ABSTRACT

The importance of gravel fabric lies in its potential aid in determination of source area and genesis of a sedimentary deposit. Gravel fabric in fluvial sediments has received scant attention in the past because its determination is overly time consuming, and because of lack of agreement among workers on the relation between the fabric pattern and the direction of stream flow. A new method is suggested for fabric determination on unconsolidated deposits which is faster than the old procedure, and can be done in the field. Another refinement includes selecting the pebbles to be plotted on the basis of shape (disc- and rod-shape). Such selection results in a clearer fabric pattern with fewer pebble measurements.

Two streams were studied to obtain the orientation of the disc- and rod-shaped pebbles with respect to the direction of stream flow. It was found that discoid pebbles shingle, with their short axes dipping downstream at a steep angle. The long axes of rod-shaped pebbles have a limited tendency to describe a plane (as outlined by the maxima in the fabric diagrams) which dips in an upcurrent direction.

In a further attempt to reduce the time involved and still obtain the sediment transport direction, apparent dip studies were undertaken on partly consolidated ancient fluvial gravel deposits. Utilizing the results from the two stream studies, readings of the dip of the apparent long axes of pebbles were made on intersecting vertical faces of flat lying gravel, to reconstruct the upcurrent dipping plane for the sedimentation unit involved, and thereby determine the direction of sediment transport. Through comparison with other means of flow direction determination, an affirmative check on the apparent dip readings was possible.