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Speculations on processes responsible for mesoscale current lineations on the continental shelf, southern California
Marine Geology, 34 (1980) M9--M18 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
M9
Letter Section SPECULATIONS ON PROCESSES RESPONSIBLE FOR MESOSCALE CURRENT LINEATIONS ON THE CONTINENTAL SHELF, SOUTHERN CALIFORNIA
H.A. KARL
U.S. Geological Survey, Menlo Park, California 94025 (U.S.A.) (Received March 28, 1979; revised and accepted September 3, 1979)
ABSTRACT Karl, H.A., 1980. Speculations on processes responsible for mesoscale current lineations o n the continental shelf, southern California. Mar. Geol., 34: M9--M18. A side-scan sonar survey of San Pedro shelf, California, reveals areas of mesoscale current lineations oriented approximately north-northeast in water depths of 20--25 m. Widths of sand ribbons range from 40 to 120 m and intervening erosional furrows, from 15 to 50 m. A conceptual model shows that the scale and orientation of current lineations agree with the dimensions and axial directions of Langmuir circulations theoretically generated by a combination either of southerly and southwesterly winds with regular trains of swell from the southern hemisphere or of two sets of wave trains crossing from the south and west. These longitudinal bedforms indicate shore-normal sediment transport at the times and on the areas of the shelf when and where they have been observed.
INTRODUCTION
A side-scan sonar system was used for a detailed survey of San Pedro Bay, California, during 8--12 December 1975 (Fig. 1). The research vessel "Vantuna", maintaining a speed of 3--4 knots, kept the bottom-towed vehicle off the bottom by about 5--10% of the total water depth. The maximum range of the recorder, which was usually set at 200 m, gave a total side-to-side cover age of 400 m. Reviews of interpretations of sonographs as well as details of the techniques involved are given by Chesterman et ai. (1958), Clay et al. (1964), Tucker (1966) and Belderson et al. (1972). In this report I describe, interpret, and speculate on the origin of certain linear features, resolved on the side-scan sonar records, that occur over parts of the continental shelf off southern California. San Pedro continental shelf, the field area, begins at Point Fermin, extends eastward for about 35 km, and terminates at Newport Submarine Canyon (Fig. 1). The shelf is a relatively fiat, featureless platform ranging in width from about 3 km at its west and east boundaries to about 20 km at its widest point. San Pedro shelf is covered with a thin (3--20 m) veneer of undifferenti-
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o r i e n t a t i o n o f c u r r e n t l i n e a t i o n s . T r a e k l i n e s d o n o t s h o w h e a d i n g d e v i a t i o n s f r o m set course.
Fig. 1. M a p o f s t u d y area, s h o w i n g side-scan survey t r a c k l i n e s , s h i p e k grab s a m p l e a n d c a m e r a t r a n s e c t s , b o x core l o c a t i o n s , and
33°35`
33°45'
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MII
ated late Quaternary sediment punctured in places by outcrops of Tertiary rocks (T.R. Nardin, oral communication, 1976). Modem surficial sediment consists of a m o n o t o n o u s blanket of silty sand and sandy silt interrupted by patches of coarser relict sediment. Currents have not been previously studied sufficiently to describe shelf circulation patterns in detail. Surface tides flood west and ebb east; tidal currents rarely exceed 20 cm/s at the surface (Emery, 1960). Surface waves approach San Pedro shelf along corridors from the west and south. Islands and banks tend to damp out long-period (15--20 s) swell from the west (Horrer, 1950) and leave short-period (7--10 s) waves to continue relatively free of interference. Long-period swell from the southern hemisphere reaches the shelf without interruption (Horrer, 1950). Although regional surface winds are generally northwesterly, San Pedro Bay is often affected by southerly and southwesterly winds generated by the Catalina Eddy (Graham, 1950; Stevens, 1977). Other localized winds are the seasonal northeasterly Santa Anas and the diurnal onshore--offshore breezes. Regional winds generated by seasonal high- and low-pressure cells strongly influence the major offshore current systems. However, the relationships between localized winds and shelf currents are very complex and not well understood in the study area. OBSERVATIONS A distinctive sonograph pattern of parallel bands of alternating dark and light streaks characterizes parts of San Pedro shelf (Fig. 2). The signature occurs in discontinuous patches at depths shallower than about 25 m on a substrate of fine to medium sand. Though identified in three areas, the lineations are well defined in only one patch on the central shelf (area A, Fig. 1). The width of dark lineations varies from about 15 to 50 m and the width of intervening light bands, from about 40 to 120 m (Fig. 2); the length of these features is at least 100--200 m. Wherever observed on the shelf, the long axis of lineations is oriented approximately north-northeast (Fig. 1); the lineations are apparently symmetrical, very low relief (> 1--2 m) features. During April 1976, 21 Shipek grab samples were collected along two transects, each approximately 1 nautical mile in length (Fig. 1). Samples show large variation in color and texture of surficial sediment (Fig. 3). Numerous photographs of the b o t t o m taken during this cruise along the same transects as the grab samples reveal abundant small-scale bedforms. Many bedforms (designated type A ripples) consist of double sets of ripples of unequal wavelength, with orthogonally intersecting crests (Fig. 4). The longer wavelength (15--20 cm) ripples strike approximately west-northwest and the shorter wavelength (5--10 cm) ripples approximately north-northeast. Ordinary symmetrical ripples, striking approximately west-northwest, were observed in deeper water and at other times of the year. Three box cores collected during November 1974 in area B (Fig. 1) show striking differences in vertical
