ABSTRACT

The Plio-Miocene subsurface geology of Belle Isle dome in coastal Louisiana was studied using electrical logs. The pattern of syndepositional folding, faulting and sand distribution at this shallow piercement salt structure reflects variations with time in the configuration of the paleotopographic mound and indicates contemporaneous intrusive movement and sedimentation. Intrusive growth proceeded from an elongate shale and salt mass (ridge stage) to a buried cylindrical salt plug (deep-plug stage) to a plug which maintained a near-surface position (shallow-plug stage).

Each growth stage has a particular style of folding and normal growth faulting which is related to the shape and burial history of the intrusive. The ridge stage is characterized by movement on tangential faults and intrusive-coincident peripheral faults. The highly convergent beds of the deep-plug stage record the main episode of radial faulting. Shallow-plug stage sediments have few faults and low sedimentary convergence rates. During both plug stages the salt-sediment interface acted as a vertical circular fault.

The shallow-plug stage is associated with slowly deposited non-marine sediments whereas the deep-plug stage is associated with rapidly deposited deltaic sediments. It is postulated that deposition rates largely determined burial depth of plug and that changes in absolute rate of salt movement during plug stages were minor.

The large topographic mound present during the ridge stage and deep-plug stage caused sands to shale-out as they approached the salt, thereby creating numerous stratigraphic traps.