Abstract Six subparallel multichannel seismic lines across the Mexican Ridges foldbelt in the southwestern Gulf of Mexico between 21ºN and 22ºN latitude show details of the folds and provide new insight into their origin. The top of the seismic unit containing inferred Jurassic salt continues relatively undeformed beneath the foldbelt, suggesting that the subparallel folds are not caused by salt tectonics. Instead, the folds appear to occur in competent beds above a possible décollement. The detachment from the underlying rocks is along a deformed zone occurring within a thick Upper Cretaceous-Lower Tertiary shale section. A domal uplift observed on only one line and seemingly unrelated to the foldbelt may be cored with salt. The uniform folding of most of the upper sedimentary section without any significant diapirism in the cores of the folds, plus the numerous imbricate thrust faults associated with the folds, suggest regional compressional stresses acting in an east-west direction. Involvement of beds as young as Pliocene and Pleistocene indicates that folding and thrusting is very young and may be continuing today. Decreases in fold amplitude and the decrease in sediment ponding in the synclines in a seaward direction suggest that the zone of maximum deformation moved seaward through time. At least two separate mechanisms adequately explain the tectonic style of the Mexican Ridges folds observed on the seismic sections: 1) massive gravity sliding possibly triggered by regional uplift and supplemented by sediment loading at the head of the slide; and 2) compressional tectonic stresses originating within the deeper crust beneath Mexico and transmitted into the fold area through deep thrust zones. In both cases, detachment and deformation take place above a decollement or deformed zone located within mobile substrata (possibly geopressured slope shales).

Abstract Preliminary results of a long-term program to investigate the tectonic history of the Gulf of Mexico are summarized principally through multichannel seismic reflection methods. Knowledge of the deep part of the Gulf contributes significantly to understanding the evolution of its continental margins. Reasonable and usual stratigraphic principles, based on marine reflection data, can be used successfully to interpret the geological history of such regions. The stratigraphic unit which contains the Late Jurassic salt responsible for the diapirs of the deep Gulf of Mexico can now be recognized throughout this region, and diapirs have now been located as far east as the base of the West Florida Escarpment. Basement structural arches which seem responsible for the reef growth controlling the Florida and Campeche scarps are also present in the Yucatan Straits. All post-Jurassic units of the deep Gulf pinch out by depositional overlap against the Florida Escarpment to the east, against the Campeche Escarpment to the south, and against the newly discovered basement feature in the Strait of Florida. There is no evidence of faulting associated with these scarps. The post-Jurassic through Miocene sediments of the deep Gulf originally continued north of the Sigsbee scarp, and completely across the Mexican Ridges to 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 late Pliocene or Pleistocene time. 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.

Abstract Multifold seismic reflection investigations have provided data pertinent to the problem of origin and mode of deformation of salt in the Gulf of Mexico. The Challenger seismic unit which contains Jurassic salt covered the Jurassic abyssal basin and onlapped the Campeche and West Florida continental margins; it is thought to have on lapped the Texas-Louisiana, RioGrande,and East Mexican margins as well. The unit has an estimated average thickness of 1.5 km and a maximum thickness of at least2.5km.On lap and pinch out of the Challenge run it and isostatic considerations suggest that the unit was deposit edona sea floor several thousand meters below sea level.The data are inconclusive with respect to the question of whether the salt was deposited indeep water or whether the surfaceof the Gulf was greatly lowered. The principal dissimilarity of the Challenger from overlying units derives from mobilization of salt within the Challenger.The causes of localization of salt mobilization are not clear,but salt mobility is developed best in are as where the lower,seismically transparent part of the Challenger is thickest. The volume of salt with in the Challenger equals an estimated 20 to 50 times the volume of salt inpresent Gulf waters.An accumulation of this magnitude required continuous replacement from the world ocean and could not have resulted from a single episode of drying-up of the Gulf of Mexico. Four modes of salt mobilization and emplacement can be recognized:(1)geographically random diapirism continuously active from Jurassic to present in the Texas-Louisiana Shelf and upperslope, Campeche Knolls,and Sigsbee Knolls;(2)formation of sinuous,sub parallel ridges beneath the Mississippi cone,probably due to differential sediment loading of the prograding delta;(3)Pleistocene over thrusting of a salt“tongue”in the central Sigsbee Scarp;and(4)late Miocene-early Pliocene mobilization of Jurassic stratiformsalt in the Campeche Knolls province. The Mexican Ridges,which some investigators have suggested are cored by salt, appear to be cored with shale.

Abstract Multichannel sections across U. S. Gulf Coast Continental Margins show structure and sedimentary layers to much greater depths than most previously published data. A section across the Perdido Escarpment, three to four sections across the Sigsbee Scarp and two sections across the Florida scarp will be used to show some of these differences. The various hypotheses about the formation of these scarps will be discussed in the light of these new data, as well as the structure of the associated continental slopes and shelves. The deep Gulf of Mexico Basin sections which bear on these areas will also be presented. Significant sedimentary accumulations occur both above and below these scarps, so that the Gulf Coast hydrocarbon province may well extend out into the deep Gulf Basin.