M12
%o CORRECTED
. ~ " ~ . ~ . 0 R~IENTATION
Fig. 2. Part of sonograph record (transect X--X', Fig. 1) showing lineations described in text.
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M13
Fig. 4. Type A ripples described in text. lithology. Each core penetrated about 15 cm of sediment; the shallowest (core 21375) contained only rust-colored sand with a mean grain diameter of 0.24 mm. Core 21376, 2 km seaward, contained reddish-brown medium (0.29 ram) sand underlain by gray-green fine (0.19 ram) sand; the deepest core (21377) consisted only of gray-green very fine (0.09 ram) sand. DISCUSSION The sonograph patterns (Fig. 2) are attributed to differences in the reflectivity of the substrate caused by changes in sediment texture. Spacing, shape, and orientation of these lineations indicate that they are not the sea-floor expression of subsurface rock units, but rather are due to changes in sediment distribution associated with mesoscale and macroscale bedforms. Observed sonograph patterns closely resemble those described by McKinney et Fig. 3. Diagram illustrating variation in mean grain size along transects I--II and HI--IV.
For the purpose of graphic representation, distances between stations are shown as equal and with no scale; actual distance between successive stations ranges between 200 and 400 m.
M14 al. (1974), which they interpreted as mesoscale current lineations. Because the bedforms on San Pedro shelf are nearly shore-normal and not shore-parallel, interpreting these bedforms as mesoscale current lineations implies that the currents responsible for their formation have an offshore and/or onshore c o m p o n e n t of flow. The orientations of ripples seen in most areas of the shelf support this sense of water movement, and the stratigraphic sequence revealed in cores 21375--21377 suggests that nearshore sediment is being transported seaward. Since the model developed in this paper is speculative and based on the interpretation of these bedforms as current lineations, it should be noted that Swift and Freeland (1978) have interpreted somewhat similar nearly shore-normal bedforms found on portions of the inner shelf of the U.S. east coast as flow-transverse sand waves. Current tineations consist of sand ribbons (light bands} and erosional furrows (dark bands), which probably form in response to fields of large-scale helical flow cells (Allen, 1966, 1968; Kenyon, 1970; Duane et al., 1972; McKinney et al., 1974). According to the model of Swift and Ludwick (1976), stronger, divergent currents of these cells, where they contact the b o t t o m , erode the narrow furrows, whereas weaker, convergent currents construct the wide sand ribbons. Furthermore, sand ribbons consist of fine sand over a substrate of coarser material, whereas erosional furrows are areas in which coarse substrate has been exposed (or formed) by winnowing of the overlying sediment. This textural contrast creates the alternating dark and light bands on the sonograph. Although sampling of individual ribbons and furrows was not possible, similar textural variations exist in the area of current lineations (Fig. 3). Helical flow cells, known as Langmuir circulations, exist in the surface mixed layer of the oceans. Several theories explain the mechanisms generating Langmuir circulations (Failer, 1969, 1971; Scott et al., 1969; Craik and Leibovich, 1976). The theoretical models of Craik and Leibovich (1976), which make use of the Stoke's drift of the wave field and the shear flow induced by the wind, as well as the laboratory experiments of Faller (1977) and Failer and Caponi {1977, 1978), which utilize a wind-wave tank to generate Langmuir circulations, provide a basis for a conceptual model to account for the current lineations observed on San Pedro shelf. While increasing our understanding of some aspects of Langmuir circulation, none of these experiments or theories have unequivocally identified the mechanisms by which Langmuir circulations are generated. The laboratory experiments of Failer (1977) and Failer and Caponi (1977, 1978) showed that both wind and waves are necessary for the generation of Langmuir circulations. They form either orthogonally to the propagation direc tion of single wave trains or at the intersection of interference patterns generated by two crossed wave trains. In the wind-wave tank a thin film of water at the surface moved in the direction of the wind. However, dye placed along the floor of the tank revealed that b o t t o m horizontal convergent currents between cells flowed counter to the wind direction owing to a subsurface return
M15
flow of water (see Failer and Caponi, 1978, fig. 4, p. 3620). These experiments suggested that the wavelengths of Langmuir circulations depend upon the depth of the mixed layer but may also be a function of the wavelength of the surface waves, and that under certain circumstances more than one scale of Langmuir circulations may exist simultaneously. It is understood that the complex phenomena which characterize natural environments, restrict the extrapolation of experimental results derived in the laboratory to oceanic scales. It may be possible, however, to apply some of the fundamental aspects of Langrnuir circulations exhibited in the laboratory to the shallow shelf waters of San Pedro Bay. All the conditions necessary to generate Langrnuir circulations of the correct dimensions and orientation to account for the current lineations observed on San Pedro shelf exist at times in San Pedro Bay (Fig. 5). With a mixed-layer depth to b o t t o m of about 25 m, Lagmuir circulation wavelengths of about 50--100 m are expected (A.J. Failer, written communication, 1978; Failer and Caponi, 1978, fig. 2). During periods of southerly and southwesterly winds, Langmuir circulations can be generated in combination with southern hemisphere swell. When two wave trains approach simultaneously from the west and south, ascending currents of Langmuir circulations can form beneath intersections of the two sets of wave crests. Orientations of symmetrical ripples indicate that they have been generated by waves from the southern hemisphere. Although the origin of type A ripples remains problematic, the characteristics of these bedforms suggest formation under a set of two crossed wave trains of unequal periods (wavelengths), with longer period waves approaching from the south and shorter period waves from the west. Axial orientation of Langmuir circulations in both situations would be
Fig. 5. Conceptual model of current lineations forming in response to Langmuir circulations. A ffi Relation of Langmuir circulations to sand ribbons and erosional furrows. B and C diagrammatically depict generation of current lineations by southerly and southwesterly winds and by two conditions o f waves and swell: B = simple case of southern hemisphere swell; C ffi one possible interference pattern composed of long-period swell from the south and shorter period waves from the west.
M16
shore-normal (Fig. 5). It is not known whether a vertical flow profile develops in response to onshore winds and waves in San Pedro Bay analogous to the shear profile characterizing the experiments of Faller and Caponi (1978). Thus, it is not possible to determine whether the convergent currents of Langmuir circulations in San Pedro Bay flow seaward or shoreward; nor is it possible to assess the magnitude of the horizontal flow component. The convergent currents, observed by Faller and Caponi (1978), flowing counter to the direction of the surface wind and waves may have been forced by the size and design of the tank; shoreward surface flow on the open shelf may or may not be compensated for by an offshore subsurface flow. Apparent seaward transport of nearshore sands suggested by the vertical lithologic sequences observed in cores 21375--21377 collected in area B (Fig. 1) could reflect some other process and current. Indeed, the Langmuir circulations generated on San Pedro shelf are probably second-order currents superimposed on the primary current systems and processes governing water circulation in San Pedro Bay. Theoretically, Langmuir circulations should occur over the whole shelf width. Among the reasons that may account for current lineations being observed only on onshore parts of the shelf are that: (1) density stratification higher in the water column may limit the cell dimensions and penetration of Langmuir circulations in deeper water, (2) Langmuir circulations may not be sufficiently intense in deeper water to move the b o t t o m sediment, and (3) the uniform sediment textures seen on other parts of the shelf may not cause reflectivity changes recognizable on sonographs. Although meteorological and oceanographic conditions in San Pedro Bay theoretically can create Langmuir circulations of proper scale and orientation to account for the observed mesoscale current lineations, it was not possible to determine whether Langmuir circulations generate currents of sufficient velocity to cause the shear stresses necessary to move fine and medium sand. Sediment particles, however, may be and are set in motion or held on the threshold of movement by currents derived from surface waves and tides. An ordered helical flow field periodically superimposed on oscillatory or random current patterns would distribute sediment so as to form low-amplitude, symmetrical current lineations. CONCLUSIONS
Distinctive patterns of alternating light and dark parallel bands on sonographs of San Pedro shelf are interpreted as mesoscale current lineations. These longitudinal bedforms consist of fine sediment (constructional sand ribbons) separated by narrower bands of coarse sediment (erosional furrows) that either underlies the field of current lineations or represents a lag deposit in very shallow troughs. A conceptual model explains the current lineations as forming in response to Langmuir circulations generated b y specific combinations of the winds and surface waves often occurring in San Pedro Bay. H o w frequently these Lang-
M17 muir circulations form has not been estimated, nor has it been determined how quickly current lineations form in response to the currents generated by Langmuir circulations, or how long the current lineations remain as recognizable features on the sea floor. Presuming that the Langmuir circulations exhibit a horizontal flow component, sediment in the area of the current lineations would be transported normal to shore. It is n o t possible from the extant set of observations to determine whether the transport is onshore or offshore nor the magnitude of transport.
