Print this pageThe Effect of Cm-Dm Scale Erosive Substrate Topography from Turbidity Flow Structure
Joris T. Eggenhuisen1, Bill McCaffrey1, Rob Butler3, and Peter Haughton2
1School of Earth and Environment, University of Leeds, Leeds, United Kingdom
2Department of Geology, University College Dublin, Dublin, Ireland
3School of Geosciences, University of Aberdeen, Aberdeen, United Kingdom
Small (cm-dm) scale erosive bases are commonly observed under turbidite beds. Though these features may seem innocuous, they are significant as a common source for shale clasts and as windows creating sand-sand amalgamation and thus vertical connectivity between extensive sheet geometries. An important, question is to what extent feedback from and interaction with this self-generated topography affects the structure of turbidity flows.
In this presentation, field and laboratory data is presented that demonstrates the increase in turbulence intensity that is generated in flows that flow over erosive scours they generated themselves. Field data from the Miocene “Macigno costiero”, Tuscany, Italy, and the Grés d’Annot, southeastern France, are presented. In an analysis of paleoflow indicators around erosive topography in these examples, it is shown that flow directions at the base of the flows are changed 10-15 degrees over horizontal distances of few meters.
Experiments with subaqueuos particulate gravity currents have been carried out in which the velocity structure of the flow is mapped using Ultrasonic Velocity Profiling (UVP). These experiments are analysed with particular attention for changes in turbulence caused by flow over substrate topography with vertical scales of a few percentage points of flow depth. Results, as with the field data, indicate that there is an increase in turbulence intensity associated with flow over topographic irregularities.
The described substrate-flow interaction is essentially an internal turbulence production mechanism that can be expected to increase sediment transport capacity and run-out distances of turbidity flows.
AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas