UNEXPLAINED GEOLOGIC FEATURES WITHIN THE HYDRATE STABILITY ZONE, UPPER SLOPE MISSISSIPPI CANYON, NORTHERN GULF OF MEXICO

The upper continental slope, Mississippi Canyon, northern Gulf of Mexico, is a region characterized by dynamic geology, including subsidence, uplift, and prograding. It contains producing gas fields, salt ridges, hydrate formations exposed on the seafloor, and an interpreted hydrated and underlying free gas zone. High-frequency seismic reflection data collected from this area reveal widespread geologic features that can be identified in the shallow subsurface from disparate locations and on different data sets but that remain unsatisfactorily interpreted or explained. Unexplained reflector patterns, apparently associated with gas hydrates, are here presented.

Heat flow data, collected from an area of numerous normal faults, include widely ranging values with greater heat flow possibly representing “open faults” conducting geo-thermal fluids to the surface. However, accompanying high-frequency seismic data do not differentiate suspected open faults from non-open faults.

High-frequency data reveal vertical lineaments in the shallow sub-surface in areas of regional inclination. Each lineament extends vertically through the same sequence of horizontal reflectors and terminates as reflectors become indistinct. These “broom” features have been interpreted as short-lived faults originating as sediments descend the inclination, as zones of escaping free natural gas, or both.

High-frequency seismic data show both homogeneity and heterogeneity of reflector characteristics in the upper slope environment. Both the shallowest and the deepest of the ubiquitous shallow reflectors remain relatively homogeneous throughout the region. An intermediate unit, however, shows variability in both thickness and reflector pattern, the latter being so unique that the unit can be correlated across faults and from one profile to another. Remarkable variability in vertical extent suggests that this unit may be accommodating volume changes, possibly due to hydrate expansion. This entire sequence, by analogy with a 28.5 m piston core interpretation in the same physiographic province, appears to represent an interval of, at most, Upper Pleistocene time, and possibly as brief as a single Holocene-glacial couplet.