ACKNOWLEDGMENTS I would like to t h a n k the officers and crew of the research vessel " V a n t u n a " for their willingness to lend a hand during the survey. Without the aid of the technicians and machinists of the Marine Facility of the University of Southern California, this cruise would not have been possible. The ideas expressed herein represent part of the author's dissertation completed under the supervision of Dr. D.S. Gorsline of the Department of Geological Sciences, University o f Southern California. The National Science Foundation funded this work under grants GA 40049, DES 75-01438, and DES 75-01438, A m e n d m e n t AO1. This report was initiated and completed during tenure as a National Research Council Research Associate with the U.S. Geological Survey in Menlo Park, California. M.E. Field and D.M. Rubin of the U.S. Geological Survey and D.J.P. Swift of the National Oceanic and Atmospheric Administration (NOAA) kindly reviewed drafts of the manuscript. T.R. Nardin of the Department of Geological Sciences, University of Southern California, corrected side-scan orientations from equations supplied by D.J.P. Swift.
REFERENCES
Allen, J.R.L., 1966. On bedforms and paleocurrents, Sedimentology, 6: 153--190. Allen, J.R.L., 1968. Current Ripples. North Holland, Amsterdam, 439 pp. Belderson, R.H., Kenyon, N.H., Stride, A.H. and Stubbs, A.R., 1972. Sonographs of the Sea-Floor: A Picture Atlas. Elsevier, Amsterdam, 185 pp. Cbesterman, W.D., Clynick, P.R. and Stride, A.H., 1958. An acoustic aid to seabed survey. Acustica, 8: 285--290. Clay, C.S., Eses, J. and Weisman,J., 1964. Lateral echo-sounding of the ocean bottom on the continental rise. J. Geophys. Res., 69: 3823--3825. Craik, A.D.A. and Leibovich, S., 1976. A rational model of Langmuir circulations. J. Fluid Mech., 73: 801--821. Duane, D.B., Field, M.E., Meisburger, E.P., Swift, D.J.P. and Williams,S.J., 1972. Linear shoals on the Atlantic inner continental shelf, Florida to Long Island. In: D.J.P. Swift, D.B. Duane and O.H. Pilkey (Editors), Shelf Sediment Transport: Process and Pattern. Dowden, Hutchinson, and Ross, Stroudsburg, Pa., pp. 447--498. Emery, K.O., 1960. The Sea Off Southern California. New York, N.Y., 366 pp.
