Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
The Stratigraphic Transition from Slope to Shelf, Karoo Basin, South Africa
(1) University of Liverpool, Liverpool, United Kingdom.
In basin fill successions at outcrop, direct regional observation of the transition from slope to shelf is typically limited by exposure size. The ~900 m thick Waterford Formation (Permian Ecca group in the Laingsburg area, South Africa) is exposed along the limbs of a series of east-west trending post-depositional synclines and anticlines that trend parallel to depositional dip. Logged sections have been correlated by walking parasequence flooding surfaces along both limbs of a fully exposed west-east trending syncline for ~33 km. This has enabled quantification of dip-related changes in facies, thickness and depositional environment, linking shoreface sandstones with upper slope channel features. Stacked coarsening and thickening upward cycles in the lower Waterford are typically 10 - 50 m thick and characterized by a range of soft-sediment deformation features (debrites, slide deposits and foundered beds) overlain by sharp-based sandstones with HCS and current ripples, indicative of a mixed influence shoreline; The upper Waterford comprises alternating 5-15 m thick sharp topped and sharp based units of massive siltstone and thick bedded sandstone with only minor soft sediment deformation. The observed divisions are interpreted as different broad environments of deposition, representing a large scale upward transition from prodelta, through stacked highly unstable shelf-edge delta, to more stable shelf deposits. Internal geometries of individual cycles reveal intrashelf scale clinoforms geometries. Correlation of multiple sedimentary cycles reveals large low angle basin margin scale clinoform geometries.