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Hydrogeologic Aspects of Structural Deformation in the Northern Gulf of Mexico Basin1
The resistance of unconsolidated clastic sediments to structural deformation, an inverse function of their water content and pore pressure (commonly defined in terms of rock density and geostatic ratio) has no uniform relation to depth of burial in the Gulf Basin. Dumped along the northwestern margin of the Gulf Coast geosyncline at a very rapid rate, the sediments of younger deltas leapfrogged older deltas and spilled gulfward upon prodelta and marine clay components of the older deltas, burying them deeply before they could drain properly in response to compaction stress of the overburden load. Contemporaneous gravity faults of major dimension occur along gulfward margins of delta front sand bodies, reflecting the effect of sediment facies distribution on structure--sand bodies are more stable than shale masses.
Such contemporaneous faults, known locally as growth faults, are the most distinctive feature of Gulf Basin geology; they are normal faults with a progressive increase in throw with depth, and across which, from the upthrown to the downthrown block, there is great thickening of correlative section. In plan and profile, faulted masses resemble land slides; fault planes are concave gulfward and upward, the dip decreasing with depth until the fault plane parallels the bedding plane. Movement on the fault rotates the block, causing reversal of the dip of beds in the block and sealing the landward ends of included aquifers to discharge of waters of compaction--producing the geopressured conditions so common in the Gulf Coastal Plain and Continental Shelf. Development of geopressure in the block early in its tectonic history reduces resistance to shear, and progressive loading on the landward side of the block sustains the rotational stress. Flow of geopressured water into the fault zone--the easiest route of escape--reduces drag and facilitates movement on the fault.
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