M18 Faller, A.J., 1969. The generation of Langmuir circulations by the eddy pressure of surface waves. Limnol. Oceanogr., 14: 504--513. Faller, A.J., 1971. Oceanic turbulence and the Langmuir circulation. Annu. Rev. Ecol. Syst., 2: 201--233. Faller, A.J., 1977. Preliminary studies of controlled Langmuir circulations: Tech. Note BN-864, Institute for Physical Science and Technology, Univ. of Maryland, College Park, Md., 21 pp. Faller, A.J. and Caponi, E.A., 1977. A laboratory study of wind-driven Langmuir circulations: Tech. Note BN-861, Institute for Physical Science and Technology, Univ. of Maryland, College Park, Md., 59 pp. Faller, A.J. and Caponi, E.A., 1978. Laboratory studies of wind-driven Langmuir circulations. J. Geophys. Res., 83: 3617--3633. Graham, R.D., 1950. Divergence and vorticity of the southern California coastal winds. Unpub. rept., WBAS, Los Angeles, Calif., 7 pp. Horrer, P.L., 1950. Southern hemisphere swell and waves from a tropical storm at Long Beach, Calif. U.S. Army Corps of Engineers, Beach Erosion Board Bull., 4(3): 1--18. Kenyon, H.H., 1970. Sand ribbons of European tidal seas. Mar. Geol., 9: 25--39. McKinney, T.F., Stubblefield, W.L. and Swift, D.J.P., 1974. Large-scale current lineations on the central New Jersey shelf: investigations by side-scan sonar. Mar. Geol., 17: 79-102. Scott, J.T., Meyer, G.E., Stewart, R., and Walker, E.G., 1969. On the mechanism of Langmuir circulation and their role in empilinion mixing. Limnol. Oceanogr., 14: 493--503. Stevens, P.M., 1977. Environmental design data for the southern California OCS region (initial estimate). Report for Cons. Div., USGS, Contract No. 14-08-0001-15988. Swift, D.J.P. and Freeland, G.L., 1978. Current lineations and sand waves on the inner shelf, Middle Atlantic Bight of North America. J. Sediment. Petrol., 48: 1257--1266. Swift, D.J.P. and Ludwick, J.C., 1976. Substrate response to hydrodynamic processes: grain-size frequency distributions and bedforms. In: D.J. Stanley and D.J.P. Swift (Editors), Marine Sediment Transport and Environmental Management. J. Wiley, New York, N.Y., pp. 255--310. Tucker, M.J., 1966. Sideways-looking sonar for marine geology. Geo-Mar. Technol., 2: 18--21.
M9
Letter Section SPECULATIONS ON PROCESSES RESPONSIBLE FOR MESOSCALE CURRENT LINEATIONS ON THE CONTINENTAL SHELF, SOUTHERN CALIFORNIA
H.A. KARL
U.S. Geological Survey, Menlo Park, California 94025 (U.S.A.) (Received March 28, 1979; revised and accepted September 3, 1979)
ABSTRACT Karl, H.A., 1980. Speculations on processes responsible for mesoscale current lineations o n the continental shelf, southern California. Mar. Geol., 34: M9--M18. A side-scan sonar survey of San Pedro shelf, California, reveals areas of mesoscale current lineations oriented approximately north-northeast in water depths of 20--25 m. Widths of sand ribbons range from 40 to 120 m and intervening erosional furrows, from 15 to 50 m. A conceptual model shows that the scale and orientation of current lineations agree with the dimensions and axial directions of Langmuir circulations theoretically generated by a combination either of southerly and southwesterly winds with regular trains of swell from the southern hemisphere or of two sets of wave trains crossing from the south and west. These longitudinal bedforms indicate shore-normal sediment transport at the times and on the areas of the shelf when and where they have been observed.
INTRODUCTION
A side-scan sonar system was used for a detailed survey of San Pedro Bay, California, during 8--12 December 1975 (Fig. 1). The research vessel "Vantuna", maintaining a speed of 3--4 knots, kept the bottom-towed vehicle off the bottom by about 5--10% of the total water depth. The maximum range of the recorder, which was usually set at 200 m, gave a total side-to-side cover age of 400 m. Reviews of interpretations of sonographs as well as details of the techniques involved are given by Chesterman et ai. (1958), Clay et al. (1964), Tucker (1966) and Belderson et al. (1972). In this report I describe, interpret, and speculate on the origin of certain linear features, resolved on the side-scan sonar records, that occur over parts of the continental shelf off southern California. San Pedro continental shelf, the field area, begins at Point Fermin, extends eastward for about 35 km, and terminates at Newport Submarine Canyon (Fig. 1). The shelf is a relatively fiat, featureless platform ranging in width from about 3 km at its west and east boundaries to about 20 km at its widest point. San Pedro shelf is covered with a thin (3--20 m) veneer of undifferenti-
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LINEATIONS
,
118o00'
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.
TRANSECT
21375,, BOX CORE
X
...........
NEWPORT! '-CANYON '
o r i e n t a t i o n o f c u r r e n t l i n e a t i o n s . T r a e k l i n e s d o n o t s h o w h e a d i n g d e v i a t i o n s f r o m set course.
