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Home , Sandstone, Sediment transport

THE AMERICAN ASSOCIATION OF PETROLEUM GEOLOGISTS BULLETIN V. S1, NO. 7 (JULY, 1967). P. 1304-I3I9. 12 FIGS.

TRANSPORT OF BY WAVES, ADRIATIC COASTAL SHELF, ITALY^ R. PASSEGA,=' A. RIZZINI,' AND G. BORGHETTP Bartlesville, Oklahoma, and Milan, Italy ABSTRACT Knowledge of marine- transport in shallow is as yet insufficient to give much help to the geologist who maps the distribution of ancient bodies. In order to improve this knowledge, the bottom sediments of the Adriatic near the Italian were sampled and ana­ lyzed, lliese sediments were transported as uniform suspensions. A close relation was found between certain grain-size parameters, particularly the 1 percentile, and sea depth, indicating that induced by waves was a major factor in the distribution. Another important factor that generally has been overlooked is percentage of sand in the sedi­ ment source for the area. If these source sediments contain little sand and consist almost entirely of and , the sand remains close to shore. If, however, sand is the predominant constituent of the source sediments, as commonly occurred in ancient , sand probably can be spread by waves across extensive areas as deep as wave base The of the sand also is an important factor in its distribution, some sand suspensions being displaced by gravity and some not. Therefore, grain size may determine the transport mechanism and, thus, the sediment distribution. An understanding of these factors by the petroleum geologist is essential in his mapping and projection of sandstone deposits in the subsurface.

INTRODUCTION Such fundamental information as the sediment Vertical and areal variations in the character­ content of bottom water during storms is not istics of are a controlling factor of sand­ available. Even less is known about the direction stone reservoir characteristics. If the reservoir is of transport, down- or upslope, of sands of a stratigraphic trap, petroleum accumulation is a different grain size. result of these variations. Studies of Recent and ancient sediments are The economic significance of the characteristics mainly descriptions of sediments as they are of sand deposits induces petroleum geologists to in­ found and observed. In order to obtain the fun­ vestigate genesis and of the sand. The damental information still lacking, there now is a present paper discusses a particular genesis: for­ need for field research on the dynamics of sedi­ mation of sand deposits by waves. mentation. In recent years considerable progress has been The present paper is an investigation of the made in research. The environment mechanism of sand distribution in a limited area of commonly, but not always, can be of the Adriatic coastal shelf along the east coast identified. The regional direction of currents in of Italy. Movement of sand was not observed di­ many cases is indicated by sedimentary struc­ rectly but was reconstructed to explain the pres­ tures. Unfortunately, little progress has been ent distribution. Texture was compared with the made in one fundamental field—that of sediment texture of other sediments to determine the transport, to which the characteristics of sedi­ transport mechanism. Information also was ob­ ments are closely related. tained by comparing the grain-size distribution in Among various modes of transport of sand, this area with similar distributions on shelves of distribution by waves formed the largest volume other seas. of ancient marine sand deposits. Yet little is known about the action of waves on the sedi­ SAMPLING ments of an open shelf. Systematic observations Adriatic bottom sediments were sampled along supporting the opinions commonly expressed on the Italian coast in an area between the ports of the maximum depth of wave action are lacking. Ortona and Pescara, from the 10- to the SO-meter isobaths. This area, shown on Figure 1, is 22 kilo­ ' Published by permission of AGIP Direzione Min- eraria. Manuscript received, October 25, 1965; ac­ meters long and 10 kilometers wide. cepted, October 19, 1966. Only the uppermost part of the sediments was ^ Consultant. sampled and it was assumed that this part was ' Geologist, AGIP Direzione Mineraria. deposited since the sea attained its present level. 1304 TRANSPORT OF SEDIMENTS BY WAVES, ADRIATIC COASTAL SHELF, ITALY 130.S

In view of the nearness of the shore and the close relation between certain sediment character­ istics and sea depth, this assumption is justified. Samples were taken from a trawler equipped with an echo meter while the trawler successively followed the 10-, IS-, 20-, 2S-, 30-, 40-, and SO-meter isobaths. Along each of these isobath lines, sampling sta­ tions were spaced at approximately l,SOO-meter intervals. At each station two or three samples were taken. Sand was sampled with a grab sam­ pler, and samples of homogeneous texture were selected for grain-size analyses. were sam­ pled with a short core barrel, but only the upper­ most part of the core was analyzed. The total number of stations is 106 and the total number of samples 267. The samples were treated by hydrogen perox­ FIG. 1.—Map of Adriatic Sea, showing area of study. ide in order to destroy the organic matter with­ out acid treatment. The fraction coarser than 31 deepest parts of the , but the mechanism microns was analyzed with sieves. The finer frac­ that triggers these currents generally is still a mat­ tion was dispersed with pyrophosphate and ana­ ter for speculation. In certain areas, however, as lyzed with pipette.* at the mouth of the Congo , it is probable The grain-size analyses of the samples taken that dense sand suspensions transported by along each isobath were represented by a CM during can overcome the density difference pattern. An attempt was made to reconstruct the between salt and fresh water and flow as turbidi­ sedimentation by using these patterns. ty currents into submarine . Waves are the transport agent that moved the BRIEF REVIEW OF TRANSPORT largest volume of ancient sands. Waves act on relatively shallow bottoms on which the orbital The purpose of this work is to give a better motion, flattened by action of the bottom, creates understanding of sediment distribution by marine an alternate to-and-fro current. agents. Reliable information supplied by the Numerous laboratory experiments and theoreti­ study of present oceans is available only for cer­ cal studies have been made to explain the action tain environments. of waves on bottom sediments. The movement of Longshore currents, fairly well known, can natural or artificial sediments also has been transport sand and for distances of more traced on nearshore bottoms. As yet this than 100 kilometers along the shore in a narrow work has not led to the formulation of well-estab­ belt only a few dozen meters wide. lished laws controlling sediment motion. Tidal currents may be swift in areas where It generally is accepted that shallowing waves are restricted by the topography. Transport induce a shoreward bottom current that tends to by these currents was studied in tidal channels keep sand near the shore. This current probably and over shallow banks. Sand waves moving at is compensated for a certain distance above bot­ present on the off the south and tom by a seaward counter-current, or laterally by east of have been attributed to rip currents. tidal action. The transport mechanism, however, At times, a seaward bottom counter-current is not well understood. may be induced by the blowing the surface Something is known about turbidity currents water onshore. which transport sand and a few pebbles to the Some oceanographers still believe that sand can 'Analyses were made in the Geochemical Labora­ not be transported by waves for considerable dis­ tories of AGIP, Direzione Mineraria. tances from shore, or much deeper than 30 me- 1306 R. PASSEGA, A. RIZZINI, AND G. BORGHETTI ters. These same oceanographers also believe that size and depth of bottom sediments sampled the position of the line, i.e., the lower limit along profiles of several coastal shelves are plot­ of sand deposition, indicates the limit of wave ted on Figure 2. With the exception of the Adri­ action on sand. atic profile (I), these profiles were constructed The study of ancient sediments casts some from published data. For stations where several doubt on these concepts. Passega (1962, p. 118) samples had been taken, the median shown on Fig­ showed that the widespread distribution of some ure 2 is the largest median. marine sands could be explained only by the con­ Profiles II, III, and IV are on the southern temporaneous distribution of sand by waves California shelf near La Jolla (Inman, 1953, across extensive areas of the sea floor. He sug­ 1957). Another California shelf, discussed by gested that a reason for the difference between Trask (1955), is represented by profile V. present-day marine transport and the distribution Profiles VI and VII are situated on the shelf of of these ancient sands is a difference in the grain the Rhone delta, discussed by Kruit (1955). size and volume of the sediments supplied to the Figure 2 shows clearly that, in the same area basin (Passega, 1962, p. 117-118; 1964, p. and at the same depth, the bottom sediment tex­ 837-838). This concept is discussed further in ture differs greatly. Medians of profile II are view of the results of the present study. twice as great as those of profiles III and IV. Ideas about the limit of wave action gradually Between profiles VI and VII, the difference is are changing. Recently silt suspensions by waves even larger. It is evident that if one profile were observed by Moore (1953) at a depth of reflects wave competency, the other profiles of 100 meters. the same area do not, and therefore are con­ In order to examine the relations between wave trolled by other factors. These factors are dis­ action and sediment distribution, median grain cussed in later paragraphs.

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\ L .___ : 5 10 15 20 25 30 35 40 45 e 0 5 5 e 0 e 6 70 75 SEA DEPTH IN METERS e c Fic. 2.—Profiles of a few nearshore shelves, showing variations of median grain size of bottom sedimeiits with sea depth. I, Adriatic area of the study; 11, La Jolla, California; III, Scripps Ocean- ograprac Institiition, California (Range D); IV, Scripps Oceanographic Institution, California (Rang* U); V, Point Conception, California; VI, Grand Rhone (mouth of Rhone River), France: VII, South Beauduc (Rhone delta), France. TRANSPORT OF SEDIMENTS BY WAVES, ADRIATIC COASTAL SHELF, ITALY 1307

