The Gulf of Mexico Basin

Edited by Amos Salvador


Eighteen chapters deal with the entire Gulf of Mexico basin. Included are significant contributions from Mexican geologists. Nine topical chapters cover regional aspects of physiography and bathymetry, structural framework, the basement crust, salt tectonics and listric faulting, igneous activity, seismic stratigraphy, oil and gas resources, mineral resources and geopressured-geothermal energy, and ground water. Six chapters summarize regional stratigraphy and paleogeography for the pre-Triassic, Triassic-Jurassic, Lower Cretaceous, Upper Cretaceous, Cenozoic, and late Quaternary. Also included is a synthesis of the origin and development of the Gulf of Mexico basin. Six 4-color plates summarize the bathymetry, natural resources, tectonics, and basement structure and subcrop of the region, and provide a stratigraphic correlation chart and geologic cross sections.

  1. Page 1

    By the year 1517, 25 years after Christopher Columbus discovered the New World, most of the Atlantic coasts of both North and South America had been sighted and reasonably well surveyed. Most of the islands of the Caribbean had been colonized by the Spaniards, and northern South America and the Caribbean coast of Central America, from Panama to Honduras, had been explored and mapped. Four years earlier, in 1513, Vasco Nuñlboa had reached the Pacific Ocean by crossing the isthmus of Panama, but the search for a sea route to the Pacific and to the fabulous kingdoms of the Orient, the original objective of Columbus' had not yet met with success. The great Gulf of Mexico—the “Sinus Mexicanus” of many old maps— also remained unknown, and there was considerable confusion concerning which of the discovered lands were islands and which were parts of the mainland.

    Juan Ponce de Leon, in his search for the legendary fountain that would restore youth to old men, had explored in 1513 the east coast of the Florida Peninsula as far north as the present location of St. Augustine, but had only ventured a short distance north along the west coast. He was convinced that he had discovered an immense island. Diego Miruelo, in 1516, explored the east coast of Florida and seems to also have sailed some distance along the west coast of the peninsula. But neither Ponce de Leon nor Miruelo realized they had sailed into the entrance to a vast gulf or

  2. Page 13

    The topographic relief and bathymetry of the Gulf of Mexico basin area reflect quite closely the geologic structure of the basin (Fig. 1 and Plates 1 and 3). Parts of the structural rims along the northern, northwestern, and western flanks of the basin are marked by mountain ranges and highlands: the southern plunge of the Appalachians and the Ouachita Mountains to the north, the Edwards Plateau and the low ridges of the Marathon area to the northwest, and the Sierra Madre Oriental to the west. From the foothills of these highlands, the coastal plains slope toward the Gulf of Mexico, a small ocean basin that occupies the central and deeper part of the basin. To the north and northwest, the coastal plains and the continental shelf of the Gulf of Mexico are widest and have a gentler slope toward the center of the Gulf, corresponding to the gentle slope of the “basement” in the region. To the west, in eastern Mexico, the coastal plain and the shelf are much narrower and steeper, as is the “basement” surface. To the southeast and east, the floor of the Gulf of Mexico, which in its deepest part reaches depths of a little more than 3,700 m, rises steeply along the Campeche and Florida submarine escarpments to the flat Yucatan and Florida carbonate platforms, under which the “basement” is similarly flat and featureless. Much of these two platforms lies submerged below the waters of the Gulf of Mexico at depths of less than 200

  3. Page 31

    The Gulf of Mexico basin is a roughly circular structural basin that has been filled with 0 to 15 km of sedimentary rocks ranging in age from Late Triassic to Holocene. The crust beneath the central part of the basin is oceanic in character; this is surrounded by continental crust, which underneath much of the basin has been greatly attenuated by rift-related extension (Worzel and Burke, 1978; Buffler and Sawyer, 1985; and Chapter 4, this volume).

    Superimposed on the basin are second-order structural features that modify the overall simple geometry (Plate 2):

    1. Basins of enhanced subsidence and deposition, as well as intervening platforms or arches (“uplifts” sensu lato), which subsided less than surrounding areas. These features were formed by spatially varying rates of lithospheric cooling related to the early synrift history of extension, and amplified by differential sediment loading.

    2. Basin-margin fault systems in the northwestern and western segments of the Gulf of Mexico basin, due to flexing of the basin rim and uplift of adjacent provinces.

    3. Structural basins and uplifts sensu stricto with resultant erosional unconformities and clastic wedges related to active tectonics: block faulting, epeirogenic doming, and the formation of fold-thrust belts.

    4. Salt diapirs and related structures formed from flow of Jurassic salt that lies at the base of the sediment column. Different original salt thicknesses and different loading histories have created distinct salt-diapir provinces characterized by their style and age of diapirism. A “peripheral graben system” formed in the northern part of the basin

  4. Page 53

    Although the sediments filling the shallow-water parts of the Gulf of Mexico basin have been extensively explored for hydrocarbons using seismic methods and are reasonably well known, the nature and distribution of the underlying crust and mantle are much less well known. Several characteristics of the basin's sediments have made deep-penetration observations difficult. Thick Cenozoic clastic sediments throughout the Gulf of Mexico basin attenuate seismic energy, while Jurassic to Upper Cretaceous carbonates and evaporites provide a large impedance contrast, through which seismic transmission is further limited. In parts of the basin, mobile salt forms pillows, domes, sills, and other structures, which focus and defocus seismic energy in ways that make imaging difficult. In the southeastern Gulf of Mexico, solution cavities and karst-like paleotopography in shallow-water carbonates severely scatter seismic signals. In spite of these difficulties, reflection seismic, refraction seismic, gravity, magnetic, and subsidence techniques have been used to resolve the gross characteristics of the crust under the Gulf of Mexico basin.

    In all cases the terms “crust” and “basement” will be used as synonyms. In areas of normal or modified continental crust, the crust and basement are defined to include all rock lying beneath a widespread unconformity at the base of the marine Mesozoic section. This surface is overlain and onlapped by Middle Jurassic salt (or equivalent rocks) as well as younger sedimentary rocks. Included within “basement” are the Upper Triassic to Lower Jurassic rift sequences (see Chapter 8, this volume). Although these “red-bed” sequences should perhaps be considered a

  5. Page 73

    Local structuring in the basins that rim the Gulf of Mexico is largely the result of gravity acting on sedimentary sections deposited on an unstable base of abnormally pressured shales and/or salt. The resulting deformation takes two primary forms, salt-flow structures and listric-normal faults that sole out at various levels above the basement.

    Salt flow in this nonorogenic environment is the result of pressure gradients created by the sediments that overlie, or load, the salt. When differential loading occurs, pressures vary laterally within the salt layer, and the salt tends to move away from areas of higher pressure toward areas of lower pressure. Structures resulting from such flow have a wide variety of forms. These include low-relief anticlines and pillows, which simply deform the overlying beds; high-relief plugs and walls, which have a piercement relation with the sediments and often breach the surface; and extensive salt sheets, which have been emplaced laterally in clastic sediments deposited in a continental slope environment.

    Listric-normal faults are the result of coherent, differential basinward movement of the sediments above some decollement layer. In the Gulf of Mexico basin, this layer may be either salt or abnormally pressured shale. The term “listric” comes from the Greek word for shovel and aptly describes the curved, three-dimensional geometry of the fault itself (Bally and others, 1981). The term was originally used to describe thrust faults; thus the adjective “normal” is needed to describe their extensional counterparts. The principal driving force for such faulting is gravity acting on

  6. Page 91

    Some 220 million years ago, in Late Triassic time, the Gulf of Mexico basin began to form in the wake of the breakup of Paleozoic megacontinent Pangea, and the opening of the North Atlantic Ocean. Igneous processes played a major role in the formation of this basin, as the common occurrence of basaltic rocks in rift basins around the Gulf of Mexico margin indicates. Geophysical evidence indicates that the central basin is floored by oceanic crust, presumably similar to that of oceanic crust elsewhere. Igneous activity was, however, not confined to the early stages of evolution of the basin. During the late Mesozoic, major volcanic fields rimmed the northern margin of the basin, probably the result of intraplate stresses due to global plate reorganization or isostatic adjustment from increased sediment loads along this margin. The eastern margin of the basin may have been affected by a late Mesozoic–early Tertiary Caribbean magmatic arc complex. Throughout the Tertiary the western margin has had a complex history of igneous activity associated with subduction of the Pacific plate beneath the North American Plate. There are numerous volcanic fields on the coastal plain and presently three active submarine volcanoes within the Gulf of Mexico.