Fig. 1. M a p o f s t u d y area, s h o w i n g side-scan survey t r a c k l i n e s , s h i p e k grab s a m p l e a n d c a m e r a t r a n s e c t s , b o x core l o c a t i o n s , and
33°35`
33°45'
118o20'
0
MII
ated late Quaternary sediment punctured in places by outcrops of Tertiary rocks (T.R. Nardin, oral communication, 1976). Modem surficial sediment consists of a m o n o t o n o u s blanket of silty sand and sandy silt interrupted by patches of coarser relict sediment. Currents have not been previously studied sufficiently to describe shelf circulation patterns in detail. Surface tides flood west and ebb east; tidal currents rarely exceed 20 cm/s at the surface (Emery, 1960). Surface waves approach San Pedro shelf along corridors from the west and south. Islands and banks tend to damp out long-period (15--20 s) swell from the west (Horrer, 1950) and leave short-period (7--10 s) waves to continue relatively free of interference. Long-period swell from the southern hemisphere reaches the shelf without interruption (Horrer, 1950). Although regional surface winds are generally northwesterly, San Pedro Bay is often affected by southerly and southwesterly winds generated by the Catalina Eddy (Graham, 1950; Stevens, 1977). Other localized winds are the seasonal northeasterly Santa Anas and the diurnal onshore--offshore breezes. Regional winds generated by seasonal high- and low-pressure cells strongly influence the major offshore current systems. However, the relationships between localized winds and shelf currents are very complex and not well understood in the study area. OBSERVATIONS A distinctive sonograph pattern of parallel bands of alternating dark and light streaks characterizes parts of San Pedro shelf (Fig. 2). The signature occurs in discontinuous patches at depths shallower than about 25 m on a substrate of fine to medium sand. Though identified in three areas, the lineations are well defined in only one patch on the central shelf (area A, Fig. 1). The width of dark lineations varies from about 15 to 50 m and the width of intervening light bands, from about 40 to 120 m (Fig. 2); the length of these features is at least 100--200 m. Wherever observed on the shelf, the long axis of lineations is oriented approximately north-northeast (Fig. 1); the lineations are apparently symmetrical, very low relief (> 1--2 m) features. During April 1976, 21 Shipek grab samples were collected along two transects, each approximately 1 nautical mile in length (Fig. 1). Samples show large variation in color and texture of surficial sediment (Fig. 3). Numerous photographs of the b o t t o m taken during this cruise along the same transects as the grab samples reveal abundant small-scale bedforms. Many bedforms (designated type A ripples) consist of double sets of ripples of unequal wavelength, with orthogonally intersecting crests (Fig. 4). The longer wavelength (15--20 cm) ripples strike approximately west-northwest and the shorter wavelength (5--10 cm) ripples approximately north-northeast. Ordinary symmetrical ripples, striking approximately west-northwest, were observed in deeper water and at other times of the year. Three box cores collected during November 1974 in area B (Fig. 1) show striking differences in vertical
M12
%o CORRECTED
. ~ " ~ . ~ . 0 R~IENTATION
Fig. 2. Part of sonograph record (transect X--X', Fig. 1) showing lineations described in text.
I00 0.90 080
0 70 E Q: 0 6 0 ul
'~ 050 o
0.40 030
0 20 Trr
/
010 000
L
J
± .....
I
/
TT
~
i
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i
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i
STATIONS (DISTANCE/NO SCALE)
M13
Fig. 4. Type A ripples described in text. lithology. Each core penetrated about 15 cm of sediment; the shallowest (core 21375) contained only rust-colored sand with a mean grain diameter of 0.24 mm. Core 21376, 2 km seaward, contained reddish-brown medium (0.29 ram) sand underlain by gray-green fine (0.19 ram) sand; the deepest core (21377) consisted only of gray-green very fine (0.09 ram) sand. DISCUSSION The sonograph patterns (Fig. 2) are attributed to differences in the reflectivity of the substrate caused by changes in sediment texture. Spacing, shape, and orientation of these lineations indicate that they are not the sea-floor expression of subsurface rock units, but rather are due to changes in sediment distribution associated with mesoscale and macroscale bedforms. Observed sonograph patterns closely resemble those described by McKinney et Fig. 3. Diagram illustrating variation in mean grain size along transects I--II and HI--IV.