FIG. 3.—Typical CM diagram, from Passega (1Q64). While , sand, and part of the slit are forming a CM pattern. Characteristics of these transported on or near bottom by waves and cur­ patterns were discussed by Passega in two papers rents, finer sediments form pelagic suspensions (1957, 1964). that can be transported by currents that do not Figure 3 shows the most general shape which a reach the bottom and are deposited as mud. Pelag­ CM pattern can assume. It should be recalled ic suspensions may be formed either by dis­ that the different segments into which the pattern crete particles or by aggregates, such as those is subdivided by letters on Figure 3 correspond to formed by flocculated clays. Grain-size analyses different transport mechanisms. NO represents were made of the muds that form the bottom of sediments transported only by rolling, OP sedi­ the northern Tyrrhenian Sea. These muds were ments transported in part by rolling and in part sampled at depths ranging from 40 to 1,500 me­ in , PQ sediments transported mostly ters (Fierro and Passega, 1965). Independently in suspension but including a few grains that are of the depth, all samples have almost the same rolled, QR sediments transported as a graded sus­ grain-size distribution. The median diameter is pension, and RS sediments transported as a uni­ 2-3 microns and the 1 percentile (approximate form suspension. The graded suspension is a bot­ value of the maximum grain size) is less than 40 tom suspension characterized by upward-decreas­ microns. Grains larger than 40 microns apparent­ ing concentration and grain size. In the uniform ly can not be transported long distances as pelag­ suspension which generally overlies the graded ic suspensions, and must be transported by bot­ suspension, concentration and grain size are rela­ tom currents. tively constant. The part T of the diagram is formed by sedi­ GRAIN-SIZE PARAMETERS ments transported as a pelagic suspension. Two parameters were used to represent the The CM diagram of a deposit can form the coarsest part of the samples: C, the 1 percentile complete pattern of Figure 3 only if sediments of of the grain-size distribution which is an approxi­ all sizes are available. mate value of the maximum grain size, and M, Other diagrams can be used to represent the the median. Samples are represented by points fine fraction of the sediments of a deposit. LM 1308 R. PASSEGA, A. RIZZINI, AND G. BORGHETTI and AM patterns represent the variations as a by the , published by the U. S. function of the median of the percentage by Waterways Experiment Station (1939), were weight of (1) the fraction finer than 31 microns, used to construct several diagrams (Figs. 4-6). L, the lutite; and (2) the fraction finer than four These illustrate the variations of the concentra­ microns. A, the clay. An example of LM and AM tion and grain size of the suspension and of the patterns is shown on Figure 12. water velocity. The concentration of the suspen­ In order to understand the sedimentation in sion, either decreasing or constant upward, was the Adriatic area under consideration, it is useful used to distinguish graded from uniform suspen­ to examine the conditions under which graded sion. On the diagrams, the points corresponding and uniform suspensions flow. For this purpose to graded suspensions were distinguished from the data on the sediments carried in suspension those corresponding to uniform suspensions.

13- • uniform suspension rising stage •

12- • uniform suspension fatling stage o graded suspension rising stage X

11- • graded suspension falling stage i^ •

10- • 9- X • • X 8-

U •• X • t-: • U. 6- • X • • z X X >- • .: X 1: 5- x°o* X u o- O 0 • • X 0 o 'o • X -J o • • 111 4_ o • > 0 "• X* 0 • • °0.0 •xx , " X 3- • X X 0 oo X >^ X 2- • Oo X o 1- o • •

0 1000 2000 3000 4000 5000 6000 7000 6000 3000 10 000 CONCENTRATION IN P.RM. G C FIG. 4.—Velocity-concentration diagram, Mississippi River near Mayersville, Missis­ sippi. Samples are taken at several stations in a section of river at various times. Each point of diagram represents series of samples taken at one station but at different depths during an observation. Concentration and velocity are average concentration and velocity of uniform suspension and of graded suspension, considered separately, for each station during an observation. Rising and falling stages of river are represented by different sym­ bols. TRANSPORT OF SEDIMENTS BY WAVES, ADRIATIC COASTAL SHELF, ITALY 1309

-7^

^^ : /

/ T^

10 20 50 MEDIAN GRAIN SIZE IN MICRONS

• unilorm suspansion rising stage graded suspension rising stage O unitorm suspansion falling stage graded suspension falling stage

FIG. S.—Velocity-median grain-size diagram, Mississippi River near Mayersville, Mississippi. Values are averages for graded and uniform suspensions at each station ob­ tained as indicated in explanation of Figure 4..

