accepted fact that water discharged from the Mississippi River is carried almost entirely to the west and that it stays relatively close to the shore. Clarke (1938, p. 91) found that measurements of transparency supported this conclusion. Geyer (1950b, p. 100) noted that the salinity of

the offshore coastal waters of Louisiana west of the delta was largely controlled by the discharge of fresh water from the Mississippi River and the westward moving littoral current. The observations of the writer between 1948 and 1951 confirm the westward movement of the fresh water entering the Gulf from the Mississippi River.

Cogen (1940, p. 2101) examined samples of sediments taken from the bottom of the Gulf near the mouth of the Rio Grande and concluded that the present bottom sediments of this region were carried into the Gulf by the Rio Grande.

Bullard (1942, pp. 1021-1043) showed that each of the principal rivers carries a distinct suite of heavy minerals. The Rio Grande sand shows its primary source by the predominance of basaltic hornblende and pyroxene and only 30 percent of the stable minerals such as garnet, rutile, zircon, tourmaline, and staurolite in the heavy mineral residue. The Nueces, San Antonio, Brazos, Trinity, and Sabine Rivers, draining areas of sedimentary rocks, have little hornblende and pyroxene and a high content of stable minerals. Since the Colorado River derives its load from both primary and secondary rocks, its suite of heavy minerals is over half green hornblende. Northward from the Rio Grande the beach of Padre Island contains the Rio Grande suite of heavy minerals, but the influence of the other rivers is clearly shown by an increased ratio of more stable minerals in the samples farther north in Texas.

The sediments of the Coastal Plain do not end at the shore but extend out under the sea, and "if the basement surface on which they rest continues to slope uniformly, the mass of sediments must increase in thickness at least as far as the edge of the continental shelf, beyond which they should thin out rapidly as they merge into the oozes of the ocean depths" (Stephenson 1926, p. 463).

Land derived sediments are not being moved in a "continuous sheet of detritus all the way from the beach to the continental slope" (Daly 1942, p. 100). If this were true, much of the continental shelf would be some fathoms shallower than at present. With continuing deposition the sea would become more shallow, and wave and current action would push the sediments nearer the edge of the shelf. When the sediments reached the edge of the continental shelf and a profile of equilibrium was attained, the shelf sur

face would have been raised several fathoms. Therefore, it appears that a profile of equilibrium does not exist on the outer part of broad Gulf of Mexico continental shelves.

Sediments carried to the Gulf of Mexico largely remain in that body of water rather than being carried into the Atlantic. The Gulf of Mexico is of no importance to the deep-water circulation of the Atlantic Ocean (Kuenen 1950, p. 44). The unnamed current that becomes the Florida current is the major current of the Gulf, and "it is essentially a direct continuation of the current through the Yucatan Channel" (Sverdrup, Johnson, and Fleming, 1942, p. 642). The waters of the Gulf mainly form independent eddies and are only to a small extent drawn into the Straits of Florida. These eddies appear to be semipermanent features with their locations determined by the contours of the coast and the configuration of the bottom (idem., p. 641).3

EARLY STUDIES OF SUBMARINE DEPOSITS The Coast Survey instituted a series of investigations on physical problems of the deep sea in 1846, with emphasis on the Gulf Stream. In 1850, L. Agassiz made an extended biological survey of the Florida reefs, and in 1867, Pourtalès and Mitchell began a more systematic deep-sea exploration. Dredging between Florida and Cuba in 1868 reached depths of 850 fathoms, and the bottom samples obtained showed a closer relationship to the cretaceous fauna rather than to organisms of the adjacent shores.

Commander Howell, U. S. N., began a systematic exploration of the Gulf of Mexico in 1872, starting in the shallow waters along the west coast of Florida, and the work was continued by Lieutenant Commander Sigsbee in 1875-78, using the United States Coast Survey steamer Blake. The specimens of bottom deposits were sent to John Murray of the Challenger for examination, and he published the results in 1885 (Murray, pp. 51-61). Excerpts from his original description are as follows:

In all the deeper deposits in the Gulf of Mexico and Strait of Florida, the crystalline mineral particles are very small, rarely exceeding one-tenth of a millimeter in diameter. They consist principally of small rounded grains of quartz, with fragments of felspars, mica, hornblende,

For a detailed discussion of circulation of water in the Gulf of Mexico see article by D. F. Leipper, Physical Oceanography of the Gulf of Mexico, in this book, pp. 119–137.

augite, magnetite, and rarely tourmaline. In a few places there were fragments of pumice, and glauconitic particles were occasionally noticed. The mineral particles and fine clayey matter appear to be almost wholly derived from North American rivers.

The carbonate of lime in the deposits of these regions is mostly made up of the shells of pelagic Foraminifera and Mollusks. In depths greater than 2,000 fathoms the Pteropod and Heteropod shells appear to be nearly, if not quite, absent the carbonate of lime then consisting of the shells of pelagic Foraminifera; in less depths the Pteropod and Heteropod shells are present, and in depths varying from 200 to 500 fathoms they make up the bulk of the deposits in many places. In several of the deposits, where the percentage of carbonate of lime is very high, the whole has a very chalk-like appearance; it appears, indeed, as if it were in the process of transformation to true chalk.

The siliceous organisms consist of Radiolarians and Sponge spicules, with a few Diatoms, but these seldom make up more than three or four percent of the whole deposit.

A study of the United States Coast and Geodetic Survey maps of the continental shelf adjacent to Louisiana shows many different materials forming the Gulf bottom such as sands, muds, clays, shells, and local reefs. These represent the surface of the Gulf floor, and little is known about the material even immediately below the surface. Some borings have been made in the erection of the platforms required for petroleum exploration, but these platforms are all located approximately within the first 30 miles off shore. The wells drilled from these offshore structures have yielded no known information of the surface formations. Likewise, crews making geophysical surveys in the Gulf are not interested in the surface or nearsurface formations (Willey 1948, p. 3).

Trowbridge (1927, p. 148) stated that the United States Coast and Geodetic Survey obtained 600 bottom samples in 1921 and that their map of 1926 included the results of this work.

RECENT STUDIES OF SUBMARINE DEPOSITS

According to Trask, Phleger, and Stetson (1947, p. 460) sediments in the Gulf of Mexico. have changed in relatively recent time. During the 1947 expedition of the Atlantis, more than 600 cores were taken along 19 lines perpendicular to the Texas and Louisiana coast, crossing both the continental platform and the continental slope and continuing into the depths of the Gulf. The complete results of this expedition have not been published to date, but some data. were discussed by Phleger (1950). It was found