Sediment transport in Washington and Norfolk submarine canyons

EVAN B. FORDE, DANIEL J. STANLEY*, WILLIAM B. SAWYER and KAREN J. SLAGLET Atlantic Oceam~,qraphic and ~letem'olo,~lical Lahoratm'ies, NOA A, ~.liami, Florida 33149, USA

A sediment study suggests that Washington and Norfolk canyons off the Mid-Atlantic States are not inactive, but have served periodically since the Late Pleistocene as conduits of sediment originating on the adjacent shelf and upper slope. Large quantities of sand occur in the canyon heads as thin beds and laminae, and on the continental slope as mixtures of sand (to > 40",,j, silt and clay that are extensively reworked by burrowing organisms. Sandy turbidites occur in the canyons on the rise. Basinward dispersal, from the outer shelf and uppermost slope, is recorded by heavy mineral suites and bioclastic components, primarily foraminifera of shallow marine origin, in the lower slope and upper continental rise canyon cores. The down-axis movement of material, presumably episodic, in the Holocene to recent results from offshelf spillover into canyon heads, failure on the steep walls bordering canyons on the slope, and resuspension by bottom currents.

I N T R O D U C T I O N

The potential role of submarine canyons off the eastern USA as conduits through which large volumes of sedi- ment are transported basinward continues to interest geologists evaluating the evolution of the northwest Atlantic continental margin. There is general agreement that these large valleys were particularly active in outer margin sedimentation during the Quaternary eustatic low stands I -3. Moreover, most studies suggest that canyons south of Georges Bank are relatively inactive at present, and do not funnel substantial amounts of coarse-grained sediment basinward 4- 6. Seismic and sedimentologic sur- veys of the Wilmington and Baltimore canyon systems, however, indicate that movement downslope of material as large as ~pebble-size has occurred since the end of the Pleistocene'. Less is known of Washington and Norfolk, two major canyons incised on the southern Mid-Atlantic Shelf seaward of the Chesapeake Bay Delmarva Peninsula (Fig. 1).

The outer continental shelf of the Middle Atlantic Bight slopes gently (0.16 ° ) seaward s , and shelf-break depths range from 120 to 130 m 9. The meandrous axes of both Washington and Norfolk canyons extend eastward across the slope toward the Hatteras Abyssal Plain. In cross- section the canyons display V-shaped profiles between their heads and the lower-slope reaches; the canyons are bound by ridges and levees on the lower slope and rise ~°. The head of Washington Canyon incises the outer shelf in a NNW direction (Fig. 2A): in contrast, the head of Norfolk is longer (cut about 16 km into the shelf) and

* Division of Sedimentology, Smithsonian Institution, Washington, D.C. 20560, USA. t US Army Corps of Engineers, SAWEN-EW, Wilmington, N.C. 28402, USA.

trends toward the west (Fig. 2B). Surficial sediments on the Mid-Atlantic Shelf adjacent

to the canyon heads consist largely of coarse to medium- grained, shelly terrigenous sand 4' 11. Comparable data for Washington and Norfolk canyon sediments are sparse. This paper, summarizing the results of a petrologic study, bears on the problem of post-Pleistocene to recent sediment dispersal in Washington and Norfolk canyons.

METHODS

Twenty-five hydroplastic cores and eleven grab samples were taken in the Washington and Norfolk canyons from the NOAA ship Researcher in 1975 and 1976. A modified hydroplastic gravity corer 12, with a diameter of 7.6 cm and PVC barrel to 3 m in length, provided high recovery rates with a minimal amount of sediment disturbance. Shipek grab samples were collected in areas where bottom sediment was too coarse to allow core penetration.

All core samples were X-radiographed within a few hours of recovery, and a study was subsequently made of the texture and mass physical properties of 169 samples. Size distribution determinations are based on Automated Rapid Sediment Analyser ~3, pipette, and Model T Coulter Counter methods: proportions of sand, silt and clay were calculated as percentages of the total sample. Aliquot samples for mineral identification were taken from 45 core intervals based on a predetermined random scheme. Composition was determined by point counts (approximately 400 grains counted) of the separated light and heavy mineral fractions. X-ray diffraction was used in some instances for identification of minerals. Tabular listings of textural and compositional data are available from the senior author.

0141-1187/81/020059 0452.00 © 1981 CML Publications Applied Ocean Research, 1981, Vol. 3, No. 2 59

Sediment tran,sport in Hitshi~l,qton and NorloIk submarine ca~u'(ms: E. B, km'de el al.

4 0 °

7 6 °

3 9 °

7 7 ' :

3 8 ° 7 8 c

3 7 c

7 9 ~ ~ o J

3 6 ° 7 9 ° 7 8 ° 3 5 ° 7 7 ° 3 4 °

Figure 1. Chart qfthe continental margin of l the US Mid- Atlantic States showing Washington and Nm:[blk c'anymts discussed in text (depth in metres)

CANYON SEDIMENTS

Sediment in both Washington and Norfolk canyons is highly variable in texture, ranging from sandy gravel to silty clay; the majority of core sections, however, consist of dark olive grey (5Y3/2) silty clay in Washington and clayey silt in Norfolk Canyon. The sand component is dominated by quartz (average 610;: maximum 987~,) and feldspar, and generally includes large proportions of foraminiferal tests (average 35'!~;: in a few instances, range to a maximum of 90'~;,), and shell hash. Heavy minerals, mica and other terrigenous components generally form minor proportions (to <1571;) of the remaining sand fraction.

Sand laminae, thin lenses and beds (to 2 cm) between finer-grained units commonly occur in cores from the heads of the two canyons. With few exceptions, cores seaward of the upper slope in both canyons generally contain sandy clayey silts and silty clays that show few sedimentary structures in split core examination. X- radiographs, however, reveal the presence of many in- tensely bioturbated sections in these cores. Biogenic structures are more prominent in the Washington Canyon cores.

The assemblages of the coarse fraction mineral com- ponent are remarkably similar in both canyons. The following minerals are present in the two canyons, in order of diminishing abundance: quartz, feldspar, mica, ilmenite and opaque heavy minerals, zircon, garnet, epidote, hornblende, staurolite and tourmaline. This assemblage is generally comparable to that mapped on the outer shelf by Kelling et al. ~. However, the pro- portion of iron-stained minerals in the canyons is some-

what lower than on the adjacent shell. Authigenic pyrite was noted in association with burrowing activity. The biologic constituents include planktonic and benthonic foraminiferal tests and shell fragments (pelecypods, echinoid spines, and othersL No microt)ssils older lhan Pleistocene were identified•

SAND DISTRIBUTION PATTERNS

Surficial sediment in both Washington and Norfolk canyons shows a general decrease in grain size in a downslope direction (Fig. 2A, B). Sediment in the head and upper slope reaches of the canyons commonly comprises more than 507{, sand, but sediment in slope areas seaward of 800 m is characterized by decreased sand content and increased proportions of silt and clay. The surficial sand content in both Washington and Norfolk canyons decreases to about 4,.~, at approximately 1000 m. Cores at and beyond the base of the slope (not shown in Fig. 2), such as N-19 (36 57.64'N, 74 I I.10'W, 2126 m deptht, N-21 36 58.79'N. 73 58.54'W, 2526 m depth), W-

A 7 4 ° 3 0 ' 3 7 " 3 0 7 4 ° 2 0 ' 74o]0

W l

3 7 ~ 30 '

WASHINGTON CANYON

: , l W 1 7 W I 3

w~2~ , • ~

: : . WlSI~-~ ""~VVl8

- - ~ 1 1 / / : ~

37 ° 20'

3 - 6

~ 10-~3

~ 2 0 - 2 3

U ° 35 -~8

z 5 0 - 5 3

~ 7 0 73

~ ~ 0 0 - I 0 3

1 4 6 149

7 4 o 3 0 '

W-5 W - 9

, ~ . ~

, . ~ ~

, , ~

. ~ . ~

~ ~ . . ~

3 7 ° 2 0 . 7 4 o 2 0 ' 74~10 '

P E R C E N T A G E S A N D

W II W-12 W ]4 W 15 W 17 W i8 W-Z1 W-22 q 1 TT~! [~! !

- ~

j ~ ~ ~ B ,̧, ~ N 2

• ~ 3

~.,

• N7

'~N6

*N~ ~

~ N ~

3 - 6

0 13

~ 2 © - ~ 3

_~ 35 3a

~> 5o 53

~o z3

I 0 O ~03

NORFOLK CANYON

. . . . . . . . . . .~,, ~.,,

P E R C E N T A G E SAND

' ~ I ~ ~

, ~ ~ ~ i ~ L' ~ - r ' ~ i ~ ,l •

~ 1 ~ 4 • ,

1~2

: , : 2 : : ::: I N ] ~ I N 1 7

~ 1 6

~.

~-21

~ ~ . . . . . , . . . .

~ 125 128 123

~ .... ' " " " ~ I . . . . ~' ~ ~ - ~ • • , , , 5 ~ - - ~

1~4 • . . . . . " " ' " • " ' . . . . 1~5.

Figure 2. Detailed charts showing coqfiguration q[ (A) ~shi~gton aml (B) No~]blk cmD'ons, and sample sites ( • = core: • = Shipek flrah samples). The proportion qlsand in core samples is shown in black

60 Applied Ocean Research, 1981, Vol. 3, No. 2

TURBIDITE

120

125

O

z I$0 "1- I-- (:L bJ C3

135

Sediment transport in Washington and No~Tlblk submarine canyons: E. B. Forde et al.

14.o

(CORE N-21)

=ND- SILT- CLAY

~RALLEL SAND LAMINAE

RADED INTERVAL

ILTY-CLAY

Figure 3. Notfolk core 21, showing a fining-upward, coarse-grained turbidite consisting of mixed bioclastic and terrigenous components. Note the sharp base, at 134 cm, truncating the underlying clayey silt unit

were found throughout the turbidite indicating down- slope displacement of material of proximal origin. The fine-grained units below and above this turbidite are comparable, i.e. intensely burrowed with a sand content of

o , about 8/,,. The temporal patterns of the coarse fraction in

Washington and Norfolk canyon cores are variable (Fig. 2). Cores recovered from the axis of Washington Canyon at depths less than 800 m display a slight down-core increase in sand content. In contrast, Norfolk Canyon axis cores at less than 800 m display decreased amounts of sand down-core. Cores deeper than 1100 m in Washington and 800 m in Norfolk canyons show variable vertical trends in sand content. Exceptions are core W-22 (2323 m), which reveals a coarsening-downward trend, and turbidite bearing core N-21 illustrated in Fig. 3.

