--> ABSTRACT: Emplacement and Evolution of Salt Sills in Northern Gulf of Mexico, by Thomas H. Nelson and Lee H. Fairchild; #91022 (1989)

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Emplacement and Evolution of Salt Sills in Northern Gulf of Mexico

Thomas H. Nelson, Lee H. Fairchild

Extensive subhorizontal bodies of salt encased in young sediments are now widely recognized beneath the outer shelf and slope of the northern Gulf of Mexico. High-quality reflection seismic data indicate that these bodies are sills formed as salt from near-surface diapirs intrudes the adjacent sediments at depths of less than 1,000 ft below the sea floor. More limited stratigraphic data suggest that the sediments into which these sills intrude are dominantly low-density shales.

Intrusion of these sills uplifts the overlying sediments and creates a bathymetric high onto which subsequent deposits thin and onlap. The development of this high and its eventual burial are recorded in the patterns of deposition and provide direct information on sill emplacement. Analysis of such data over more than 50 sills shows that the sills spread rapidly to their maximum extent as fairly thin sheets. After spreading, sills continue to thicken, with final thicknesses ranging from about 2,000 to 11,000 ft. Increase in sill thickness is accommodated partly by compaction of the underlying sediments, partly by a rise in the top of the sill, and, in many cases, by collapse of the underlying section resulting from deep withdrawal of salt to feed the developing sill.

We conclude that the formation of salt sills is a normal part of the evolution of buoyant diapirs that reach the surface in a slope environment where rapid deposition of low-density, low-strength shale is occurring. In the progradational environment of the Gulf, we should thus expect to find remnants of similar sills in the deep section beneath the shelf and perhaps even onshore.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.