TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractSands penetrated at shallow depths (< 1 km) in the Gulf of Mexico can flow to the surface and undermine seafloor installations. Here we explore aspects of natural fluid expulsion structures from these "shallow-water sands". Highresolution seismic data shows no obvious sand injection structures from the Blue Unit, a shallow-water sand, in the Ursa area, but perhaps nascent dike formation. Also, channel sands stratigraphically above the Blue Unit are overpressured, with a possible pressure source in the latter. In the Ursa area the maximum principal stress is vertical and constrained by the integrated unit weight of the sediments at IODP Site U1324. The stress necessary to form hydrofractures (the minimum principal stress plus negligible tensional strength of the sediment) is about 85% of the maximum principal stress indicating an approximately isotropic state of stress. High Poisson's ratios (0.46-0.49), determined from wireline compressional and shear wave velocities at Site U1324, are also consistent with a nearly isotropic state of stress at shallow depths. Estimated maximum fluid pressures are about 80% of the total overburden stress, less than necessary to hydrofracture the sediment. Therefore, the fluid pressure is insufficient to form sand injectites. The formation of sand injectites, as observed in the geologic record, may require very high fluid pressures, approaching lithostatic values. Diverse orientations of ancient injectites support the necessity of very high fluid pressures and a nearly isotropic state of stress.

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