Lateral Variability of Turbidite Beds Close to Confining Slopes
Turbidity currents can be diverted or ponded by sea floor topography. The resulting onlap of turbidite beds against confining slopes can display a wide range of geometries. Two end-members have been recognised; the sandstones can abruptly pinch-out or they can show an aggradational geometry thinning and draping up-slope. Different controlling factors have been suggested (e.g. flow magnitude, ratio of slope aggradation to turbidite aggradation, flow incident angle) to explain the various stratal architectures. The onlap of turbidites onto slopes can provide effective stratigraphic traps for hydrocarbons. A key issue is whether it is possible to predict sand quality and heterogeneity close to sand bed pinch-outs.
Other factors that are important in considering onlap style are the slope morphology and the character of incident flows. A number of examples from the Annot Sandstone (SE France) and the Tabernas Basin (SE Spain) provide insight into the interplay between these controls. Three different scenarios were examined: 1) a slope characterised by terraces and steps (due to tectonism), 2) a shallow slope with high rugosity (the upper surface of a remobilised unit), and 3) a steep slope (fault-related topography). A wide range of flow types impinged against these confining bathymetries, including low and high density turbidity currents, debris flows and transitional flows, with both turbulent and laminar behaviour.
Onlap style is described and interpreted for each scenario. In case 1, sandstone beds thin gradually over the terraces and rapidly pinch-out against the steps. Abundant transitional flows results in high facies variability over a wide area next to the slope. In the second scenario, individual bed thickness is very variable as sandstones can thin or pinch-out against the rugosity, leading to poor lateral sand connectivity. Slope rugosity promotes enhanced flow turbulence resulting in highly structured parallel and ripple laminated thick beds with repetitions of characteristic sequences of sedimentary structures. In the third example, little or no thickness variation occurs before beds terminations. Narrow confinements relative to the flow scale reduce the likelihood for lateral facies variation.
This study provides a series of ‘rules’ relating slope geometry and flow types to lateral facies and thickness variation in turbidite beds pinching out against lateral confining slopes.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009