Classification of Bedforms and Cross-Bedding
This publication uses a new classification scheme that relates the geometry of cross-bedding directly to the morphology and behavior of the bedforms that deposited the beds (Fi g. 1). This scheme was developed because existing classifications of bedforms are generally not applicable to cross-bedding, and because existing classifications of cross-bedding do not adequately relate bedding geometry to bedform behavior. The approach toward both the modeling and classification in this publication emphasizes the shape and behavior of bedforms rather than size, flow, or fluid medium, and, consequently, bedforms are not subdivided into such categories as ripples, dunes, or sand waves, and the term bedform is used in a broad sense that includes all cyclic topographic features.
Most classifications of modern bedforms cannot be applied to cross-bedding because the morphologic and behavioral properties that can be determined from observation of bedforms are significantly different from the properties that can be determined from cross-bedding. Specifically, instantaneous observation of bedforms gives a detailed view of morphologic properties such as height, spacing, asymmetry, crestline sinuosity, and trough profile, but gives no indication of changes through time in bedform morphology or of transport-related characteristics such as the relative migration speeds of the main bedforms and superimposed bedforms, spurs, or scour pits. In contrast, cross-bedding commonly contains less information about the morphology of bedforms that existed at any one time but contains more information about morphologic history and transport-related behavior of bedforms. Existing bedform classification schemes generally cannot be applied to ancient bedforms because the ancient morphology is usually too imprecisely known, and, even in those cases where the classification schemes are imprecise enough to be applied or where the deposits are exceptionally revealing, the bedform morphologic history and behavior cannot be included in the classification.
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.