Laterally extensive, sub-horizontal salt sheets are now widely recognized beneath the continental slope of the northern Gulf of Mexico. Because they overlie significant sections of Tertiary clastics which elsewhere in the region produce oil and gas, these sheets are of great interest to the petroleum industry.

Study of a number of salt bodies in the eastern part of the Louisiana slope has led to the conclusion that they are sills. The sills formed as salt from the tops of near-surface diapirs intruded through the shallow, low density slope sediments at depths of less than 1000 feet below the seafloor. Evidence supporting this conclusion is shown in figure 1. In this figure, a series of reflection terminations is evident to the left of the edge of the salt sill (lower arrow). The reflection terminations represent depositional onlap onto what was once a bathymetric high. Intrusion of the sill uplifted the overlying sediments and created the bathymetric high. An estimate of the depth below the sea-floor at which intrusion occurred can be obtained by tracing the onlap surface (upper arrow) onto the top of the sill and measuring the thickness of sediments between the onlap surface and the top of the salt. Similar measurements made at 53 different sill edges showed that, in all cases, intrusion occurred at burial depths of less than 1000 feet with a mean emplacement depth of 390 feet below mud-line.

Structural and depositional patterns within the sediments above and around the sills, combined with limited well control, indicate that the sills preferentially intrude into nearly pure muds and spread initially as thin sheets which reach their maximum extent rapidly.

A proposed sequence for sill formation is shown in figure 2. This sequence begins when the density directly over the source salt exceeds the salt density and a diapir begins to form. Assuming the average slope sediment densities, this should occur at about 4900 feet below mud-line. Given an adequate supply of salt, the diapir reaches the surface only when the average density of the sediments above the source salt equals or exceeds the density of the salt. On average this requires about 12,000 feet of sediment overburden.

At this point, observation suggests that a sill will be initiated only if the near-surface sediments around the diapir are nearly pure mud. Sediment strength most likely

Figure 1.

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plays a significant role in controlling sill initiation. Once started, the sill spreads rapidly to its maximum extent, probably as a thin sheet on the order of a few hundred feet thick.

Further salt addition increases the thickness of the sill, but rarely appears to change the extent of the sill. Thickening continues until either the source of deep salt is depleted or a new density inversion is established above the sill.

In either case, when silling stops, remoblization begins. If the salt source is depleted and no density inversion has been established, irregularities on the top of the sill tend to dampen out or become inverted. If a density inversion has been established, irregularities on the sill become focal points for the growth of new diapirs and the cycle begins again.

Figure 2.

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