Abstract: Nonbuoyant Diapir Emplacement

Richard S. Bishop

The buoyancy theory long has been regarded as the principal mechanism of diapir emplacement; this theory requires a density inversion for diapirism to occur. However, if the overburden is cohesive rather than viscous, diapirism may occur without a density inversion. The pressure causing nonbuoyant diapirism depends on the ratio of densities between the overburden and the mobile substrate (^rgr_o/^rgr_ms) rather than the difference of densities (^rgr_o - ^rgr_ms). The nonbuoyant theory describes piercement diapirism in unfaulted clastic overburdens overlying laterally extensive, viscous mobile substrates.

Piercement diapirism occurs in three ways: extrusion, vertically unconstrained intrusion, or vertically constrained (forceful) intrusion. Extrusion can occur only with a density inversion, i.e., a ratio of overburden to mobile substrate density greater than 1.0. Intrusion, on the other hand, will occur with a ratio less than 1.0. Vertically unconstrained intrusions are free to grow as overburden accumulates. Vertically constrained intrusions are not; they grow by fracturing and then shoving aside the overburden. The depth to an intrusion determines whether it is constrained or unconstrained. For a constrained diapir, the overburden thickness also is important because it determines the direction of overburden failure. Thin overburden results in vertical but not horizontal growth; thick overburden results in horizontal expansion before vertical growth.

This theory shows that salt diapirs can pierce thicker overburdens than shale diapirs. During burial, the ratio of salt to the overburden density increases, and the salt diapir remains unconstrained vertically throughout sedimentation. In contrast, the ratio of shale density to its overburden may decrease during burial, causing intrusion at a slower rate than sedimentation. This results in deeper burial vertical constraint and perhaps cessation of shale diapirism.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC