SMALL-SCALE SPATIAL VARIABILITY IN TURBIDITY-CURRENT FLOW CONTROLLED BY ROUGHNESS RESULTING FROM SUBSTRATE EROSION: FIELD EVIDENCE FOR A FEEDBACK MECHANISM

Analysis of turbidity-flow processes from turbidite outcrops remains difficult. In this paper the distribution of orientations of paleoflow indicators around scours a few centimeters to a few decimeters deep is analyzed. Two examples are presented; one from the Gres de Champsaur Formation in southeastern France and one from the Macigno Formation in northern Italy. The orientations of paleoflow data sets on either side of the scours are analyzed and differ 10-15 degrees over horizontal distances of only a few meters. This mismatch is argued to represent a rotation of the flow whilst it flowed over the scour. The reconstructed rotation is a new example of a feedback mechanism involving interaction between the current and its substrate. Paleoflow indicators only record flow direction at the base of the flow. However, the flow-substrate interaction can be expected to affect the complete turbulent velocity structure of the flow, so that erosion by frontal parts of the current influences the suspended-sediment transport processes of hindward regions of the current.