ABSTRACT

Progressive deformation of prograding wedges over a ductile layer above a flat or stepped basement is clearly illustrated by a series of analog model experiments. Regardless of density relations, initially planar sand wedges were deformed into sigmoidal clinoforms by differential loading and salt expulsion. Using observations from these experiments, we reinterpret similar structures in various basins and compare them with salt structures imaged with reflection seismic data in the Gulf of Mexico basin. In the Fundy Basin, Nova Scotia, outcrop structures resemble our models in which the base of the ductile layer is flat. The ductile layer is an Upper Triassic mudstone that flowed below the fluvio-deltaic sandstone. The sandstone was deformed into expulsion rollovers and turtle anticlines. Locally, mud diapirs extruded over the overlying fluvio-deltaic sandstone. Palinspastic restoration of a section from the Norwegian Danish graben illustrates the effect of the basement steps on the evolution of salt structures. A series of southwest-younging salt structures formed above the northeast-dipping basement steps during southwestward progradation of the lower Mesozoic wedges over the Permian Zechstein Salt. Expulsion rollovers formed in the proximal flanks of these structures. In the Cantabrian basin in northern Spain, lower Jurassic to lower Tertiary wedges over the Triassic Keuper Salt triggered diapirism that decreases in age, amplitude, and maturity toward the interior foreland basin. Wedges in the proximal flanks of highly mature diapirs were deformed into expulsion rollovers; wedges above more distal, immature anticlines were less deformed and preserved their depositional downlap geometry.