    Throughout the evolution of the Gulf of Mexico basin, igneous rocks have also been a significant component of sediment being deposited within the basin. In the Late Cretaceous, local basins along the northern margin had adjacent volcanic sources, and during the early Tertiary uplift, these volcanic terrains were the source of

  7. Page 109

    The thick Mesozoic-Cenozoic fill of the Gulf of Mexico basin was deposited on a floor of Paleozoic and older “basement” that is still poorly known. Fewer than 250 wells out in the basin, away from the structural rim, have penetrated pre-Mesozoic rocks; fewer than a dozen of these wells drilled as much as 1,000 m of this older section. Basinwide, the average penetration of these older rocks is only 220 m, and data from many of the wells are sketchy and incomplete. The pre-Triassic has long been considered “economic basement” by the oil industry, and stratigraphers have paid little attention to these rocks. Whereas a great part of the pre-Triassic rocks was subjected to strong deformation and metamorphism, it is now known that unaltered sediments of Cambrian(?) to Devonian age underlie the Mesozoic in the southeastern U.S. states of Alabama, Georgia, and Florida. Sediments of early Pennsylvanian to Permian age, some possibly older, are known to underlie the Mesozoic in the northwestern part of the basin, and seismic data suggest thicknesses of 5 km or more in some areas for these young Paleozoic sediments. Unmetamorphosed pre-Triassic rocks are also known from the western flank of the Gulf of Mexico basin, in eastern Mexico, and from its southern flank in southern Mexico, Guatemala, and Belize.

    One of the earliest recorded penetrations of pre-Triassic “basement” out in the basin was reported in 1928. A well drilled in Marion County, Florida (location 1, Fig. 1), encountered rocks first called “basement schist and quartzite.”

  8. Page 131

    Geological and geophysical evidence—and confidence in the postulates of plate tectonics—indicates that in the dawn of the Mesozoic what would become the Gulf of Mexico basin area was part of a very large land mass, the supercontinent of Pangea. It is generally believed that toward the end of the Paleozoic and the beginning of the Mesozoic, Pangea grouped all the continental plates of the Earth. How all these plates were assembled is still a subject of considerable controversy, particularly how and where the North American and South American Plates fit together in the area that would eventually become the Gulf of Mexico basin and surrounding positive tectonic elements.

    Many reconstructions of the Gulf of Mexico basin area during the late Paleozoic or earliest Mesozoic have been proposed (Bullard and others, 1965; Freeland and Dietz, 1971; Owen, 1976, 1983; Carey, 1958, 1976; Smith and Briden, 1977; Pilger, 1978; Salvador and Green, 1980; and many others). They all differ in the manner in which the authors interpreted how the plates were assembled, but agree, at least, in indicating that the North American and South American Plates were joined in some manner during the late Paleozoic and earliest Mesozoic as part of the emergent and stable supercontinent of Pangea.

    The first recorded active events of the Mesozoic geologic history of the Gulf of Mexico basin area correspond with the beginning of the breakup of Pangea, probably during the Late Triassic, and the drifting of the North American Plate away from the African and

  9. Page 181

    Throughout most of Early Cretaceous time, the Gulf of Mexico basin was a major site of continental and marine deposition surrounded by the Appalachian and Ouachita uplands on the north, the Llano and Marathon uplifts on the northwest and the Chiapas massif and Maya Mountains to the south. During this time there were marine connections to the Pacific Ocean to the west and to the Atlantic Ocean to the southeast. The Gulf of Mexico basin was tectonically stable except for continuing slow subsidence of its central part, growth faulting on the margins of some depocenters, and local deformation related to underlying Jurassic salt. Shallow-marine water covered its rims and peripheral shelves, and progressively deeper waters its slope and abyssal plain.

    Lower Cretaceous rocks form a continuous disc of sediments, which thin and pinch out updip along the periphery of the Gulf of Mexico basin. Lower Cretaceous sequences crop out along the northwestern, western, southwestern rims of the basin. No Lower Cretaceous outcrops are known east of the Mississippi River nor in the Florida and Yucatán Peninsulas (Fig. 1).