The sediments represented on the diagrams, all 1957). It can therefore be stated that, as velocity sampled in suspension, were distinguished ac­ increases, some rolled grains pass into the graded cording to whether they were sampled during ris­ suspension while some graded suspension particles ing or falling stages of the river. The crest stage, are lifted into the uniform suspension. during which a few samples were taken, was con­ This concept has been mentioned in different sidered to be a part of the rising stage. terms. Sundborg (1956, p. 218) discussed the The first diagram (Fig. 4) represents the varia­ passage of sediment from "" to "sus­ tions of the concentration of the sediment in sus­ pended load" with increasing velocity. However, pension as a function of the water velocity. The the terms "bed load" and "" are diagram shows that uniform suspension is present vague, graded suspension being considered at both during rising and falhng stages of the river, times as part of the bed load, and at other times whereas graded suspension generally is present as part of the suspended load. It is preferable to only during rising stages. Graded suspension can subdivide the load into rolled load, graded sus­ form at any water velocity greater than 2 feet pension, and uniform suspension. per second. Graded suspension concentrations Figure 6 shows the variations of the suspended range from 1,000 to 8,500 ppm. Uniform suspen­ sediment's median as a function of the concentra­ sion concentrations are highest during rising tion. This figure is particularly interesting be­ stages, but do not exceed 3,500 ppm. cause it shows that, if the concentration of the The diagram of Figure 5 shows the variations suspended matter exceeds 1,000 ppm., the coarsest of the median grain size of the suspended sedi­ particles of the uniform suspension pass into the ment as a function of the water velocity. As the graded suspension. velocity increases, the finest particles of the grad­ From these remarks some characteristics of ed suspension pass into uniform suspension. The sediment suspensions can be inferred. A bottom maximum grain size which can be transported in current transports sediments as a graded suspen­ the graded suspension is a function of turbulence sion where particles of appropriate size range are and therefore of current velocity (Passega, available. This range differs to a certain extent 1310 R. PASSEGA, A. RIZZINI, AND G. BORGHETTI with the characteristics of the current. Graded A, the percentage of clay defined as the fraction suspensions seem to flow discontinuously and finer than 4 microns. probably are likely to be formed during periods On the maps, at stations where several samples of strong upstream . The presence of a were taken, only the maximum values of C and uniform suspension is more continuous, although M and the minimum value of L are. shown. the concentration of the suspension may change Figures 7-9 show the areal variations of C, M, with time. As the velocity decreases, particles of and L in the area studied. These parameters grad­ the uniform suspension tend to settle and, if the ually change with the sea depth, the most regular concentration is sufficiently high, may form a change being that of C. With the exception of the graded susjiension. If the concentration of a uni­ area near Ortona, C contours coincide almost ex­ form suspension increases above a certain limit, actly with the bathyraetric lines. The variations the coarsest particles tend to settle and may form of L show a general increase in lutite percentage a graded suspension. with depth, the mud line being at a depth of 30-40 meters. The lutite map, however (Fig. 9), GRAIN SIZE OF ADRIATIC SEDIMENTS shows a certain irregularity in lutite deposition Grain-size characteristics of the bottom sedi­ which is reflected also in the variations of M ments are represented on a set of maps and di­ (Fig. 8). agrams. The parameters used to define the grain The good correlation between C and bottom size of each sample are: C, the 1 percentile; M, depth indicates that C, although it is an extreme the median; L, the percentage by weight of lutite parameter, also is a significant one. As a matter defined as the fraction finer than 31 microns; and of fact, in this particular area, C appears to be

\ \ i^. \ X A. .\ \ S8- ^ 0 0° \ \ '\ o o u o-

5 10 20 60 MEDIAN GRAIN SIZE IN MICRONS # uniform suspension rising stage X graded suspension rising stage o uniform suspension falling stage (^ graded suspension falling stage

FK. 6.—Concentration-median grain-size diagram, Mississippi River near Mayersville, Missist|ippi. Values are averages for graded and uniform suspension at each station ob­ tained as indicated in explanation of Figure 4. TRANSPORT OF SEDIMENTS BY WAVES, ADRIATIC COASTAL SHELF, ITALY 1311

