Environmental Studies Program Direction:
Accelerated Deepwater Activities
Deepwater: Geology
Twenty-five years ago our knowledge of processes
on the continental slope in the northern Gulf of Mexico was so limited that it was viewed
simply as an accreting sedimentary structure. It is now known to be extremely dynamic and
is the international center of deepwater development and expertise. The slope is also
marked by huge troughs or canyons cutting across the slope. These are conduits for
sediment movement.
On the long-term, northern Gulf slope
features are controlled by an interaction of rapid sedimentation and salt tectonics. On
the shorter scale, this long-term complex is greatly modified by erosion, mass movement,
and fluid expulsion. Both long and short-term processes pose significant geohazards.
Shelf edge progradation is an interplay of
rapid sedimentation and salt migration. In Pliocene and Pleistocene periods, sea level
cycled, causing pluvial systems to prograde to the edge of the continental shelf
depositing low sea level deltae. Thus, the edge of the shelf is stacked depocenters. The
Mobile River delta off Alabama and the Brazos-Trinity complex off Texas are prime
examples. The continued transport downslope today from these old shelf edge deltae pose
considerable geotechnical problems to development. The processes carrying material to
deepwater are numerous and poorly understood. Recent advances are linked to new
technology. Processing of seismic signals to see through salt and back stripping have
proven invaluable. These have produced a more realistic picture of salt tectonics.
Superimposed upon the regional complex or
topography the dominating allocthonous salt sheets structure slope basins and major
faulting. Sediment stripping, mass wasting, local faulting, and fluid expulsion are the
main processes of short-term modification. Mass movements are caused by instabilities that
are relatively shallow in the sediment column. Even slowly accumulated hemipelagic
sediment drape, once thought quiescent, has proven to cause thin skin slumping and
sliding. Faulting at all scales is intense over topographic highs.
Fluid expulsion areas, which include
hydrocarbon seeps, contain a range of structures and processes that pose geohazards. These
systems may be viewed as evolutionary in the sense that the feature spectrum can be
related to an age and flux rate model. Expulsion areas may begin as mud-prone systems
which then progress to mineral-prone.
For more information, contact Keith Good.