    The Lower Cretaceous sediments are primarily carbonates and evaporites on the circum-Gulf shelves, and carbonates in the bathyal areas. Continental and shallow-marine terrigenous clastic sediments occur primarily around the northern and northwestern rims of the basin, from northeastern Mexico to the Florida panhandle. They are most prevalent in the lower part of the Lower Cretaceous section (Berriasian to Barremian), and represent the sediment load of rivers draining the continental interior and the

  10. Page 205

    In the Gulf of Mexico basin and contiguous areas the Late Cretaceous history is marked as a general time of oceanic high stand. The Late Cretaceous here begins with a short period, characterized over the northern margin, by basin fill. This interval was followed by major transgression, during which marine waters inundated the basin margins and eventually linked the Gulf of Mexico with the great epicontinental Western Interior Seaway. The following is a summary of these and the subsequent events that transpired during the Late Cretaceous in the Gulf of Mexico basin region.

    Upper Cretaceous rocks form a virtually continuous blanket over the Gulf of Mexico basin. Overall, the inner edge of the northern outcrop parallels the deeply buried Paleozoic Ouachita orogenic belt. Conspicuous projecting features, such as the Rio Grande and Mississippi embayments, are situated on pronounced salients of the older Ouachita orogenic belt. At the updip pinch-out, the present-day outcrop pattern follows a trebly arcuate path (Fig. 1). The easternmost Gulfward-trending arc extends from Georgia westward, wrapping around the southwest end of the Appalachian trend across Alabama, and thence northward through Mississippi, Tennessee, and Kentucky, to terminate at the head of the Mississippi embayment in southern Illinois. Because of local overlap by Tertiary deposits, the second and complementary outcrop arc is intermittent, but wraps around the Arkansas platform through Missouri and Arkansas, then trends westward and terminates north of Dallas, Texas. From this point a third outcrop arc trends first south, then southwestward across Texas, generally paralleling the

  11. Page 245

    While carbonate and evaporite deposition continued over the stable Florida and Yucatan Platforms, terrigenous clastic deposition dominated the rest of the Gulf of Mexico basin during the Cenozoic.

    The position of the Cretaceous shelves and platforms determined to a great extent the shape and size of the basin at the beginning of the Cenozoic. This stratigraphic and structural framework was modified during the Cenozoic by the vast influx of terrigenous clastic sediments from the north and west, and by the structural impact of the Laramide orogeny during the Paleocene and Eocene. Subsidence of the central part of the basin continued during the Cenozoic, but it was the result more of sedimentary loading than of the thermal cooling of the oceanic crust.

    The immense volumes of terrigenous clastic sediments that entered the Gulf of Mexico basin, particularly along its northern and northwestern margins, caused rapid basinward migration of shoreline deposition across the shelves, ultimately to positions considerably beyond the trend of the Cretaceous shelf margins. When large volumes of terrigenous clastics began to accumulate basinward of the Cretaceous shelf margins, an offlapping depositional style was developed along the northern and northwestern Gulf of Mexico basin that characterizes the Cenozoic. Very thick sedimentary sections began to accumulate over the continental slopes, and to fill the deeper parts of the basin, loading and depressing the attenuated continental crust and the oceanic crust.

    The same three distinct stratigraphic-structural provinces in existence during the Mesozoic can still be recognized during the Cenozoic around the center

  12. Page 325

    The Gulf of Mexico basin (Fig. 1) is the largest semi-enclosed depositional basin in North America and has been the site of extensive hydrocarbon exploration and exploitation since the turn of the century. Since Late Jurassic times, the drainage basin of the Mississippi River system has been delivering sediments to the Gulf of Mexico (Worzel and Burke, 1978; Chapter 8, this volume). Mesozoic and Cenozoic deposits are estimated to have attained a total thickness in excess of 15 km (Martin and Bouma, 1978; Bouma and others, 1978a). Thus, the river system has been operative over relatively long periods of time, constantly feeding sediments to the receiving basin and building a thick Jurassic, Cretaceous, Tertiary, and Quaternary sequence of interfingering deltaic, nearshore coastal brackish water, and marine sediments, which have prograded the coastal plain shoreline seaward. Relatively little sediment yield has occurred during the Quaternary from the southern rim of the Gulf of Mexico basin. Through time, depocenters have shifted within the northern flank of the basin, forming a relatively thick sequence of Tertiary and Quaternary clastic sediments. The zone of maximum thickness trends roughly east-west near the present-day coastal plain of Louisiana and west toward Texas. Rapid subsidence associated primarily with sediment loading and salt and shale diapirism has been responsible for unusually thick, localized sedimentary accumulations and for the complex bathymetry on the continental slope (Fig. 1; Plate I, this volume). Throughout the Tertiary and Quaternary, minor and major transgressions and regressions have occurred, although the major depositional component

