AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Canopy Evolution: Deformation Processes and Subsidence Patterns
(1) Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX.
It is well known that salt canopies form by the coalescence of individual salt diapirs, but the details of canopy growth are poorly known. Our physical model investigates how diapirs merge to form a canopy and react to subsequent sedimentary loading. Fourteen unevenly spaced stocks evolved into five canopies through four stages.
1. Diapirs grew vertically by downbuilding. Withdrawal basins around individual diapirs merged to form a composite withdrawal basin that eventually encompassed the entire field of diapirs. As diapirs continued to grow, this withdrawal basin deepened to form a regional autochthonous weld.
2. Diapirs spread asymmetrically seaward in response to a 1° seaward tilt. The composite withdrawal basin widened downdip as salt in the source layer flowed seaward inflating its distal region. Because of this additional supply of salt, the seaward diapirs rose most vigorously and created the deepest part of the composite withdrawal basin.
3. Salt sheets coalesced as they advanced seaward and radially expanded. The spreading canopies gradually blocked sedimentary fairways. Salt flow directions depended on local topography, a function of diapiric vigor, and on regional dip. Salt sutures formed between coalescing salt sheets, and fold belts formed in the canopy roof upstream of these sutures. Where a salt sheet overrode its less vigorous neighbor, the suture between them bowed in the direction of override.
4. Progradational loading of the canopies drove salt seaward. During sedimentary loading, distal parts of the canopies inflated at the expense of proximal parts, where minibasins formed. Each inflation-deflation pair defined a flow cell in the canopy. Sutures that formed in Stage 3 as a result of local flow against regional dip became everted after progradational loading reversed salt flow in the canopy. As a result the suture became bowed in an opposite direction.
In summary, the model shows how canopy coalescence is influenced by the differing rates of individual diapiric growth, which can cause local canopy flow to be counter or oblique to regional dip. During subsequent progradational loading, canopy salt is redistributed primarily by seaward flow, but the disposition of individual feeders within a canopy can cause local salt flow oblique to the progradation front.