Geologic Development and Characteristics of the Continental Margins, Gulf of Mexico

ABSTRACT The continental slope of the Gulf Basin covers an area of more than 500,000 sq km and consists of smooth and gently sloping surfaces, prominent escarpments, knolls, intraslope basins, and submarine canyons and channels. It is an area of extremely diverse topographic and sedimentologic conditions. The slope extends from the shelf break, roughly at the 200 m isobath, to the upper limit of the continental rise, at a depth of 2800 m. The most complex province in the basin, and the one of most interest to the petroleum industry, is the Texas-Louisiana slope, occupying an area of 120,000 sq km and in which bottom slopes range from less than 1 degree to greater than 20 degrees around the knolls and basins. The near-surface geology and topography of the slope are functions of the interplay between episodes of rapid shelf edge and slope progradation and contemporaneous modification of the depositional sequence by diapirism. Development of discrete depo-centers throughout the Neogene results in rapid shelf edge progradation, often in excess of 15-20 km per million years. This rapid progradation of the shelf edge leads to development of thick wedges of sediment accumulation on the continental slope. Slope oversteepening, high pore pressures in rapidly deposited soft sediments, and changes in eustatic sea level cause subaqueous slope instabilities such as landsliding and debris flows. Large scale features such as shelf edge separation scars and landslide related canyons often result from such processes. Application of sedimentary load to pre-existing sediments results in salt and shale diapirs and associated faulting. Slope sediments are uplifted, folded, fractured, and faulted by diapiric action. Local oversteepening on the diapiric flanks and near faults causes additional slope instabilities. Petrogenic and biogenic gas seepage along faults and diapiric induced discontinuities lead to gas and clathrate accumulations in the near-surface sediments. Sea floor erosion and development of low sea level carbonate bioherms often occur on the summits of the diapirs. The intraslope and interdiapiric basins form contemporaneously with diapiric growth, resulting from salt and shale withdrawal. They are commonly the sites of thick accumulations of Neogene sediments derived from the outer shelf and flanks of the neighboring diapirs. The base of the continental slope is marked by prominent features such as Escarpments and fan lobes. The Sigsbee Escarpment is the expression of the lobate frontal edge of the northern Gulf diapiric province and is underlain throughout its length by a series of complex salt ridges, overthrust tongues, and steep-sided salt massifs. The continuity of the escarpment is broken locally by several large pronounced reentrants and diapiric outliers.