isobaths in meters

isovalucs ot C in microns profiie I of fig. 2

sampling stations witii value of C in microns

a CAFILACCHIANI

FIG. 7.—Areal distribution of C (approximate maximum grain size of bottom sediment) in area of study. less sensitive to sampling errors than M. This CM patterns of the 10-, 20-, 25-, and 30-meter supports the contention of the senior writer (Pas- isobaths are uniform susjjension patterns. On the sega, 1957) that the good correlation, not affect­ IS-meter pattern, a few points have C values ed by sampling errors, between C and M which is ranging from 200 microns to 5 millimeters and characteristic of certain modes of transport, are scattered. These large grains are an artificial justifies the use of C for characterizing a deposit. sediment resulting from dismantling by waves of In the area of the study, grain size of lutite is constructions built during the First constant and independent of the percentage of World War. The coarse grains are rolled on a lutite in the sample as a whole. This is illustrated sand probably transported as a uniform suspen­ by Figure 10 on which the percentage by weight sion. These coarse grains are interesting because of clay in each sample (A) is plotted against the they behave like pebbles eroded by waves from a percentage of silt finer than 31 microns (L-A). It rocky coast. is clear that A is approximately proportional to The patterns of the 40-, and 50-meter isobaths L-A. may be considered to be patterns formed by pelag­ Several CM diagrams (Fig. 11 A, B, and Fig. ic suspensions. The absence of sand in the sedi­ 11 C, D, E, F, G) were constructed by grouping ments indicates that sand can not be transported on each diagram the points representing all sam­ a certain distance by currents having no contact ples taken along the same bathymetric line. As with the bottom. It must be noted, however, that can be seen, the CM patterns show little scatter. the limit between uniform and pelagic suspen- 1312 R. PASSEGA, A_ RIZZINF, AND G BORGHETTl sions is not sharply defined. graded suspensions in which, for values of M No pattern clearly indicates transport of sedi­ greater than 80 micnuis, L and A generally are ment as a graded suspension. The 10- and constant whereas M decreases with C as the grain 15-meter isobath patterns show a concentration of size of the coarsc-l p;ir' of the sediment de­ points near the line C=M that could be interpret­ creases. ed as a graded suspension. However the points Summarizing, wiih (he e.xception of the few with similar values of C but much smaller medi­ rolled grains of I he I -'-meter isobath, the bottom ans, which never represent graded suspensions sediments along each isobath are formed by a (Passega, 1964), suggest transport as uniform mixture i in various |)ri)))ortions) of lutite and suspensions. The patterns probably represent the well-sorted sand th(- >and being represented on limit between uniform and graded-suspension the CM diagram b\ points situated near fine C = transport, a limit which is not sharply defined. M. The grain size D! ihe lutite does not change The fine fraction of the sediments was investi­ with the bathymetry l>ut that of the well-sorted gated by means of LM and AM patterns. These sand does. The i-pen entile (' is closely related to show the variations of the lutite and clay con­ the sea depth, vvhcira- M is less closely related to tents with the median. Typical LM and AM pat­ the depth. terns, constructed for the samples of the 20-meter The mud line lorinmg the lower limit of the isobath, are shown on Figure 12, L increases fair­ sandy bottoms is ^iluated between the 30- and ly sharply as M decreases and A is proportional to 40-meter isobaths, liclow this depth the sediment L. This distinguishes uniform suspensions from consists almost inliriK ul lutite.

sobaths n meters

bovaiucs of M Ti microns

sampling stations with va'ue of M !- microns

FIG. 8.—Areal distribution of M (median grain size of bottom sediment • in area of study. TRANSPORT OF SEDIMENTS BY WAVES, ADRIATIC COASTAL SHELF, ITALY 1313

isobaths in meters 40m~ b^^'VN'v \^' " isovalucs of pcrcantags by weight of lutito en ^' ^*^^^ \\ \ 0^ (fraction finer than 31 microns) in bottom sediment °U sampiing stations with percentage of lutite ^

FIG. 9.—Areal distribution of L, percentage by weight of lutite in bottom sediment in area of study.

