Abstract

Ripples are the smallest sedimentary structures formed on sand by moving fluids. In the past, they have been interpreted only as current-formed or wave-formed. Interpretation can be significantly improved by careful examination of several aspects of ripples that are preserved on bedding surfaces or in stratification viewed on vertical sections. Water velocity or energy can be estimated from current ripples; wave dimensions and periods can be deduced from wave ripples; and relative importance of currents and waves can be evaluated from combined-flow ripples. These indications of hydraulic environment obtained from ripples can be used in stratigraphic interpretations in much the same way that ecologic interpretations of fossils are used. For example, ripples may indicate that the environment was dominated by waves and that those waves probably formed in an open sea because the wave periods are long and the dimensions large.

Interpretation of ripples depends upon observations of continuity of crests, degree of orientation of crests, variability of height and spacing, and symmetry, angularity, and slope of profiles. These characteristics are readily observed on bedding surfaces but can also be inferred from fragmentary evidence preserved in small samples of ripple cross-stratification.

Ripples formed by currents just capable of moving sand grains have relatively continuous but sinuous crests, uniform height and spacing, and asymmetric, angular profiles with downstream faces that slope at the angle of static repose. As current energy increases, crests become shorter and more curved and height and spacing become more variable. At still higher current energy, ripple-covered small dunes with heights exceeding 3 cm form. Wave ripples have long, straight crests, symmetrical rounded profiles, and very uniform height and spacing. Ripples produced by combinations of currents and waves bear some characteristics of both hydraulic influences. Their profiles are asymmetric but rounded; other characteristics such as spacing, slope of steeper faces, and continuity of crests reflect the relative energies of the current and wave components. Ripples experimentally formed under a variety of hydraulic conditions are illustrated in Plates 1 to 6 and described in Part I of this paper.

In Part II, other experiments and theoretical considerations are used to develop an interpretation scheme for a broader range of sand sizes and hydraulic conditions. This scheme is summarized in Figure 14. Limitations on interpretation remain; ripples composed of silt or very fine sand are difficult to interpret, and the influences of complex combined flows and rapid deposition are not adequately known. However, the improvement in interpretation described here allows the geologist to suggest conditions of ripple formation and test depositional hypotheses by means that have not been previously available.

First Page Preview

First page PDF preview
You do not currently have access to this article.