Hodgson, David1, Stephen Flint1, Richard Wild1, David Hodgetts1, Nicholas Drinkwater2
(1) University of Liverpool, Liverpool, United Kingdom
(2) Schlumberger Cambridge Research, Cambridge, United Kingdom
ABSTRACT: Sequence Hierarchy in Lowstand Deposits of the Karoo Basin, South Africa
Large-scale exposures of deepwater deposits in the Permian Karoo basin of South Africa allow detailed documentation of lowstand basin floor and submarine slope deposits and establishment of a sequence hierarchy. Although biostratigraphic control is insufficient to be certain of sequence durations, the hierarchy of carefully mapped physical stratigraphic units and surfaces has enabled the development of a sequence hierarchy scheme. Presumed 3rd, 4th and 5th order sequences, dominated volumetrically by their individual lowstand components, stack into sequence sets and composite sequences.
Fourth order lowstand basin floor fans 1-4 are 35-60 m thick and stack in an overall progradational manner, with complications due to lateral switching. They are separated by 20-50 m of muddy turbidites and hemipelagic shales, interpreted as transgressive and highstand systems tracts. Each fan is built of 3-17 higher frequency (presumed 5th order) sequences, each comprised of a 2-20 m thick sandy lowstand and 0.5-5 m thick shaly transgressive/highstand systems tracts. These 5th order sequences typically stack into a progradational-aggradational-retrogradational pattern and exert an important control on shale geometries at reservoir scale.
A condensed shale above basin floor fan 4 is overlain by a 100m thick succession of channel and inter-channel deposits. Slumps and creep deposits are common in this Unit 5, which is volumetrically the largest deepwater sandstone complex in the succession. It is interpreted as a lower slope wedge, related to a low order (?3rd order) sequence boundary that represents the abrupt basinward stepping of the slope that restricted basin floor deposition in the Tanqua area through increased accretion.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.