ABSTRACT

The cross-bedded (internal) structure of a laboratory delta delineates successive profiles of the delta front as the latter progresses downstream. Some of the basic variables affecting the frontal profile of laboratory deltas (tabular units of cross-bedding) were studied in a series of controlled flume experiments. The variables isolated for study were: (a) stream velocity and bed shear stress, (b) the ratio of stream depth to basin depth (depth-ratio), and (c) sediment type, as expressed by relative fall velocity. The results of the investigation demonstrate that bed-load transport deposits an angular type of cross-bedding with planar foreset abutting the basin floor with an angular contact. A low velocity of flow or a bed shear stress just above the threshold for particle mov ment favors the development of a planar type of internal structure. Even at higher velocities characterized by a moderate amount of suspension transport, an angular basal contact may still persist provided the basin is relatively deep with respect to the stream (low depth-ratio). The transition stage from an angular to a tangential basal contact is characterized by the deposition of laminae that alternate from planar to incipiently tangential, that is, with the bottomset. As velocity and depth-ratio increase, the frontal profile evolves through the following stages: angularincipiently tangentialtangentialstrong y tangentiallow-angle concave. The comparative behavior of different sediment types in similar hydraulic environments can be evaluated on the basis of relative fall velocity. The latter is defined as the ratio of the median fall velocity of the sediment particles to the average velocity of streamflow. Thus a coarse-grained sediment transported by a current of given strength may yield a planar foreset with an angular basal contact, whereas a fine-grained sediment transported and deposited under comparable conditions may yield a strongly tangential contact. Wave action and small oscillations of base level may produce frontal profiles that are either convex or sigmoidal (S-shaped) in vertical section.