DISCUSSION OF WAVE SIEDIMENTATION ON eral variations in the bottom turbulence induced ADRIATIC COASTAL SHELF by waves. Textural information suggests the following re­ The first explanation that comes to mind for construction of the sedimentation. the maximum grain-size distribution is that at CM patterns of Figure 11 show that graded- each place are deposited the largest grains that suspension deposits generally are absent. This, as wave turbulence can lift. The mud line therefore mentioned earlier, is caused by the deficiency in would mark the lower limit of action of this tur­ the area of sediments of appropriate grain size. bulence on bottom sand. This explanation, how­ Sand is fine and forms uniform suspensions. Silt ever, is not in agreement with the indications of and clay are transported either in uniform or in the profiles of Figure 2 or with the recent obser­ pelagic suspensions. A few coarse grains resulting vations of wave transport, discussed previously. from erosion by waves of a concrete construction Another explanation is therefore suggested. are rolled as indicated by Figure 11-B. During a storm, in shallow water, waves could The regular areal variation with sea depth of erode the sea floor deeply, if this action were not texture parameters, and particularly of C, illus­ limited by the maximum concentration of sedi­ trated by Figures 7-9, indicates the absence of ments that waves can hold in suspension. As was local currents, such as rip currents, capable of mentioned in a preceding paragraph, in a uniform transporting sediments seaward. Variations of the suspension, if lutite concentration increases above texture with depth can be attributed only to lat­ a certain limit, the coarsest sand is forced out of 1314 R. PASSEGA, A. RIZZINI, AND G. BORGHETTI the suspension. This sand probably forms a grad­ by CM, LM, and AM patterns (Figs. 11, 12), ed suspension just above bottom. well-sorted sand settles first, followed by mixtures It is assumed that the sediment concentration of the same sand with increasing amounts of lu­ in this graded suspension is not much greater tite. Shallow-water turbulence forces most of the than in the uniform suspension and is sufficiently lutite into deeper water. low to prevent transport' of the suspension by The result of the proposed sedimentation gravity. mechanism is that the coarsest grains of the uni­ The bottom current induced by waves or, more form suspension are swept into shallow water and generally, the lateral flow of turbulent water the lutite is deposited preferentially in deeper moves the graded suspension shoreward, where water. Sand distribution is therefore controlled the increased turbulence again lifts the sediments by differential bottom turbulence and by the pro­ into the uniform suspension. portion of lutite in the available sediment, which When the storm abates, turbulence decreases determines the depth to which the uniform sus­ and the uniform suspension settles. As suggested pension can contain sand. Sand distribution is

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QI 10 20 30 4-0 50 60 SILTIinar Ihan 3T micron °/o G.C. FIG. 10.—Clay-silt diagram showing for each bottom sample percentage by weight of clay (fraction finer than 4 microns) plotted against percentage of silt finer than 31 microns (fraction between 31 and 4 microns). TRANSPORT OF SEDIMENTS BY WAVES, ADRIATIC COASTAL SHELF, ITALY 1315

B - 15 m scrooo •

.

•

CO z 0 oc u 1 A - 10 m • 20000 z 1 u Z^ 1 ' •KTOOO • ; • ^00

B 10 30 1000 1

1! 1

z ! 300 o u

z • \K • 4 u f\

• K >

J^S 1

3 5 10 30 M IN MICRONS -G.C.- FIG. 11 A, B.—CM patterns. Patterns represent samples taken along following isobaths: A.—10 m.; B.—IS m.

controlled by the maximum turbulence only if having a low median and a value of C corre­ lutite is absent. Then bottom sand probably ex­ sponding to the bottom depth. tends to wave base, which presumably is consid­ The proportion of lutite in the bottom sedi­ erably deeper than 30 meters. In the extreme op­ ment changes somewhat along an isobath as indi­ posite condition, where the sediment as a whole is cated by Figure 9. This explains why C is more lutite, the entire sea floor and even the closely related to the sea depth than M. C there­ are covered by lutite. fore gives a good indication of the relative values Between two storms, the lutite which is trans­ of the sea depth. These indications differ from ported as a pelagic suspension settles. one area to another because they are dependent As lutite is deposited, it becomes mixed with on the grain size of the available sediment, which some of the bottom sand and forms a sediment may be expressed as the "sand percentage" by 1316 R. PASSEGA, A. RIZZINI, AND G. BORGHETTI weight in the part of the available sediment that whereas sand is transported largely by longshore can be transported in suspension. currents. Therefore, in the area of profile VII, 20 The sand distribution illustrated by the profiles kilometers west of profile VI, the sand percentage of Figure 2 supports the above explanation. in the available sediment is much higher than at The Rhone, as most present rivers, transports the mouth of the river. On profile VII, the sedi­ relatively little sand. Therefore, at its mouth, the ment with a median larger than 30 microns ex­ "sand percentage" is low. Profile VI (Fig. 2) be­ tends to a depth of 30 meters, as in the Adriatic gins at the mouth of the river. Sand is present area of this study. only in water a few meters deep, and at 13 me­ Near La Jolla, California, the area of profile II ters the median of the sediment is only 30 mi­ (Fig. 2) is supplied with sediment eroded by crons. Lutite is transported mainly as a pelagic waves from the rocky Point La Jolla. This sedi­ suspension from the mouth of the river seaward. ment is moderately coarse. Sediments of profiles

^*;H:CL ••-1 M JN MICRONS r ' >

•so

M IN MICRONS

F-

/ 100 M IN MICRONS / 1 / •/v •i' y ) ?• / / M IN MICRONS G- 50m

V) z / O 1 1 Z ) 7 /. • • /5 C Vr •. / • / M IN MICRONS

FIG. 11 C, D, E, F, G.—CM patterns. Patterns represent samples taken along following isobaths: C—20 m.; D.—2S m.; E.—30 m.; F.—+0 m.; G.—SO m. TRANSPORT OF SEDIMENTS BY WAVES, ADRIATIC COASTAL SHELF, ITALY 1317

