Wave-Base, Marine Profile of Equilibrium, and Wave-Built Terraces: Discussion

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

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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

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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. Even Confusion perhaps has resulted from the fact where attained in either modern or ancient seas, that some investigators refer to topography or a clean separation of particle sizes is not pro- morphology in speaking of a profile of equili- duced because silt- and clay-size particles are brium, whereas other investigators have re- not resuspended easily. Nevertheless, there is a ferred to the nature of the bottom sediments. gradation from coarse to fine in most modern Beyond the nearshore profile of equilibrium, continental shelf sediments where they are in discussed by Dietz, many shelf surfaces of relict equilibrium with present conditions. morphology are indeed covered with a veneer The old concept of wave-base then is grossly of modern shelf-facies sediments in equilibrium oversimplified but should be modified rather with the present condition (Curray, 1960). than rejected. We submit that over the natural The sediments are thus in equilibrium, although cyclical period of a region, an equilibrium a morphological profile of equilibrium has not occurs between deposition and removal for yet been attained. each particle size. This depositional equilibrium is a function of wave and current conditions Wave-Built Terrace and the rate or supply of sediments. Dietz (1963) reviewed the origin of the term wave-built terrace (Gilbert, 1890). Gilbert Marine Profile of Equilibrium clearly referred to shore line and nearshore The marine profile of equilibrium, a direct deposits of sand formed by wave action and corollary to the wave-base concept, has been longshore drift. Regardless of the internal shown by Dietz to apply to modern seas in the sedimentary structure of the continental ter- nearshore region. Beyond this nearshore region, race (coastal plain, continental shelf, and con- many continental shelves are relict both in tinental slope), it should not be called a wave- surface sediment distribution and in morphol- built terrace, in view of Gilbert's original ogy. The effects of wide, rapid Pleistocene definition. Furthermore, today we have better fluctuations of sea level are far reaching. The terminology for shore line and nearshore fea- present configuration of the continental shelf is tures. We urge then, that the term wave-built a result of the transgressions and regressions terrace be abandoned and that the continental accompanying glacial advance and retreat. terrace be referred to simply as the continental Some shelves show a truncation of bedrock terrace or its component parts. In suggesting produced by a migration of the surf zone back the discarding of this term, we concur with and forth across the shelf. Other shelves, in Dietz; however, our reasons for discarding the predominantly subsiding regions or in regions term are different. Dietz proposes abandoning of more abundant sediment supply, show the the term because of a misconception about the effect of deposition of paralic, nearshore, and internal structure of the continental terrace. open marine shelf sediments as the shore line By his reasoning, if a continental terrace struc- and the river sources of the sediments have ture consisting of both shelf and slope deposits moved back and forth across the shelf. In any existed and was formed by transgressions and event, the configuration of the shelf is pri- regressions of the sea, he would apply the term marily a result of these fluctuations in sea level. wave-built terrace. We will show that this is There has been insufficient time since sea level indeed the internal structure of some con- has reached its present position for the attain- tinental terraces, but we see no necessity for ment of any kind of profile of equilibrium applying the term wave-built terrace, in con- except in localized areas of rapid deposition. tradiction to the original definition by Gilbert. These localized areas primarily include nearshore areas or the shore face of the beach, as Delta Terrace pointed out by Dietz (1963) and as discussed Dietz's (1963) discussion of the delta terrace by Inman and Bagnold (1963, p. 530). Beyond correctly differentiates processes that form a this nearshore area, the relict topography is true river delta and those of the so-called wave- in places covered with a thin veneer of modern built terrace. We feel that Dietz did not go far shelf-facies muds and in no sense can be called enough in his critical appraisal of the use of the a profile of equilibrium. Elsewhere modern terminology topset, foreset, and bottomset beds. shelf sediments are known to attain thicknesses Topset, foreset and bottomset are classical