  13. Page 353

    The geology of the deep Gulf of Mexico basin can be interpreted only by means of geophysical data, since few direct geologic data are available. The purpose of this chapter, therefore, is to review the seismic stratigraphy and geologic setting of the deep Gulf of Mexico basin and adjacent margins as inferred mainly from the interpretation of seismic reflection data. The deep Gulf of Mexico basin as used in this chapter is bathymetrically the deepest part of the basin (Fig. 1). The area is also part of the structurally deepest part of the basin and is underlain by a thick section (as much as 9 to 10 km) of generally undeformed sedimentary rocks overlying basement (Fig. 2). The term deep, therefore, refers both to the structural configuration of the basin as well as the depth of water. The margins of the deep basin are defined either by areas of deformed sedimentary rocks or steep escarpments that disrupt the seismic record and limit the correlation of strata in the deep basin with the better defined geology of the adjacent shallower parts of the basin (Figs. 1 and 2). The areas of deformation include the Campeche-Sigsbee Knolls to the southwest, the Mexican Ridges to the west, and the Sigsbee Escarpment to the north, which marks the southern limit of the extensively deformed Texas-Louisiana Slope (Fig. 1). The steep escarpments include the Florida Escarpment to the east and the Campeche Escarpment to the south (Figs. 1 and 2). In the southeastern Gulf the

  14. Page 389

    For more than 100 years, geologists have speculated about the initial steps of the formation of the present Gulf of Mexico basin. A review of the literature, most likely incomplete, uncovered more than 70 publications on the subject by close to 80 authors.

    Early workers (Schuchert, 1909; Willis, 1909) considered the Gulf of Mexico to be an ancient feature, a deep-water body in existence since the Precambrian. Willis (in Schuchert, 1935, p. 72) regarded the Gulf as representing “a mass of basalt which was erupted in Pre-Cambrian time…,” a basin “of great antiquity.”

    With increased information on the area opinions progressively changed, and from the late 1910s to the early 1930s most contributors to the controversy came to favor a much later beginning for the Gulf of Mexico basin (Dumble, 1918; Miser, 1921; Schuchert, 1923, 1929, 1935; Sellards, 1932; and a number of others). They believed that during most of the Paleozoic a continental landmass or borderland, most commonly referred to as “Llanoria,” occupied the northwestern part of the present Gulf of Mexico basin, from northeastern Mexico to Mississippi, and included a large but undetermined part of the present northwestern Gulf of Mexico (Fig. 1), an idea first implied by Edward Suess (1888) in his celebrated Das A ntlitz der Erde. In the southeastern part of the present gulf, the borderland was thought to have been submerged and covered by a shallow sea; the entire area was regarded as a “neutral area, or better, a slightly negative one,” an “ancient

  15. Page 445

    The oil and gas resources of the Gulf of Mexico basin are a major contribution to the geologically derived wealth of the basin. This chapter provides a concise, essentially quantitative description of those resources.

    The description is divided into four parts. The first part indicates the overall amount of known petroleum resources within the basin and the distribution of these resources by product (crude oil, natural gas, and natural gas liquids), by geographic area (subprovince), and by gross field size category. The second part discusses the history of petroleum exploration, discovery, and production within the basin.

    The third part, the bulk of the chapter, describes the petroleum resources of the basin by stratigraphic unit. This description is divided into five major units: the Upper Jurassic, Lower Cretaceous, Upper Cretaceous, Paleogene, and upper Cenozoic (Neogene/Pleistocene). Within each of these units the discussion is organized by series, stage, or groups of stages. The description of each grouping covers the known amounts of petroleum, the distribution of these resources by field size categories, the producing trends within the grouping, and the fundamental factors of petroleum accumulation—reservoir, trap, seal, and source— within these trends. The chapter concludes with a brief discussion of why the Gulf of Mexico basin is so productive.

    The limits on the size of this chapter do not permit discussions of even a few individual fields. For those who are interested in such discussions, many excellent publications (other than the references cited here) on oil and gas fields in the Gulf

  16. Page 495

    The Gulf of Mexico basin is best known for its vast and widespread oil and gas resources. They have been described in the preceding chapter of this volume. The basin, however, also contains important deposits of phosphate, lignite, and sulfur and small deposits of uranium. In addition, salt from several salt domes is produced by underground and solution mining and is used principally as a chemical feedstock for the manufacture of many industrial products. Large volumes of geopressured-geothermal water are also known from the Tertiary sediments of the Gulf of Mexico basin, particularly around its northern margin. It often contains natural gas in solution. This overpressured, gas-bearing hot water may someday be an important source of thermal and kinetic energy; it is now just a gleam in the eye of imaginative energy tacticians.

    The phosphate deposits of Florida and southeastern Georgia, the Florida Phosphogenic Province, represent about 75 percent of the total domestic phosphate production, and ranged between 34 and 28 percent of the total world production between 1983 and 1987.

    Important lignite deposits, for the most part of Eocene age, are known from the Gulf of Mexico basin. Two-thirds of the lignite is found in Texas, but it occurs also in parts of northeastern Mexico, Louisiana, Arkansas, Tennessee, Mississippi, and Alabama. Modest resources of Upper Cretaceous bituminous coal are found in northeastern Mexico.

    Once an important industry, the production of sulfur from the caprocks of some of the many salt domes in the U.S. Gulf Coastal Plain and shallow

  17. Page 529

    Ground water is an important natural resource in the Gulf of Mexico region. Although the region as a whole is relatively rich in water resources, water availability and quality vary dramatically throughout the area. Water resources include rivers—the Chattahoochee, Alabama, Mississippi, Sabine, Trinity, Brazos, Colorado, Rio Grande, and Grijalva—and the immense groundwater resources that are the focus of this chapter. Many major cities in the Gulf of Mexico basin (e.g., Miami, in Florida; Memphis, in Tennessee; Gulfport and Baton Rouge, in Louisiana; and Houston and San Antonio, in Texas) rely chiefly on ground water. Climate varies from moist temperature or subtropical to semiarid conditions, and while population densities (and water-resource demands) vary from minimal to intense, it can be stated that fresh, potable water is the mineral resource in greatest demand. Its continued availability in the region will require special attention in the near future. As elsewhere, the foremost challenge is the provision of adequate quantities of good-quality water, but several special problems exist in the region, including salt-water intrusion and subsidence. An understanding of the hydrogeologic setting of aquifers in the Gulf of Mexico basin is required to preserve and fully utilize this valuable resource.

    Aquifers in the Gulf of Mexico basin area (Fig. 1) may be grouped into the following categories: clastic sediments dipping toward the center of the basin; the major carbonate systems of Florida, Texas, and Yucatan; and less importantly, major alluvial aquifers, island aquifers, and volcanic aquifers.

    The thick section of predominantly Cenozoic clastic sediments

  18. Page 545

    The information contained in the preceding chapters of this volume is clear evidence of the significant progress that has been made in understanding the geologic and geophysical composition and geologic history of the Gulf of Mexico basin. Many uncertainties remain, however; many questions are still unanswered, and many fundamental problems are yet to be solved.

    Our current advanced knowledge of the basin is the result of the persistent collection during the last 100 years of a great volume of geological and geophysical information, most of it by the petroleum industry in its search for oil and gas accumulations, but also by geologists and geophysicsts from national and state geological surveys and academic institutions.

    It is safe to say that, for many years now, the Gulf of Mexico basin region has been the home of the largest concentrations of geologists and geophysicists anywhere in the world. The region probably can claim to have the densest seismic coverage—unfortunately not all of it in the public domain—and close to 700,000 wells have been drilled throughout the basin, many of them to considerable depths. The amount of geological and geophysical information on the basin is, therefore, voluminous, as are the number of publications on its geologic composition and history.

    As mentioned in Chapter 1 of this volume, the collection of geological and geophysical information on the Gulf of Mexico basin progressed from the periphery to the center, from the investigation of the rock outcrops around its margins by the geologists of the oil companies

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