ABSTRACT: Deformation of Prograding Wedges above a Ductile Layer--Applications of Physical Models to Geologic Examples

GE, H., M. P. A. JACKSON, B. VENDEVILLE, M. MALER, and J. HANDSCHY

Physical modeling illustrated progressive deformation of prograding wedges above a ductile source layer (evaporites or shale) overlying a flat or stepped basement. Initially planar wedges were deformed into sigmoidal clinoforms by salt expulsion by differential loading regardless of density relations. Using these models and new concepts, we reinterpreted similar structures in various basins and compared them with salt structures in the Gulf of Mexico basin.

In the Fundy basin, Nova Scotia, outcrop structures strikingly resemble those in our model whose base of the source layer was flat. A Late Triassic mudstone layer flowed below the fluvio-deltaic sandstone wedges. These wedges were deformed into expulsion rollovers and turtle anticlines that locally welded to the basement. Locally, mud diapirs extruded above the overlying sandstone.

Restoration of a section from the Norwegian Danish graben illustrated the effect of the basement steps on the location and evolution of salt structures. A series of southwest-younging salt structures formed above the northeast-dipping basement steps during progradation of the early Mesozoic wedges above the Permian Zechstein salt. Expulsion rollovers formed in the proximal flanks of these structures.

In the Cantabrian basin in northern Spain, Early Jurassic to early Tertiary wedges above the Triassic Keuper salt triggered diapirism whose age, amplitude, and maturity decrease toward the interior foreland basin. Wedges in the proximal flanks of the highly mature diapirs were deformed into expulsion rollovers, wedges above immature salt anticlines were less deformed and preserved their depositional downlap geometry.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana