Sedimentary Structures and Bedding along Ephemeral Streams: ABSTRACT

Abstract

Sedimentary structures are useful for interpreting conditions of deposition of sediments. Experimental studies utilizing flumes indicate the possibility of quantitative interpretations that are based on sedimentary structures. In order to bridge the gap between experimental studies and sedimentary rocks, ephemeral streams in northeastern Utah have been observed. Ephemeral streams serve as natural experiments, exhibiting End_Page 357------------------------------ all the complexity of real processes and some of the control of the laboratory. A variety of structures was observed along stream courses. With few exceptions, all of the structures resulted from infrequent floods and were developed during waning phases. In contrast, most of the bedding was deposited during maximum flood conditions. Erosional structures observed include various types of scours, rills, tool marks, microterraces, and mud balls. Structures produced by transportation and deposition were ripple marks (10 varieties), lineations (3 varieties), and imbrication. Postdepositional structures include desiccation cracks (6 varieties), surface markings, salt-crust features, algal mats, and sand volcanoes. Bedding types observed were horizontal parallel and discontinuous, small- and medium-scale trough cross-stratification, medium- and large-scale low angle wed e cross-stratification, inclined channel fill-bar complex, ripple stratification, high angle avalanche front cross-stratification, and convolute bedding. Within the channel, inclined bedding is dominant, but on bars, low angle cross-stratification is the most abundant type of bedding. Several factors combined to control the occurrence of specific structures and bedding types. Stream velocity is the most important factor, but particle size, time, water depth, and local channel irregularities are also significant. End_of_Article - Last_Page 358------------