DAVID G. MOORE JOSEPH R. CURRAY WAVE-BASE, MARINE PROFILE OF EQUILIBRIUM, AND WAVE-BUILT TERRACES: DISCUSSION Abstract: Some concepts presented in a recent marine profile of equilibrium, a direct corollary, has paper by Dietz (1963) can be enlarged upon with an not yet been attained on most continental shelves attempt to emphasize what are believed to be because of the effects of rapid, wide Quaternary shortcomings. Other concepts can be negated by fluctuations of sea level. newer field evidence. Use of the terms delta, topset, foreset, and bottom- The old concept of wave-base should not be set should be restricted to known marine or lacus- completely discarded but should be modified in trine deltas. In particular, continental slope deposits accordance with present knowledge of the trans- should not be termed foresets unless they are of portation and deposition of sediments. Where known deltaic origin. modern rates of supply are high enough, sediment Dietz's model of the structure of the continental is transported to and deposited on the inner and terrace is incorrect for the surveyed portions off the central parts of continental shelves, generally with coasts of North America. The basic sedimentary decrease of grain size offshore. The gradient in framework of most of these continental terraces particle size cannot be predicted merely by con- appears to be upbuilding by shelf and paralic facies sidering instantaneous conditions but may theoreti- and outbuilding by slope deposition. This basic cally be predicted over the net cyclic period of the structure is locally modified by salt-dome tectonics, region, perhaps in the range between 1 and 100 folding, faulting, and mass slumping. The continen- years. The effects of variations in energy and supply tal slope is believed to be predominantly a realm of and the occurrence of extreme flood or storm con- deposition rather than of turbidity-current de- ditions are thus averaged out. The morphological gradation. abrupt base level below which sedimentary Introduction particles were not stirred by wave action and In a recent paper, Dietz (1963) attempted to above which particles could not come per- update the concepts and use of the terms manently to rest because of wave action. wave-base, marine profile of equilibrium, and We know that this oversimplified concept is wave-built terrace. He very convincingly not valid, but in re-evaluating it, we should not pointed out original definitions, examples of swing the pendulum too far and disregard what misuse, and a recent opinion of the proper we know about the relationship between application and status of these terms. We sub- orbital velocity associated with waves and mit that, whereas much of this discussion was depth of water. apropos and certainly necessary to clarify these Dietz (1963), following Dietz and Menard concepts, Dietz did not sufficiently consider (1951), rejected "wave-base" in favor of newer field data. As a result, he has held to "surf-base" or "surge-base," defined as the some very questionable concepts, particularly approximate depth of effective wave abrasion. regarding the structure of the continental He cites evidence that this effective depth of terrace, and he has failed to reconcile these wave abrasion is a maximum of approximately concepts to the present state of our knowledge. 5 fathoms or 10 m, although wave-cut terraces on bedrock are formed essentially at sea level. Wave-Base We concur in this concept but believe it is The concept of wave-base, popular during important to emphasize that this depth of the early part of this century as an explanation effective wave abrasion is dependent upon the of the origin of the continental shelf, has fallen type of material to be eroded and upon the into disrepute. (See, for example, Dietz and length of time available for erosion. For Menard, 1951; Dietz, 1952; Fairbridge, 1952; example, "surf-base" would be essentially sea Bradley, 1958; Dietz, 1963, Shepard, 1963.) level for a short period of time on a resistant The old concept of wave-base implied an bedrock base; "surf-base" would be much Geological Society of America Bulletin, v. 75, p. 1267-1274, 2 figs., December 1964 1267 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/75/12/1267/3442577/i0016-7606-75-12-1267.pdf by guest on 30 September 2021 1268 MOORE AND CURRAY—WAVE-BASE: DISCUSSION deeper for a longer given period of time or over Once the silt- and clay-size particles were soft, unconsolidated sediment. deposited and remained on the bottom for a Confusion surrounding the wave-base con- significant period of time, higher velocities cept has stemmed partly from two factors. would be required to resuspend them because of First, some earlier investigators spoke of cohesion between the particles. Further com- wave-base in regard to erosion of pre-existing plications in nature are the extreme variability material; others used wave-base in referring to from day to day in wave conditions, the pre- a base of deposition of sediment; and still others sence of and periodic variations in tidal cur- equated these two depths. The processes of rents, and the presence of permanent currents deposition of sediment and erosion of rock and other wind-induced currents. obviously should be considered separately. It is futile, therefore, in dealing with an Secondly, earlier investigators attempted to accumulated column of sediment to speak of equate wave-base, in connection with both instantaneous wave-base or wave conditions; deposition and erosion, with the present edge of we must consider instead the net effect over the continental shelf. The depth of the edge of what we might define as the natural cyclic period the continental shelf is now, however, generally of a region, (Curray and Moore, 1964). This regarded by most marine geologists as relict natural cyclic period of a region, perhaps in the from conditions of lower stands of sea level range of 1-100 years, is that period of time over during the Pleistocene. (See, for example, which we must integrate sedimentary processes Shepard, 1948; 1963; Dietz and Menard, in order to average out variations in wave and 1951; Dietz, 1952.) Also relict from lower other current conditions and variations in rate stands of sea level are many of the coarse of sediment supply. The variations in wave and sediments now found on continental shelves current conditions include normal waves, where not yet covered with sediments in extreme storm or hurricane waves, and coin- equilibrium with present conditions. As cidence of extreme storm conditions on periods pointed out by Emery (1952) much confusion of spring tidal range. These extreme, although has resulted from failure to distinguish these rare, conditions may affect the nature of relict sediments from modern detrital sedi- deposition more than do mild, normal con- ments. ditions. For any simple wave train acting in deep If we now consider this natural cyclic period water, the orbital velocity decreases exponen- and the net rate of sediment supply over this tially with depth of water. As the waves ap- period of time, we can arrive at depositional proach shallow water, this exponential de- equilibrium for a given region. Where the sedi- crease with water depth is modified and dimin- ments involved in this equilibrium are thick ished but can still be approximated theoreti- enough to cover underlying older sediments, cally. Good data are now available on the they normally show a gradation to relatively threshold water velocities necessary to pick up finer sediment farther from shore or source. particles from the bottom and on the energy The process of transportation is discontinuous conditions necessary to maintain them in within the basin of deposition throughout the suspension (Hjulstrom, 1939; Sundborg, 1956; natural cyclic period. It consists of many tem- Inman, 1963; Inman and Bagnold, 1963). porary periods of deposition and "erosion" or If we assume the simplest possible conditions at penecontemperaneous resuspension before sedi- any instant of time, that is, a wave train of mentary particles come to an ultimate resting single wave height and period and a given place and burial. For any given particle size, depth of water, we can approximate the largest there will be an equilibrium depth at which the size of the bed load particles. Thus, over a net rate of deposition will equal the net rate of range of depths, we would have an effective removal, the point of depositional equilibrium. zone of wave-base corresponding to a range of Below this depth zone, net deposition will particle sizes. This effective, instantaneous occur out to the range limit of the transporting "wave-base" would be "shallower" for gravel processes (Curray, 1964). Beyond the range and coarse sand than for medium to fine sand. limit of the transporting processes the present If these simple wave conditions persisted for a shelf surface may be covered with sediments period of time, silt- and clay-size particles relict from previous equilibrium conditions. would be maintained in suspension until they Above the point of depositional equilibrium, reached their effective wave-base on the con- only net transportation of this given particle tinental shelf where they would be deposited. size will result. In actual fact, this situation has Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/75/12/1267/3442577/i0016-7606-75-12-1267.pdf by guest on 30 September 2021 SHORT NOTES 1269 been attained in our modern seas only on nar- of many tens of feet (Moore, 1960), and the row shelves or on the inner parts of some wide shelf may be starting to approach a profile of shelves because of the recency of sea level equilibrium. changes associated with the Pleistocene.