Ian Davison¹
(1) Earthmoves Ltd, Surrey, United Kingdom

Abstract: Lithological controls on drag zone shape adjacent to salt diapirs

Drag zones are produced by frictional interaction between sedimentary overburden and salt diapirs and are often the sites of trapped hydrocarbons. The highly-deformed zones are usually less than 500 m wide, similar to the width of drag zones adjacent to large extensional faults. Broad drag zones with large vertical relief ( 100 m) are produced in anisotropic semi-lithified sequences. Weaker layers buffer the drag deformation and protect more external parts of the drag zone from intense faulting. Narrow drag zones (10-50m) with relatively high relief (10-100 m) occur in unlithified sediments of any composition and in claystones, which deform in a plastic manner. Strong layers, such as well-cemented sandstones or limestones, produce narrow drag zones of tens of metres with limited vertical upturn (10’s metres scale). Broader drag zones are often affected by outward-dipping extensional faults and the onset of drag usually coincides with a fault. Large stratigraphic dip changes can occur across these faults with steeper dips in the footwalls. This indicates pervasive shear and bed rotation has occurred on the footwall side, closer to the diapir wall. The innermost fault usually shows the largest dip change, and separates an intensely deformed inner drag zone from a less-deformed zone. Inner drag zones show evidence of intense fluid migration. Examples of diapir walls from Cape Breton Island, Nova Scotia, Paradox Basin Utah, and the Red Sea, Yemen will be compared with extensional fault drag zones from various localities in the UK.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana