ABSTRACT

Some cross-beds in coarse-grained sandstone and pebble conglomerate of the Coso Formation (lower Pleistocene) are inclined against the direction of flow of depositing currents. Textural gradation, mineralogical changes, and structural considerations show that the depositing currents flowed westward. About one half the cross-beds are inclined to the east, and the others are inclined to the west. The diametrically opposed directions do not fit the pattern expected of meandering streams. The eastward-inclined cross-beds are called backset beds.

Backset beds are the planar (inclined) type and make an angle of 25° or less with the normal bedding planes. Cross-laminations are long tapered wedges with the wide part of the wedge at the base. Material within the wedge is coarse near the bottom and grades to finer material at the top. Normal foreset laminations are either the inclined or festoon (trough) type, and material within each lamination is well sorted and not graded.

The backset beds are believed to have formed during the antidune phase of sediment transport described by G. K. Gilbert. The factors contributing to the antidune phase of sediment transport are high load, high velocity, high resistance, and shallow depth of water. Inasmuch as the deposits were probably deposited in a manner analogous to the heavily overloaded streams of the Basin and Range province today, all these factors could well have been present.