Utility and Variability of Bed Thickness Stacking Pattern in Submarine Lobes: Examples from the Laingsburg Formation, Karoo Basin, South Africa
Within small-scale outcrops, core, or well log datasets,
thickening upward packages of sandstone beds are often used as a criterion to
interpret lobe deposits in submarine fan systems. However, terminal lobe
deposits from the Laingsburg depocentre, south west Karoo Basin, demonstrate
that a full range of bed stacking patterns exists within stacked lobes (lobe
complexes). The exposures allow accurate measurements of individual bed
thicknesses and therefore an insight into bed thickness trend at lobe scale. The
definition and recognition of lobes is based on the nature of bounding surfaces
across which there are abrupt sedimentary facies changes and their lateral
extents, rather than on the thickness, dominant lithofacies and bed thickness
trend of the package between these two surfaces. Three outcrop sections through
Unit A of the Laingsburg formation were chosen at various localities along the
Baviaans syncline (called Krans, Wilgerhout and Buffel) along with a research
borehole (BAV 1B). The five end members of bed stacking patterns recognised and
described in this study are named: thickening upward, thinning upward,
thickening-then-thinning upward, thinning-then-thickening upward, and constant
bed thickness. Observations show that bed stacking patterns within a lobe can
vary over short distances (<km) and that it is not related to the location
of sampling. The bed stacking pattern of lobes is interpreted to be controlled
by the internal organisation of smaller scale components, i.e. individual beds
- bedsets and lobe elements, rather than simple widespread and ubiquitous
progradation as implied by thickening upward patterns. An understanding of the
significance of bed stacking pattern at lobe scale is crucial to better help
recognition and interpretation of individual lobes within poorly exposed
outcrops and subsurface dataset.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California