TOPOGRAPHY OF THE CONTINENTAL SLOPE IN NORTHWEST GULF OF MEXICO

Topographic charts of the continental slope in the northwest Gulf of Mexico were constructed largely from soundings from unpublished hydrographic surveys of the U. S. Coast and Geodetic Survey. The configuration of the slope and the nature of its sediments suggest a genetic interpretation for its varied topography and high relief.

Twenty-one profiles show that the break in slope at the edge of the continental shelf is marked only by a gradual steepening. A hummocky zone on the upper part of the slope shows features resembling landslide scars and deposits, sediment-flow scars, and surface expressions of faults. These features indicate that the upper part of the slope is underlain by a mass of relatively unstable sediments, probably deposited during the Pleistocene when sea level was near the present 75-fathom curve.

Steep slopes and a lack of hummocks on the lower part of the slope indicate that most of the underlying sediments are relatively stable. Many steep-sided troughs cut this portion of the slope. Their position and configuration suggest a history which may involve (1) subaerial erosion, (2) submergence and tilting, (3) masking of the upper part of the slope by Pleistocene sediments, and (4) extensive modification by the failure of sediment under the stress of gravity.

The Sigsbee Scarp, at the base of the slope, is at least 340 miles long and as high as 4800 feet. It may represent one of a series of large faults surrounding the Gulf of Mexico and downthrown toward the Sigsbee Deep.

The Sigsbee Deep is a basin of gentle slopes and subdued relief extending from the base of the Sigsbee Scarp, at 1700 fathoms, to a maximum depth of about 2200 fathoms.

Although salt domes may underlie a number of features of low relief on and near the edge of the continental shelf, there is no evidence which indicates they have been an important geomorphic factor on the continental slope.

The topography and sediments suggest that in the northwest Gulf of Mexico there is a large mass of sediment under the stress of gravity and failing by landslides, faults, creep, sediment flows, and intermediate types of failure.