Print this pageAnatomy, Geometry and Volume of Submarine Lobes, Karoo Basin, South Africa
Terminal submarine lobes are the down-dip depositional record of sediment gravity flows transported through continental margins. Modern and shallow seismic datasets provide excellent images of planform timeslices but limited information on their internal architecture. This work aims to construct the depositional architecture of a submarine lobe in terms of the distribution of lithofacies and stacking patterns, and interpretation of sedimentary processes using well exposed lobe deposits in the Karoo Basin. This research utilizes several behind outcrop wells to demonstrate how understanding the anatomy of a lobe can refine subsurface interpretations.
Lithofacies, facies associations and their lateral variations define several environmental settings in a single lobe: axis, off-axis, fringe, and distal fringe. Facies transitions in lobes generally show an increase in bed stratification and tractional structures from axial to fringe settings. This is not a linear transition as multiple zones of increased amalgamation (axes) are identified along strike in a single lobe. The transition in lithofacies across a lobe is controlled by the stacking of lobe elements, the topographic template inherited from the previous lobe, and the points of erosion at the base of the lobe. Bed thickness trends in a lobe are controlled by the stacking of lobe elements and the environmental setting, such that thinning- and thickening-upward tends can be identified in the same lobe along strike. The anatomy of a lobe is not a simple stack of sand-prone sheets; rather a complicated lithofacies pattern that indicates control by subtle and dynamic surface topography on sediment gravity flow behaviour.
Results from the Tanqua depocentre are compared to other high resolution datasets, from different basin configurations, sediment supply (calibre and rate), tectonic setting, seabed topography, age, and delivery system. Despite these differences in all cases lobe deposits are characterised by two distinct planform shapes that are related to the amplitude of basin floor topography. Secondly, average volumes of lobes from the different systems are comparable and they are not influenced by the scale of the system, which indicates a depositional control on the size of a lobe before a new one is generated. Finally, compensational stacking patterns between different elements of the hierarchy is typical but the exact organisation of individual elements seems to be specific to a system.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009