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The Transfer of Channel-Lobe Transition Zones Into the Stratigraphic Record: A Synthesis of Exhumed Examples From the Karoo Basin, South Africa

Abstract

Seabed imagery indicates that modern submarine channel-lobe transition zones (CLTZs) are characterized by a distinctive assemblage of erosional and depositional bedforms in base-of-slope settings. In the subsurface, base-of-slope settings are attractive reservoir targets because abrupt decrease in the confinement of turbidity currents across CLTZs is thought to result in rapid deposition, and the accumulation of amalgamated sandstone successions. Outcrops and research boreholes from well-constrained base-of-slope settings in the Karoo Basin, South Africa, are used to consider the transfer of CLTZs into the stratigraphic record and to discuss their potential as reservoir targets. The studied base-of-slope settings contain several distinctive features. Erosional bedforms are common, and range from meter- to 20 m-deep. The orientation and shape of the basal surface with respect to paleoflow, and the facies and architecture of the infill, support an interpretation as scour-fills. Distinctive depositional bedforms that thicken then thin downstream contain individual beds with abrupt changes in sedimentary facies that are cut by upstream-facing internal erosion surfaces. Their sedimentology suggests net deposition from turbidity currents with large hydraulic fluctuations. These beds stack to form lenticular sandstone packages interpreted as sediment waves. Packages of relatively tabular sandstones, interpreted as base of slope lobes, differ from basin-floor lobes in their stacking patterns and dimensions, the absence of extensive lobe fringe facies, and the high amount of banded sandstone and climbing ripple sandstone. Commonly, successions comprising these distinctive erosional and depositional bedforms are 10's of meters thick, can be mapped for several kilometers downstream, and separate underlying basin-floor lobes from overlying channel-fills. This indicates that the CLTZ in base-of-slope settings can be net depositional. In other areas, channel-levee successions erosionally overlie basin-floor lobes, indicating transfer of the CLTZ into the rock record as a surface. The highly variable stratigraphic expression of CLTZs is associated with different accommodation conditions and rates of system progradation. This synthesis of CLTZs has established a range of criteria for their subsurface identification, and highlights the complexity of their depositional architecture, which needs to be captured in geological models of base-of-slope reservoir targets.