Abstract: Testing the Concept of Diapiric Rise and Fall Using 3D Seismic Data from the U.K. Southern Gas Basin

Robert J. Hooper, Colin More, John Colleran

The U.K. Southern Gas basin experienced a long and complicated developmental history - involving extension, salt tectonics, inversion, and subsidence - that began in the early Triassic and continued into the middle Tertiary. In our study areas, faulted Paleozoic rocks (Rotliegendes Formation and older) are separated from a faulted overburden sequence (Bacton Group to Chalk) by the salt-dominated Zechstein Group Overburden structure is dominated by a series of broadly NW-SE trending graben and associated salt diapirs and walls.

New insights into salt tectonics derived from scaled analog and numerical model studies, reveal that in areas with originally thick salt, regional extension of the overburden is important in triggering and controlling diapirism and graben growth. Diapiric development typically occurs in two stages - rise (or piercement) and fall (collapse). Diapiric piercement proceeds in a predictable series of modes - reactive, active, and passive. It must be appreciated that the mode of piercement can change along the trend of a graben-diapir system and can also change through time. As extension continues, if for some reason the salt supply becomes restricted, the diapir can no longer grow and begins to fall. The diapir can then no longer support the graben floor and the graben collapses into the f lling diapir. The change from diapiric rise to diapiric collapse is often marked by fundamental changes in sediment distribution patterns.

The general developmental history of our study areas can be explained in terms of concepts derived from experimental data. We interpret graben and diapiric growth to be initiated by gravity-driven thin-skinned brittle extension of the overburden. Continued extension led to a widening of both graben and underlying diapirs. Most diapirs developed in a reactive mode though we observe considerable variation in diapiric "style" both along the trend of any given graben-diapir system and also through time. The supply of salt to the developing diapirs eventually became restricted and continued extension lead to diapiric collapse. The magnitude of collapse varies along the trend of any given graben-diapir system ranging from relatively minor (accompanied by the development of small salt horns) to total (with graben floors and flanks welded to the underlying basement). Collapse is marked by fundamental changes in sediment distribution patterns. While all graben continued to act as depocentres, many graben flanks, areas that were originally high, became syn-collapse depocentres; many intergraben troughs, areas that had acted as depocentres during diapiric rise, became starved and received little sedimentation during diapiric collapse.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France