Mexican Ridges Research Articles

Post-Middle Cretaceous Seismic Stratigraphy of Abyssal Southwestern Gulf of Mexico Basin: ABSTRACT

The post-middle Cretaceous sedimentary section in the abyssal Gulf of Mexico (7 to 8 km thick) is divided into seven major depositional sequences. The sequence boundaries represent widespread unconformities which are best observed along the base of the Campeche Bank. These depositional sequences provide a framework for reconstructing the geologic history of the area. The oldest sequence (middle Cretaceous-early Tertiary) displays strong to weak, discontinuous reflections, interpreted to represent fine-grained pelagites and hemipelagites. The overlying three sequences (early Tertiary-Middle Miocene) are characterized by strong, high-amplitude, continuous to distcontinuous reflections, and probably consist (predominantly) of sandy turbidites, as evidenced from cores recovered from the bottom of Deep Sea Drilling Project hole 90. Seismic facies analysis of these three sequences shows large convex-upward depositional buildups and numerous relict channels, suggesting deposition in large submarine fan complexes. The three youngest sequences consist of fine-grained turbidites and pelagites and display such features as prograding foreset beds deposited in the distal parts of submarine fans, and large-scale, dune-like features with wavelengths of 2 to 4 km. Westward thickening of most of the sequences indicates the source of the detrital sediments was to the west in eastern Mexico. The southwestern Gulf of Mexico possesses excellent potential for hydrocarbon entrapment, especially in buried stratigraphic features such as the fan channel systems, and also in structural traps along the Mexican Ridges foldbelt. End_of_Article - Last_Page 796------------

Geologic History of Deep Gulf of Mexico Basin: ABSTRACT

New multifold seismic reflection lines (approximately 15,000 nmi; 27,780 km) and ocean-bottom seismometer (OBS) refraction lines collected by The University of Texas Marine Science Institute, Galveston Geophysical Laboratory, in the deep Gulf of Mexico basin and its adjacent margins show a sedimentary section up to 10 km thick overlying an acoustic basement. The thick section is divided into two major depositional sequences separated by a prominent gulf-wide unconformity, tentatively assigned a middle Cretaceous (Cenomanian) age on the basis of seismic ties with core holes and comparison with global sea-level cycle charts. Below the middle Cretaceous unconformity, the early geologic history of the gulf basin is complex. A rifted, block-faulted, and attenuated continental crust is inferred to underlie the deep gulf just north and west of the Campeche Scarp and eastward into the Straits of Florida area. A thick salt basin overlies this continental crust in the area north and west of the Campeche Scarp, where it forms a band of salt structure 50 to 100 km wide (Sigsbee salt dome province). Early deformation of the salt and the overlying thick Jurassic sedimentary section suggests a period of gravity sliding associated with early rapid subsidence of the basin. Refraction data indicate that an oceanic crustal layer underlies the rest of the deep gulf basin. A Late Jurassic (post-salt) period of rapid seafloor spreading or oceanization probably provided a mechanism (thermal cooling) for the rapid Late Jurassic-Cretaceous subsidence. A younger undeformed sequence of rocks onlaps and fills in on top of the oceanic crust, the outer basement high, and the early salt structures in the central deep gulf. This sequence represents the deeper water, offbank equivalent of the Late Jurassic-middle Cretaceous carbonate banks, shelves, and platforms that built up around the gulf basin as it subsided. The sequence thickens eastward to over 3 km beneath the Florida Scarp and Straits of Florida. Relief developed on these carbonate banks by middle Cretaceous time formed the proto-Florida and Campeche Straits. Location of the carbonate banks appears to be, at least in part, controlled by basement structure. Along the Florida and Campeche Scarps, there was a major middle Cretaceous shift from shallow- to deep-water sedimentation as the outer banks subsided. The post-middle Cretaceous section in the deep Gulf is divided into five depositional sequences or seismic units, defined by major unconformities along the base of the northwestern Campeche Scarp and tentatively correlated with global unconformities and sea-level changes as follows: early Tertiary, middle Oligocene, late Miocene, and the Pliocene-Pleistocene boundary. The main source of sediment supply to the basin was on the west in Late Cretaceous-early Tertiary time, but shifted more to the north during the late Tertiary and culminated in deposition of the huge Pleistocene Mississippi fan. End_Page 1277------------------------------ The Oligocene-Miocene section along the deep western gulf apparently was deposited as part of an older deep-sea fan complex with a western source. The lower part of the fan sequence (Oligocene-middle Miocene) is characterized by strong, discontinuous reflectors and is interpreted as relatively coarse-grained material deposited in a channelized midfan environment. The upper Miocene part of the fan consists of fine-grained laminites and is characterized by prograding clinoforms, deposited as lobes seaward of a midfan zone of bypass. The overall upper Tertiary fining-upward sequence in the western gulf and the gradual cessation of turbidites from a western source probably were due to the late Tertiary development of the Mexican Ridges foldbelt. This foldbelt apparently formed, or is still forming, owing to large-scale downslope gravity sliding of a more competent Tertiary section over incompetent, possibly geopressured, shales. The northern margin of the deep Gulf is defined by the Sigsbee Scarp, which represents the southern extent of a zone of salt deformation along the entire Texas-Louisiana slope. The western scarp bulges southward and is characterized by salt wedges thrust 10 to 15 km seaward over Pleistocene rise sediments. East of the bulge area the scarp is formed by the seawardmost series of vertical salt ridges that have uplifted and deformed Pleistocene sediments. The salt deformation along the scarp probably is continuing today as a result of both downslope gravity forces and massive sediment loading in a large Pliocene-Pleistocene depocenter farther upslope. End_of_Article - Last_Page 1278------------