For the purpose of graphic representation, distances between stations are shown as equal and with no scale; actual distance between successive stations ranges between 200 and 400 m.
M14 al. (1974), which they interpreted as mesoscale current lineations. Because the bedforms on San Pedro shelf are nearly shore-normal and not shore-parallel, interpreting these bedforms as mesoscale current lineations implies that the currents responsible for their formation have an offshore and/or onshore c o m p o n e n t of flow. The orientations of ripples seen in most areas of the shelf support this sense of water movement, and the stratigraphic sequence revealed in cores 21375--21377 suggests that nearshore sediment is being transported seaward. Since the model developed in this paper is speculative and based on the interpretation of these bedforms as current lineations, it should be noted that Swift and Freeland (1978) have interpreted somewhat similar nearly shore-normal bedforms found on portions of the inner shelf of the U.S. east coast as flow-transverse sand waves. Current tineations consist of sand ribbons (light bands} and erosional furrows (dark bands), which probably form in response to fields of large-scale helical flow cells (Allen, 1966, 1968; Kenyon, 1970; Duane et al., 1972; McKinney et al., 1974). According to the model of Swift and Ludwick (1976), stronger, divergent currents of these cells, where they contact the b o t t o m , erode the narrow furrows, whereas weaker, convergent currents construct the wide sand ribbons. Furthermore, sand ribbons consist of fine sand over a substrate of coarser material, whereas erosional furrows are areas in which coarse substrate has been exposed (or formed) by winnowing of the overlying sediment. This textural contrast creates the alternating dark and light bands on the sonograph. Although sampling of individual ribbons and furrows was not possible, similar textural variations exist in the area of current lineations (Fig. 3). Helical flow cells, known as Langmuir circulations, exist in the surface mixed layer of the oceans. Several theories explain the mechanisms generating Langmuir circulations (Failer, 1969, 1971; Scott et al., 1969; Craik and Leibovich, 1976). The theoretical models of Craik and Leibovich (1976), which make use of the Stoke's drift of the wave field and the shear flow induced by the wind, as well as the laboratory experiments of Faller (1977) and Failer and Caponi {1977, 1978), which utilize a wind-wave tank to generate Langmuir circulations, provide a basis for a conceptual model to account for the current lineations observed on San Pedro shelf. While increasing our understanding of some aspects of Langmuir circulation, none of these experiments or theories have unequivocally identified the mechanisms by which Langmuir circulations are generated. The laboratory experiments of Failer (1977) and Failer and Caponi (1977, 1978) showed that both wind and waves are necessary for the generation of Langmuir circulations. They form either orthogonally to the propagation direc tion of single wave trains or at the intersection of interference patterns generated by two crossed wave trains. In the wind-wave tank a thin film of water at the surface moved in the direction of the wind. However, dye placed along the floor of the tank revealed that b o t t o m horizontal convergent currents between cells flowed counter to the wind direction owing to a subsurface return
M15
flow of water (see Failer and Caponi, 1978, fig. 4, p. 3620). These experiments suggested that the wavelengths of Langmuir circulations depend upon the depth of the mixed layer but may also be a function of the wavelength of the surface waves, and that under certain circumstances more than one scale of Langmuir circulations may exist simultaneously. It is understood that the complex phenomena which characterize natural environments, restrict the extrapolation of experimental results derived in the laboratory to oceanic scales. It may be possible, however, to apply some of the fundamental aspects of Langrnuir circulations exhibited in the laboratory to the shallow shelf waters of San Pedro Bay. All the conditions necessary to generate Langrnuir circulations of the correct dimensions and orientation to account for the current lineations observed on San Pedro shelf exist at times in San Pedro Bay (Fig. 5). With a mixed-layer depth to b o t t o m of about 25 m, Lagmuir circulation wavelengths of about 50--100 m are expected (A.J. Failer, written communication, 1978; Failer and Caponi, 1978, fig. 2). During periods of southerly and southwesterly winds, Langmuir circulations can be generated in combination with southern hemisphere swell. When two wave trains approach simultaneously from the west and south, ascending currents of Langmuir circulations can form beneath intersections of the two sets of wave crests. Orientations of symmetrical ripples indicate that they have been generated by waves from the southern hemisphere. Although the origin of type A ripples remains problematic, the characteristics of these bedforms suggest formation under a set of two crossed wave trains of unequal periods (wavelengths), with longer period waves approaching from the south and shorter period waves from the west. Axial orientation of Langmuir circulations in both situations would be
Fig. 5. Conceptual model of current lineations forming in response to Langmuir circulations. A ffi Relation of Langmuir circulations to sand ribbons and erosional furrows. B and C diagrammatically depict generation of current lineations by southerly and southwesterly winds and by two conditions o f waves and swell: B = simple case of southern hemisphere swell; C ffi one possible interference pattern composed of long-period swell from the south and shorter period waves from the west.
