Grain fabric of experimental gravity flow deposits

Abstract

Grain fabric of deposits accumulated from a high-density surge-type gravity (turbidity) current in an experimental flume was measured. Vertical sequential change (0.2-mm interval) in imbrication shows that a bed can be divided into lower, middle, upper and uppermost parts. The lower part is characterized by both up-current and down-current imbrication with a wide range of angles. Dominant up-current imbrication and rare down-current imbrication characterize the middle part. The imbrication angle of this part tends to be smaller than in the lower part. The upper part is represented by intervals with up-current imbrication. Nearly flat imbrication is dominant in the uppermost intervals. Statistically significant preferred orientation was observed from the lower, middle and upper parts, and it deviates up to 13° in both clockwise and anticlockwise directions from the current direction. The lower and middle parts of the bed may correspond to the Bouma A-division judging from the wider range of imbrication angles and the presence of down-current imbrication which have been reported from natural turbidite beds. The upper part, which is characterized by up-current imbrication, is interpreted to be the Bouma B-division. The uppermost interval may coincide with the D-division. The episodically appearing down-current imbrication in the lower and middle parts can probably be attributed to oscillation of an interface between a denser basal layer and superjacent low-density layer of the turbidity current. Measurement of grain orientations in this and previous studies implies that at least 30° of deviation from the flow axis should be considered for paleoflow analyses based on grain fabrics.