Structural Geology and Evolution of the Mississippi Fan Fold Belt, Deep Gulf of Mexico (1)

Abstract

The Mississippi Fan fold belt, located in the deep Gulf of Mexico beneath the upper and middle Mississippi Fan, is characterized mainly by basinward-verging anticlines and associated thrust faults. The fold belt extends approximately 300 km eastward and is approximately 50 km wide. Based on correlations with deep Gulf of Mexico seismic sequences (Challenger, Campeche, Lower Mexican Ridges, Middle Mexican Ridges, and Upper Mexican Ridges), the folded strata are interpreted to be Upper Jurassic through Miocene. Salt tongues or sheets in the lower slope have overridden and partly masked parts of the fold belt. The approximate minimum age of the folded strata suggests that the major deformation took place mainly during the middle to late Miocene. South of the fold belt, strata downlap onto a major middle Miocene surface (15.5 Ma) at the top of the Middle Mexican Ridges sequence in the deep basin. This downlap represents sediment derived locally from the growing anticlines, and marks the beginning of growth of the fold belt. Within the fold belt, the coeval reflector represents the oldest identifiable onlap surface for turbidites deposited in the developing synclinal basins. Onlap of younger (uppermost Miocene-lower Pleistocene) Mississippi Fan strata onto the anticlines indicates that the fold belt formed major topographic barriers after its formation and influenced the distribution of early fan sediment. The structurally highest folds have experienced at least several hundred meters of truncation by deep-marine erosion. In the northern and eastern part of the fold belt, the entire Challenger sequence (Upper Jurassic-Lower Cretaceous) has been deformed with 2000-3000 m of structural relief. This uplift involves a structural thickening of stratigraphic section in the core of the anticlines, suggesting that the structures are cored with mobilized salt. Some of the younger fan sequences locally decrease in thickness across the folds, indicating recurrent movement and growth of the salt in the core of the folds throughout the Pleistocene. In some places, salt has remobilized and completely penetrated the overlying fan sediments to the surface, thus forming diapirs. Basement rocks were not involved in the deformation. Thrust faults beneath the folds sole out into the lower part of the Mesozoic section (pro ably into Middle Jurassic salt). The folds and thrust faults are most likely the result of compressive stresses caused by updip sediment loading during the development of Miocene shallow-water depocenters to the north. These stresses were transmitted to the salt basin edge along a deep decollement surface or surfaces within, or at the top of, the Jurassic salt section. Thus, the original distribution of salt overlying transitional crust controlled the location of the fold belt, which formed where the salt pinches out and the decollement surface ends.