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terms used to describe the fundamental beds of cept that he has added the continental embanJ^ a delta complex. Deltas, by definition, require ment concept. Briefly, Dietz's concept mini- subaerial, fluvial, and marine or lacustrine mizes permanent significant continental slope deposition in combination. We feel strongly deposition and suggests that the slope is a realm that these terms should be restricted to their of general degradation by sliding and turbidity original meaning and should not be used to currents. He further minimizes the importance describe purely marine deposition. In particular, of open continental shelf deposits and would the use of foreset beds to describe continental have the body of the terrace built up of shallow slope deposition is unfortunate and confusing. water and other paralic deposits accumulated as There is a clear distinction between delta foreset a result of subsidence. He explains the sub- beds and slope deposits, and this distinction sidence as isostatic adjustment to the load of should be recognized in the terminology. Off continental rise deposits accumulating at the the Costa de Nayarit, Mexico (Curray and base of the initial, structural continental slope Moore, 1964), for example, both kinds of as turbidites. In an old-age terrace, he postu- deposits are present, and in fact adjoining lates that these turbidites cover the eroded (Fig. 1). In this area foreset beds of a Pleistocene slope face by embankment. delta have nearly prograded across the pre- At the time of its initial publication in 1952, existing continental shelf. Below the foreset Dietz's theory on geomorphic evolution of the beds and beyond the old shelf break are similar- continental terrace was new and challenging. ly dipping, but genetically different, deposits of It explained some known instances of ancient the upper continental slope, which in this case rock outcrops in deep submarine canyons and are the bottomset beds of the delta complex. was in tune with contemporary ideas of tur- It is true that in cases where deltas have built bidity-current action. Re-affirmation of this completely across the shelf and deposited foreset same theory in the 1963 paper, however, shows beds on the upper continental slope, there a disregard of data collected during the past exists the possibility of confusing the two kinds few years. of deposits. However, where deposits are of The recent development of continuous re- known deltaic origin, they should retain delta- flection profiling techniques (see, for example, bed terminology; similarly, where beds are Hersey, 1963) has made possible the observation known to be nondeltaic continental slope of the internal structure of the continental deposits, they should be termed continental terrace in two of the key regions discussed slope deposits, not foreset beds. by Dietz. In the northwest Gulf of Mexico In the case of topset beds, the terminology (Moore and Curray, 1963a), reflection records can be similarly analyzed. Sediments on or un- show conclusively that, at least as far back as der and parallel to the present surface of the the Late Tertiary, the structure is formed by continental shelf may consist of marine, paralic, simple sedimentary upbuilding and outbuilding or continental facies, but they should not be with accompanying subsidence. No evidence of called topset beds unless known to be deltaic. extensive erosion has been found on the upper Likewise, deposits of the continental rise or slope, although complex slope topography has abyssal depths at the base of the continental developed as a result of salt-dome tectonics. slope should not be referred to as bottomset Uniformity and continuity of bedding suggest beds. The continental terrace may contain that open-shelf and slope deposits are the deltaic portions, but it is not, in itself, a giant predominant facies. No structural evidence of deltaic wedge. a facies change from prograded paralic beds to turbidites of an embanked continental rise has Continental Terrace Structure been found. We have thus far attempted to enlarge upon Present data on the stability of submarine Dietz's (1963) discussion of the concepts of sedimentary slopes (Moore, 1961), while ad- wave-base, marine profile of equilibrium, wave- mittedly scant, are not in accord with one of built terrace, and delta terrace. Our difference Dietz's (1963, p. 985) basic presumptions; i.e., with Dietz is more a question of emphasis than that a 6-degree continental slope is unstable and disagreement. It is the final section of Dietz's subject to degradation by slumping and tur- paper, however, which we feel to be most open bidity currents, whereas a 2-degree slope can be to criticism. His theory of the development of prograded to form a vast sedimentary em- the continental terrace is essentially unchanged bankment. This arbitrary division seems un- from that of his stimulating paper of 1952, ex- realistic when the complex factors controlling

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L J 2 MILES Figure 1. Continuous reflection record (using 2000 Joule spark sound source) from outer continental terrace off west-central Mexico (Curray and Moore, 1964), showing sub-bottom strata. (A) Continental slope deposits, the bottomset beds of the delta shown at E, (B) Former surface of the continental slope, predating the delta, (C) Former shelf break, predating the delta, (D) Former surface of the continental shelf, predating the delta, (E) Foreset beds of the Pleistocene, lowered sea level delta, (F) Multiple reflections of the sea floor and sub-bottom layers, (G) Continental shelf deposits overlying the delta

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slope stability are considered. Slope stability open shelf deposition accompanied by sub- is a function not only of declevity but of sidence. This record shows structures indicating physical properties of the sediments and rates accumulation of slope deposits to a thickness of deposition. These factors control cohesion, of at least 5000 feet (assuming an average angle of internal friction, and degree of con- velocity of 6700 feet/sec.). solidation of the slope deposits. They can re- Outbuilding and upbuilding by slope and sult in stable deposits on 20-degree slopes, or shelf deposition is by no means unique to these unstable deposits on slopes of less than 1°, such two areas. Detailed reflection profiling records as those that occur off the modern Mississippi (Fig. 1) from the Costa de Nayarit off west- Delta (Shepard, 1955; Moore, 1961). central Mexico (Curray and Moore, 1964) show In introducing the continental embankment extensive terrace progradation by slope deposi- idea, Dietz (1963) adapted his 1952 theory to tion, and unpublished records from northern comply with the known extensive deposits on California show the same structure. We do not the continental slope of the Gulf of Mexico. suggest this to be the only method of conti-

NORFOLK, VIRGINIA MILES

1200 Figure 2. Line drawing of continuous reflection record (Electro-Sonic Profiler, Arcer) across continental shelf and slope off Norfolk, Virginia. Note the thick sections of sediments underlying both the shelf and slope and the general conformity of bedding with the present surface.

This adaptation involves some conflicting con- nental terrace development, but it is, at least, cepts and unacceptable models of the postu- a common type. Furthermore, this fundamental lated embankment structure. In describing the sedimentary structure is known to be locally development of the "continental embank- modified by massive slumping on the upper ment," Dietz suggests (1963, p. 985) that ". . . slope. We do not believe, however, that beds its development was not controlled by wave- thus exposed, by slumping, or faulting, con- base. Instead, material is dumped on the con- stitute proof that the slope is an erosional realm. tinental slope as delta foreset beds and by other Continental terraces, like mountain ranges, sediments swept across the shelf." What is this must be regionally different, depending on the if not wave-base? Furthermore, the schematic balance between such factors as rates of supply diagram by which Dietz represents the struc- of sediment, intensity of oceauographic proc- ture resulting from this sweeping of material esses, rates of subsidence or uplift, and influence across the shelf to be dumped on the slope (Fig. by tectonics (van Andel and Curray, 1960). 16B), closely resembles his Figures 3B, 6A, and 9 which were intended as examples of a struc- Acknowledgments ture that would result by control of wave-base. The origin of ideas is difficult to trace; some Further evidence contradicting Dietz's are most certainly generated in casual discussion "mature" terrace structure (Dietz, 1963, Fig. with colleagues. Many marine geologists have 14) is found in the continuous reflection pro- basically the same ideas on matters they con- filer records collected from the continental sider "concepts," but most have never taken terrace off Norfolk, Virginia (Moore and Cur- the time to write them down. When they do, ray, 1963b). This is the type section of Dietz's they differ in principal emphasis. We gratefully terrace of prograded paralic beds cropping out acknowledge the assistance of many of our on an erosional slope. Figure 2 is a line drawing colleagues for discussion of "concepts" before of reflecting horizons from continuous re- and after preparation of this note and for flection profiling records. The structure in no critical review of the manuscript, especially way resembles that of Dietz's model. Instead, R. S. Dietz, F. P. Shepard, Tj. H. van Andel, the fundamental structure again suggests out- E. L. Hamilton, E. C. Buffington, G. H. Curl, building by slope deposition and upbuilding by and D. L. Inman.

