Laboratory Experiments on form and Structure of Longshore Bars and Beaches1
Published:January 01, 1961
Beaches and bars have been formed during experiments conducted in a 46-foot wave tank at the Sedimentation Laboratory of the U. S. Geological Survey in Denver. By changing one variable factor at a time, elements responsible for major differences in primary structure and in shape of sand body have been determined. These elements are—differences in slope of sand floor (expressed in terms of water depth), intensity of wave action, and supply of sand. Stages in the growth of the bars and beaches were marked with dark layers of magnetite, and cross sections were preserved on masonite boards coated with liquid rubber, thus recording cross-stratification patterns and sand-body shapes.
Longshore bars are produced at the point of wave break. In very shallow water an emergent bar commonly forms; in somewhat deeper water a submarine bar is built; and in still deeper water no bar forms. Increase in intensity of waves tends to build a bar toward, and even onto, the beach. Weaker waves build bars upward to form barriers, with lagoons to shoreward. Abundant sand furnished on the seaward side of a growing bar simulates conditions caused by some longshore and rip currents, and causes gentle seaward-dipping beds to form. In contrast, a limited sand supply results in growth of bars that characteristically have shoreward-dipping strata of steeper angle.
Beach strata normally dip seaward at low angles from the crest to a point below water level. Offshore, the seaward extensions of these gently dipping beds include fore-set beds with relatively high angles which form a shoreface terrace. The sand body comprised of both sets of bedding builds outward if a large supply of sand is furnished. In shallow water, however, or at moderate depth where waves are strong, the period of beach growth is limited by the deposition of longshore bars which eliminate wave action as they grow into barriers and form lagoons. Under conditions in which no bar is built, growth of the beach and shoreface terrace is controlled by the amount of sand available; the proportion of top-set to fore-set beds is determined by the strength of waves.
Figures & Tables
Geometry of Sandstone Bodies
This volume contains the eight papers presented as a symposium of the Research Committee of The American Association of Petroleum Geologists at the 1960 annual meeting in Atlantic City, New Jersey. One paper presented in the General Session at that meeting, one reprinted paper, and three other solicited papers are also included.
The choice of “Geometry of Sandstone Bodies” as a timely and pertinent subject lor the 1960 symposium was made after an extensive canvass of Research Committee members and about fifty other geologists vitally interested in research in petroleum geology. From a group of about 15 proposed subjects, this one was selected as first choice by almost all those canvassed. Partly because of this high level of interest, the decision was made to attempt publication of the symposium as a special volume of The American Association of Petroleum Geologists. The word geometry in the title probably had several different meanings among the selectors, and for this reason an attempt was made to define the term adequately in order to establish uniformity of communication among symposium participants.
The dictionary definition of the word “geometry” is the science of magnitudes in space. In applying the term to the symposium theme, some modification and interpretation of its formal meaning were needed, and the following definition was therefore proposed for use in this volume—
Geometry of Sandstone Bodies—Spatial relationships of sandstone deposits within the sedimentary framework.
As used in this book, the subject is more than just a three-dimensional study in which thickness is added to areal distribution.