III and IV, situated 1 and 3 kilometers, respec­ o CO tively, north of profile II, are supplied by long­ • • » shore currents from the north. The sand is much o • finer than the sand of profile II. The sand per­ , >• o centage probably is high and deposits with a < (0" median larger than 30 microns extend to depths u . of more than 75 meters. In the area of profile V, 0 0 it also can be assumed that the sand, transported >:- t_ o u t by longshore currents around Point Conception, ) • 1 0 • is reasonably free of lutite. The median decreases < 3 '^ less with increasing depth than on the other c 1 *' profiles and is 200 microns at a depth of 20 me­ CJ 6" o o ters. The present sea-level is recent. With time o ft, • the sands of Point Conception should be able to 0 00 0 construct an extensive sandy shelf. The Adriatic coastal shelf is representative of J67«aW ! 3 4 S S 7 e 9 la' • lutltl! M(MEDIAN) IN MICBONS conditions existing on many present shelves. Sedi­ ° '"^ .S.C, ments suppUed to the oceans by present rivers generally are fine. The sand percentage is low. It Fig. 12.—LM and AM diagrams for 20-meter iso­ bath, showing variations of lutite (percentage by is approximately 5 per cent for the Niger weight of fraction finer than 31 microns in bottom (NEDECO, 1959, 1961) and S for the Mississippi. samples) and clay (percentage by weight of fraction finer than 4 microns) with median. The abundant clay transported to the sea by riv­ ers flocculates as it comes into contact with the sea water and largely settles near the coast where et al., 1965). In some, however, where the water it builds a coastal shelf. On this shelf, fine sand was shallow, the distribution of graded suspension forms uniform suspensions, and is kept in a nar­ sands by waves was similar to the distribution of row coastal belt by the high proportion of lutite . in the sediments. The sand percentage in the In ancient basins uniform suspensions com­ available sediments can be increased by transport monly are associated with graded suspensions, in longshore currents. This could lead to the con­ probably as a result of wave action on the upper struction of extensive shelves as suggested by finer part of the graded suspension. Differential profiles III and IV (Fig. 2). bottom turbulence seems again to control deposi­ The preceding discussion is limited to sedi­ tion as the graded suspension generally is ments transported by waves as uniform suspen­ deposited in more turbulent areas than the uni­ sions. This is far from being the only mode of form suspension. wave transport. In numerous ancient shallow- Rolled grains are common on ancient shelves in water basins, sand predominantly transported as association with uniform suspensions. If the a graded suspension extensively covered the sea grains are rolled and deposited individually they floor. Passega (1964) indicated that gravity prob­ are represented by scattered points on CM pat­ ably was a controlling transport factor for many terns similar to the pattern of Figure 11-B. sand and sandstone deposits. As an example, in Rolled grains may be concentrated in coarse­ the Tertiary basins of the Po , during cer­ grained offshore bars. Because of their rugosity, tain periods of , rivers eroded consider­ these bars generally are free of fine sand. able volumes of sand from the uplifted areas and Differential bottom turbulence causes the deposi­ transported dense graded suspensions to the sea. tion of uniform suspensions around the . These suspensions consisted mostly of sand. They Finally, it may be noted that in some ancient flowed on the sea bottom, seeking the depressions Italian basins uniform suspension deposits and which were filled with sand deposits, commonly rolled grains were deposited on shelves while several hundred meters thick. In most of these graded suspensions settled on slopes or in depres­ basins, faunas indicate that the water was deep sions. Selective transport probably removed the and that the deposits were turbidites (Byramjee sediments which were more subject to the action 1318 R. PASSEGA, A. RIZZINI, AND G. BORGHETTI of gravity. The residual sediments are thereby the sand percentage in the available sediments. "stabilized." This process probably explains the This sand probably would have built extensive stability of sand banks on some Recent shelves shelves if time had not been short since the sea swept by storm waves, such as the Gulf of Mexi­ attained its present level. co shelf. Transport of uniform suspensions on the Adri­ CONCLUSIONS atic coastal shelf is not affected by the action of greatly suffers from a lack of under­ gravity. standing of marine transport of sediments. Strati- In ancient seas, at times, conditions were graphic trap and reservoir investigations and, to a different. Rivers, flowing in