Anatomy and
Stratigraphic Development of a Lower Slope to Base-of-Slope Siliciclastic Wedge
from the Tanqua Depocentre, SW Karoo Basin, South Africa
Van der Merwe, W.1, David
Hodgson1, Richard Wild1, De Ville Wickens2,
Stephen Flint1 (1)
Permian deepwater deposits of the Tanqua
Depocentre, SW Karoo Basin, South Africa, include Unit 5, a 100m thick lower
slope to base-of-slope sand-rich succession, which overlies a 0.2m thick
mudstone and is capped by a 12m thick regionally-developed mudstone. Unit 5
lies above point-sourced sand-rich basin-floor fans, and is overlain by
progradational shelf-edge deltaics. As such, Unit 5 represents the transition from
a bypass- to accretion-dominated slope system. Unit 5 is exposed for 50km down
depositional dip and >20km across strike, which has allowed an extensive
outcrop-based dataset to be collected. This is augmented by four research
boreholes sited away from outcrop.
Unit 5 consists of six discrete stages of
stratigraphic growth. Several points of channelisation occur across the outcrop
belt and indicate a change from point- to line-source sediment supply.
Channel-fills are dominated by structureless amalgamated sandstone, with basal
mudstone clast conglomerates. Deposits away from channel-fills are thin-bedded,
ripple-laminated layered sandstones, and are interpreted to represent a
combination of frontal splay deposits from earlier up-dip slope feeder systems,
and lateral spill deposits formed during channel incision and aggradation. The
ratio of frontal: lateral splay deposits at any section depend upon the
stratigraphic position and spatial location with respect to the genetically
related channel-complex. Sedimentological criteria for establishing the
depositional setting of thin-bedded facies include stratigraphic packaging and
bed thickness (thickening upward trends and greater average bed thickness in
frontal deposits), abrupt changes in palaeoflow, and beds with wedge geometries
and isolated channel forms (lateral deposits).
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California