Miocene Submarine Fans in Deep Western Gulf of Mexico as Interpreted from Seismic Reflection Profiles: ABSTRACT

Sequences of regularly spaced, seaward-dipping reflections seen on east-west seismic profiles along the lower slope of the western Gulf of Mexico are interpreted to be prograding clinoforms in the distal parts of large submarine-fan complexes. North-south lines define the lobate nature of the fans, some of which are over 50 km across. Some fans appear to be composites of overlapping, individual fan lobes. DSDP hole 90 recovered upper Miocene laminated, silty mud at the top and bottom of one of the prograding sequences, suggesting that any sand in the system must have been deposited farther upslope in the more proximal parts of the fan. Deposition of these sequences may have occurred during a major late Miocene lowering of sea level. The late Miocene fans appear to be part of an overall fining-upward sequence along the lower slope beginning with the middle Miocene sandy turbidites and continuing through deposition of late Miocene silty clay laminites into Pleistocene deposition of hemipelagic and pelagic sediments. The sandy middle Miocene turbidites were derived from the Mexican mainland on the west and correspond to a zone on the seismic profiles characterized by chaotic and discontinuous reflections. These reflection patterns suggest a fan environment more proximal than the younger fans above--perhaps a suprafan environment with fan channels. The overall fining-upward sequence and gradual cessation of turbidite deposition probably were due to the late Tertiary growth of the Mexican Ridges foldbelt, which apparently trapped terrigenous sediment and prevented turbidity currents from reaching the deep sea. End_of_Article - Last_Page 426------------

Gravity-Slide Origin for Mexican Ridges Foldbelt, Southwestern Gulf of Mexico: ABSTRACT

The geometry of a part of the Mexican Ridges foldbelt in the southwestern Gulf of Mexico as detailed by multifold seismic data suggests an origin by large-scale gravity sliding. Here, apparently competent, lower Tertiary-Holocene strata, probably sandstones and shales, have moved downslope along an underlying deformed zone probably consisting of weak and incompetent Upper Cretaceous-lower Tertiary geopressured shales. Gravity-generated horizontal compressive stresses apparently caused uniform folding and imbricate thrusting within the competent strata. This deformation is probably an important mechanism for overcoming resistance to basal sliding within the incompetent layer. Large-scale submarine sliding is also suggested by the grouping of the fold trends into two large obate areas. Sliding apparently has continued periodically since about middle Tertiary time. This massive sliding may have been triggered by periods of uplift and seaward tilting and perpetuated by sediment loading in a large growth-fault system just landward of the main foldbelt along the upper slope. End_of_Article - Last_Page 426------------

Margins of Gulf of Mexico

Preliminary results of a long-term program to investigate the tectonic history of the Gulf of Mexico principally through the use of multichannel seismic reflection methods indicate that knowledge of the deep part of the Gulf contributes significantly to understanding of the evolution of its continental margins. Reasonable and usual stratigraphic principles, based on marine reflection data, can be used successfully to interpret the geologic history of such regions. The stratigraphic unit which contains the Upper Jurassic salt responsible for the diapirs of the deep Gulf of Mexico now can be recognized throughout this region, and diapirs have been located as far east as the base of the West Florida Scarp. Basement structural arches which seem responsible for the reef growth controlling the Florida and Campeche Scarps also are present in the Yucatan Straits. All of the post-Jurassic units of the deep Gulf of Mexico pinch out by depositional overlap against the Florida Scarp on the east, the Campeche Scarp on the south, and the newly discovered basement feature in the Florida Strait. There is no evidence of faulting associated with these scarps. The post-Jurassic through Miocene sedimentary rocks of the deep gulf originally continued north of the Sigsbee Scarp, and completely across the Mexican Ridges on the west. Deformation by detachment sliding and diapirism progressed southward on the United States margin throughout the Cenozoic and occurred suddenly in the folded Mexican Ridges in the late Pliocene or Pleistocene. There is no evidence in the deep Gulf of Mexico of pre-Pleistocene deep-sea cones. The large deep-sea cone of the present Mississippi River is a prominent and unique feature of the Gulf of Mexico.

Geology and genesis of the Mexican Ridges: Western Gulf of Mexico

The western Gulf of Mexico sea floor contains numerous linear, parallel, anticlinal features that comprise the Mexican ridges province. The development of this ridge province essentially began during the Mesozoic with evaporite deposition in the gulf basin. These evaporites were subsequently buried by a thick sediment cover. Increasing overburden pressure and tectonic activity produced salt flowage on an irregular basement and thereby triggered salt anticline formation, the primary cause of ridge development. The ridges first developed during late Mesozoic or early Tertiary as salt anticlines and diapirs beneath the overburden. By Early Pliocene, salt tectonism had produced incipient surface ridges that effectively stemmed the seaward transport of terrigenous sediments. The ridges apparently are still undergoing uplift. Ridge trends are predominantly north-south but are not consistent and clearly have been influenced by basement uplifts. We present here a hypothesis for the development and trends of the Mexican ridges based on extensive geophysical data to show that the ridges are due to salt tectonism or flowage and the influence by basement highs.