mobile areas, eroded large extent, are studies of sand considerable volumes of sand which formed dense characteristics. Yet the factors of these charac­ graded suspensions in the sea. These flowed by teristics are practically unknown. gravity either as turbidity currents or as wave Shape of sand bodies and sedimentary struc­ suspensions and formed extensive blanket sands. tures, frequently investigated, give inconclusive Grain size is therefore a major factor of ma­ information concerning the genesis of marine rine sedimentation as it may determine the trans­ sand deposits. port mechanism and control the sediment dis­ This work is an attempt to reconstruct a par­ tribution. Textural studies disclose close relations ticular type of marine sedimentation; i.e., dis­ among basic factors such as orogeny, grain size tribution by waves on a coastal shelf having a of river loads, grain size of marine suspensions, uniform suspension consisting of fine sand and submarine topography, and distribution of sand. lutite. Investigation of these relations is essential to the Textural parameters of the deposits were found understanding of the dynamics of sedimentation. to be an orderly function of sea depth. The maxi­ This is a relatively new field which offers to the mum grain size of the uniform suspension sedi­ petroleum geologist the possibility of making sub­ ments regularly decreases with depth. The rela­ stantial progress in mapping and projecting the tion between median and depth is not as close, characteristics of terrigenous deposits. because of irregularities in the distribution of lu­ tite. REFERENCES CITED Competency of waves does not directly control Byramjee R. S., M. A. Chierici, and R. Passega, 196S, Utilisation de la paleoecologie en exploration petro- sediment distribution. The main factors of this liere: Revue de Giogr. Physique et de G60I. distribution are differential bottom turbulence D)Tiamique, v. 7, fasc. 1, p. 11-20. and grain size of the available sediments, which Curray, J. R., and D. G. Moore, 1964, Pleistocene deltaic of continental terrace, Costa may be expressed as "sand percentage" in these de Nayarit, Mexico, in Marine geology of the Gulf sediments. The lutite contained in the suspension of California: Am. Assoc. Petroleum Geologists limits the seaward distribution of the sand by Memoir 3, p. 193-21S. Fierro, G., and R. Passega, 1965, Studio sedimento- forcing it out of the uniform suspension. As a logico di 32 carote prelevate nel Nord Tirreno: pa­ consequence, in the same area, at the same depth, per presented at 14th Congr. of Associazione Geo- and under the same wave regime, deposits may fisica Italiana, Rome, April, 196S, 12 p. Inman, D. L., 1953, Areal and seasonal variations in be sand or mud depending on the sand percentage and nearshore sediments at La JoUa, Califor­ in the available sediments. nia: Beach Erosion Board Tech. Memo. no. 39, 82 p. The maximum grain size of uniform suspension 1957, Wave generated ripples in nearshore deposits may, therefore, in a limited area, indi­ sands: Beach Erosion Board Tech. Memo. no. 100, cate the variations of the sea depth, but does not 42 p. Kruit, C, 1955, Sediments of the Rhone delta—1°, give the absolute value of this depth. Grain size and microfauna: The Hague, The Nether­ The sand percentage is extremely low in the lands, Mouton & Co., p. 357-500. Moore, D. G., 1963, Geological observations from the sediments supplied to the sea by most modern bathyscaph Trieste near the edge of the continen­ rivers. In the sea, lutite flocculates and builds tal shelf off San Diego, California: Geol. Soc. coastal shelves on which fine sand is distributed America Bull., v. 74, no. 8, p. 1057-1062. NEDECO, 1959, River studies and recommendations on in a narrow coastal belt. Longshore currents improvement of Niger and Benue: Amsterdam, transporting sand rather than lutite may increase North-Holland Publ. Co., 1000 p. TRANSPORT OF SEDIMENTS BY WAVES, ADRIATIC COASTAL SHELF, ITALY 1319

1961, The of the Niger delta: The 34, no. 4, p. 830-847. Hague, Nedeco, 317 p. Sundborg, A., 1956, The river Klaralven, a study of Passega, R., 19S7, Texture as characteristic of clastic : Stockholm, Geografiska Annaler, deposition: Am. Assoc. Petroleum Geologists Bull., V. 38, no. 2, 316 p. V. 41, no. 9, p. 1952-1984. Trask, P. D., 1955, Movement of sand around south­ 1962, Problem of comparing ancient with re­ ern California promontories: Beach Erosion Board cent sedimentary deposit: Am. Assoc. Petroleum Tech. Memo. no. 76, 60 p. Geologists Bull., v. 46, no. 1, p. 114-117. U. S. Waterways Experiment Station (Vicksburg, 1964, Grain size representation by CM pat­ Miss.), 1939, Study of materials in suspension— terns as a geological tool: Jour. Sed. , v. Mississippi river: Tech. Memo. no. 122-1, 27 p.