Bedforms and cross-bedding can be simulated mathematically using sine curves. SiSimulated structures, like real structures, fall into four categories: invariable two-dimensional bedforms and cross-bedding, variable two-dimensional bedforms and cross-bedding, invariable three-dimensional bedforms and cross-bedding, and variable three-dimensional bedforms and cross-bedding. The structures in each category can be distinguished in three-dimensional outcrops and in polar plots of cross-beds and bounding surfaces, but the structures often are indistinguishable in two-dimensional exposures.
Variability of bedforms can result from two processes: flow fluctuations that cause systematic changes to entire populations of bedforms, and migration of superimposed or intersecting bedforms in flows that may be steady. Cross-bedding produced by these two processes can often be distinguished by the relations between the dip directions of cross-beds and bounding surfaces within the cross-stratified beds. Structures produced by bedforms that change morphology or path of climb in response to flow fluctuations have cross-beds and bounding surfaces with the same strike. Structures produced by superimposed bedforms have cross-beds and bounding surfaces with differing strikes, because the crestlines of the superimposed bedforms are unlikely to parallel the crestlines of the main bedforms exactly.
Cyclic cross-bedding produced by cyclic flow fluctuations is useful for determining flow velocities. Cyclic cross-bedding produced by superimposed bedforms is useful for determining flow directions; structures deposited by bedforms that were oriented obliquely to the sediment transport direction can be recognized by along-crest migration of superimposed bedforms.
Figures & Tables
The computer modeling that forms the basis for this publication was undertaken to relate the geometry of cross-bedding to the morphology and behavior of bedforms. Using computers for this purpose is necessary only because sedimentologists cannot adequately visualize the geometry arising when complex, changing surfaces move through space. Images range from the complicated, which will be of interest to sedimentologists who have had experience interpreting cross-bedding or studying the behavior of bedforms, to simpler illustrations the can convey an understanding of the origin of cross-bedding geometry even to those not experienced in the field.