M16
shore-normal (Fig. 5). It is not known whether a vertical flow profile develops in response to onshore winds and waves in San Pedro Bay analogous to the shear profile characterizing the experiments of Faller and Caponi (1978). Thus, it is not possible to determine whether the convergent currents of Langmuir circulations in San Pedro Bay flow seaward or shoreward; nor is it possible to assess the magnitude of the horizontal flow component. The convergent currents, observed by Faller and Caponi (1978), flowing counter to the direction of the surface wind and waves may have been forced by the size and design of the tank; shoreward surface flow on the open shelf may or may not be compensated for by an offshore subsurface flow. Apparent seaward transport of nearshore sands suggested by the vertical lithologic sequences observed in cores 21375--21377 collected in area B (Fig. 1) could reflect some other process and current. Indeed, the Langmuir circulations generated on San Pedro shelf are probably second-order currents superimposed on the primary current systems and processes governing water circulation in San Pedro Bay. Theoretically, Langmuir circulations should occur over the whole shelf width. Among the reasons that may account for current lineations being observed only on onshore parts of the shelf are that: (1) density stratification higher in the water column may limit the cell dimensions and penetration of Langmuir circulations in deeper water, (2) Langmuir circulations may not be sufficiently intense in deeper water to move the b o t t o m sediment, and (3) the uniform sediment textures seen on other parts of the shelf may not cause reflectivity changes recognizable on sonographs. Although meteorological and oceanographic conditions in San Pedro Bay theoretically can create Langmuir circulations of proper scale and orientation to account for the observed mesoscale current lineations, it was not possible to determine whether Langmuir circulations generate currents of sufficient velocity to cause the shear stresses necessary to move fine and medium sand. Sediment particles, however, may be and are set in motion or held on the threshold of movement by currents derived from surface waves and tides. An ordered helical flow field periodically superimposed on oscillatory or random current patterns would distribute sediment so as to form low-amplitude, symmetrical current lineations. CONCLUSIONS
Distinctive patterns of alternating light and dark parallel bands on sonographs of San Pedro shelf are interpreted as mesoscale current lineations. These longitudinal bedforms consist of fine sediment (constructional sand ribbons) separated by narrower bands of coarse sediment (erosional furrows) that either underlies the field of current lineations or represents a lag deposit in very shallow troughs. A conceptual model explains the current lineations as forming in response to Langmuir circulations generated b y specific combinations of the winds and surface waves often occurring in San Pedro Bay. H o w frequently these Lang-
M17 muir circulations form has not been estimated, nor has it been determined how quickly current lineations form in response to the currents generated by Langmuir circulations, or how long the current lineations remain as recognizable features on the sea floor. Presuming that the Langmuir circulations exhibit a horizontal flow component, sediment in the area of the current lineations would be transported normal to shore. It is n o t possible from the extant set of observations to determine whether the transport is onshore or offshore nor the magnitude of transport.
ACKNOWLEDGMENTS I would like to t h a n k the officers and crew of the research vessel " V a n t u n a " for their willingness to lend a hand during the survey. Without the aid of the technicians and machinists of the Marine Facility of the University of Southern California, this cruise would not have been possible. The ideas expressed herein represent part of the author's dissertation completed under the supervision of Dr. D.S. Gorsline of the Department of Geological Sciences, University o f Southern California. The National Science Foundation funded this work under grants GA 40049, DES 75-01438, and DES 75-01438, A m e n d m e n t AO1. This report was initiated and completed during tenure as a National Research Council Research Associate with the U.S. Geological Survey in Menlo Park, California. M.E. Field and D.M. Rubin of the U.S. Geological Survey and D.J.P. Swift of the National Oceanic and Atmospheric Administration (NOAA) kindly reviewed drafts of the manuscript. T.R. Nardin of the Department of Geological Sciences, University of Southern California, corrected side-scan orientations from equations supplied by D.J.P. Swift.
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