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References Cited Bradley, W., 1958, Submarine abrasion and wavecut platforms: Geol. Soc. America Bull., v. 69, p. 967-974 Curray, J. R., 1960, Sediments and history of Holocene transgression, continental shelf, Northwest Gulf of Mexico, p. 221-266 in Shepard and others, Editors, Recent Sediments, N. W. Gulf of Mexico: Am. Assoc. Petroleum Geologist, 394 p. 1964, Transgressions and regressions, p. 175-203 in Miller, R. L., Editor, Papers in Marine Geology: Shepard Commemorative Volume, New York, Macmillan, 531 p. Curray, J. R., and Moore, D. G., 1964, Pleistocene deltaic progradation of continental terrace, Costa de Nayarit, Mexico, in van Andel and Shor, Editors, Marine Geology of the Gulf of California: Am. Assoc. Petroleum Geologists, p. 193-215 Dietz, R. S., 1952, Geomorphic evolution of continental terrace (continental shelf and slope): Am. Assoc. Petroleum Geologists Bull., v. 36, p. 1802-1820 1963, Wave-base, marine profile of equilibrium, and wave-built terraces: A critical appraisal: Geol. Soc. America Bull., v. 74, p. 971-990 Dietz, R. S., and Menard, H., 1951, Origin of abrupt change in slope at continental-shelf margin: Am. Assoc. Petroleum Geologists Bull., v. 35, p. 1994-2016 Emery, K. O., 1952, Continental shelf sediments of Southern California: Geol. Soc. America Bull., v. 63, p. 1105-1108 Fairbridge, R., 1952, Marine erosion: 7th Pacific Sci. Cong., Pacific Sci. Assoc. Proc., v. 3, p. 347-358 Gilbert, G. K., 1890, Lake Bonneville: U. S. Geol. Survey Mon. No. 1, 438 p. Hersey, J. B., 1963, Continuous reflection profiling, Chap. 4, p. 47-72 in Hill, M. N., Editor, The Sea, v. 3: New York, Interscience Publishing Co. Inc., Div., John Wiley and Sons, 963 p. Hjulstrom, F., 1939, Transportation of detritus by moving water, p. 5-31 in Trask, P. D., Editor, Recent Marine Sediments: Am. Assoc. Petroleum Geologists, 736 p. Inman, D. L., 1963, Sediments: Physical properties and mechanics of sedimentation, Chap. V, p. 101-147, in Shepard, F. P., Submarine Geology (2d ed.): New York, Harper and Row, 557 p. Inman, D. L., and Bagnold, R. A., 1963, Beach and nearshore processes, Part II, Littoral Processes, Chap. 21, p. 529-553, in Hill, M. N., Editor, The Sea, V. 3: New York Interscience Publishing Co. Inc., Div., John Wiley and Sons, 963 p. Moore, D. G., 1960, Acoustic reflection studies of the continental shelf and slope off southern California: Geol. Soc. America Bull., v. 71, p. 1121-1136 • 1961, Submarine slumps: Jour. Sed. Petrology, v. 31, p. 343-357 Moore, D. G., and Curray, J. R., 1963a, Structural framework of the continental terrace, northwest Gulf of Mexico: Jour. Geophys. Research, v. 68, p. 1725-1747 • 1963b, Sedimentary framework of'the continental terrace off Norfolk, Virginia, and Newport, Rhode Island: Am. Assoc. Petroleum Geologists Bull., v. 47, p. 2051-2054 Shepard, F. P., 1948, Submarine Geology: New York, Harper Bros., 348 p. • 1955, Delta-front valleys bordering the Mississippi distributaries: Geol. Soc. America Bull., v. 66, p. 1489-1498 1963, Submarine Geology (2d ed.): New York, Harper and Row, 557 p. Sundborg, A., 1956, The river Klaralven—a study of fluvial processes: Geografisk Ann., Stockholm, v. 38, p. 127-316 Van Andel, Tj. H., and Curray, J. R., 1960, Regional aspects of modern sedimentation in northern Gulf of Mexico and similar basins and paleogeographic significance, p. 345-364 in Shepard and others, Editors, Recent Sediments, Northwestern Gulf of Mexico: Am. Assoc. Petroleum Geologists, 394 p.

U. S. NAVY ELECTRONICS LABORATORY, SAN DIEGO, CALIF.; SCRIPPS INSTITUTION OP OCEANOGRAPHY, LA JOLLA, CALIF. MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 22, 1963 CONTRIBUTION OF SCRIPPS INSTITUTION OF OCEANOGRAPHY, LA JOLLA, CALIF.

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