ABSTRACT

In fluvial, cross-bedded sediments, the orientation and plunge of the long axes of pebbles are found to be guided, among other factors, by the initial slope of the depositing bed; and this plunge and orientation may vary in the different portions of the cross-strata. Elongate pebbles, in general, tend to be arranged transversely to the current direction on the topset and bottomset units having extremely low initial dips, and arranged longitudinally on the foreset units having greater initial dips.

Plunge and imbrication of pebbles are controlled primarily by the development of "current crescents" around the pebbles in a stream bed. Plunges of the long axes of the transverse pebbles on the topset and bottomset units are usually low and are not generally related to the current direction. On the foreset units, long axes of pebbles plunge markedly towards the general current direction. Various factors operating in the uppermost and lowermost parts of the forest unit destroy the preferred orientation in these zones.

Interpretation of paleocurrent direction from pebble data should take into consideration these findings. In a cross-stratified rock-body, pebble-orientation data obtained only from the central part of the foreset would be most dependable for paleocurrent interpretation.