Diagenesis of the Miocene Obispo Formation, Coast Range, California

Abstract

Transportation affects sedimentary particles both by abrasion and by progressive sorting. The latter factor has not received the attention it deserves, and probably is more important than abrasion under some conditons. The effects of abrasion and sorting may lead to similar or to opposite results; in either case, studies of the effects of transportation under natural conditions are incomplete unless an attempt is made to distinguish between these factors and to evaluate their relative importance. Sorting according to shape has been especially neglected in sediment studies and the property of shape has too often ben confused with that of roundness. It is essential that these properties be distinguished and separately measured in investigations of the effects of transportation on the form of sedimentary particles. Our knowledge of the effects of transportation, and especially of the causes of these effects, is far from complete. We know that most sediments show a progressive decrease in mean grain size in the direction of transport. Though this effect has often been considered to be due almost entirely to abrasion, it may equally well result from sorting on the basis of size. Stream and beach gravels appear to become progressively rounder with transportation, hut this might be partly the result of sorting according to shape,at least under some conditions, rather than being entirely an effect of abrasion. The predominance of flat pebbles on beaches has also been ascribed both to abrasion and to sorting according to shape. The concept that streams round their transported sands does not appear to be generally true; the available data indicate that stream sands usually remain angular or even become more angular if somewhat rounded previously. Theoretically, stream sands would be rounded if little or no coarse material were present and conditions of wear were gentle, but the writer does not know of a published description of such a case. The rounding of sand grains appears to occur chiefly in areas of sand concentration, that is, in dunes and on beaches. The evidence, however, is not sufficient to prove whether wind or wave action s most effective in rounding sand grains. Dune sands appear to show the best rounding, on the average, of any type of sediment, but this is not valid evidence in favor of the effectiveness of wind abrasion. Some data indicate that the roundness of dune sands is a property largely inherited from beach sands, the higher mean roundness of dune sands resulting from selective transportation of previously rounded grains. Regardless of which agent is most effective, experiments show that grains of sand size are rounded so slowly that well rounded grains probably have been subjected to more than one period of rounding. Progressive changes in the mineral composition of sediments in the direction of transport seem to result from the interaction of a number of factors, besides the addition of material from new sources of supply. These are abrasion, and sorting according to size, shape, and specific gravity. Of these, abrasion appears to be one of the least important with sand-sized grains, but probably becomes increasingly significant with progressively larger particles. Evidently only a few of the commonly accepted concepts regarding the effects of transportation on sedimentary particles can be considered as definitely proved. Some appear to be false; others, though possibly true, have not been firmly established by sufficient evidence. Further research is necessary on almost all phases of this problem