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