21 (37'~16.93'N, 7343.69'W, 2230 m depth) and W-22 (37°10.76'N, 73 54.63'W, 2323 m depth) contain some- what higher proportions of coarse-grained sediment in their near-surface intervals.

Some cores contain low percentages of sand in near- surface sections, but variable proportions at different depths below the core top (Fig. 2). Core W-14, for example, contains less than 3'~'/o sand at the surface but 43~o at 50 cm below the core top. Other cores with substantial proportions of coarse sediment below the surficial section include: W-12, W-15, W-17, W-18, W-21, W-22, N-14, and N-21. With the exception of N-21, the high sand fraction in these cores is well mixed with the silt and clay rather than forming distinct sand layers or lenses.

A graded sequence about 85 cm thick was recovered on the uppermost rise at Norfolk canyon core site 21 (Fig. 3). This unit, interpreted as a turbidite, has a sharp basal contact at 134 cm and an indistinct fine-grained upper boundary near 50 cm from the core top. The lower graded sequence contains gravel, shell fragments, terrigenous sand-sized components (> 90~o) and negligible amounts of silt and clay. A distinct laminated sequence above the graded interval, to 121 cm, consists of fine-grained sand, and the section above the laminated zone grades upward to a silty clay. Shallow marine microfossils from the midshelf region (Bock, W. 1980, personal communication)

DISCUSSION

Seismic reflection surveys reveal that Washington and Norfolk canyons have been shaped by numerous episodes of erosion and deposition 1°'1s. That these submarine valleys funneled large amounts of sediment across the slope and rise during Late Quaternary is shown by fan development at their lower extensions at their entry point on the margin of the Hatteras Abyssal Plain 16. There is little obvious evidence of coarse sediment transport through and out of these canyons in the post-Pleitocene.

Several arguments suggest that a downslope sediment dispersal role, albeit diminished, has continued to be important in the Holocene to the recent. (1) It would be expected that the relief of these valleys on the rise would be attenuated as a result of the large volumes of sediment transported in suspension toward the SW by the contour- following currents. However, the cross-axis profiles of these canyon systems on the rise at depths well below our coring sites show sharply defined thalwegs 16'1v. This indicates that the lower extensions of valleys have re- mained sedimentologically active, either as a result of through-canyon flow or of intensified erosion due to increased bottom current energy levels at these depths, or both.

(2) Current meters at 3 m above the bottom at 600 m depth in Washington and 30 m above the bottom at 573 m depth in Norfolk recorded velocities that periodically reached 20 cm/sec in the former and 30 cm/sec in the latter. These data from the upper slope reaches of the two canyons call attention to potential displacement of sedi- ment by bottom currents 5. The movement of sand by currents was actually observed by the first author during submersible dives at approximately 300 m on the floor of Norfolk Canyon.

(3) The proportion of sand found in some Washington and Norfolk canyon cores on the lower continental slope is higher than that recorded in previous studies ~. Much of this sand includes bioclastic material of unspecified Pleistocene to recent age, reworked from the shelf and uppermost slopes. The time of emplacement of this sand remains unknown. We do call attention to the relatively high sand fraction only 3 cm below the core top in Washington core 21 and in other slope cores, and to the sandy turbidite in Norfolk core 21. The mineralogy of these sands is identical to that of sand in the heads of the canyons and adjacent shelf.

Applied Ocean Research, 198L Vol. 3, No. 2 61

Seditnem transport in Washin.qton aml NmTlblk submarine

The steep (commonly 10 to 20 and locally > 3 0 ~ canyon walls and levees bound ing both Washington and Norfolk canyons deeply incised on the cont inenta l shelf (relief >900 m in Norfolk Canyon) provide favourable sites of sediment failure. Processes generally invoked for the displacement of large volumes of sediment of canyon margins are those associated with slumping, sediment flows including turbidi ty currents, and creep ~s'~'. We propose that periodic, down-flank gravity-induced trans- port is probably the dominan t process in the canyons at present. Indirect evidence for this is based on observat ions made during submersible dives in Washington and Norfolk canyons by Malahoff et al. 2° and in Norfolk Canyon by the first author: bioerosion has clearly wea- kened canyon walls and probably has triggered, at least on a local scale, slumps in the post-Pleistocene. Although specific slumps have not been identified in these two canyons, geotechnical tests, which arc often inconclusive in cores of such short length, suggest that these sediments are metastable. Addit ional evidence for the role of or- ganisms in this downwall t ransport process in both Washington and Norfolk canyons is recorded by the mixing of sand with silt and clay fractions, worm burrows, and the presence of fecal pellets. We believe that canyon head and canyon wall bioerosion-induced slumping, coupled with creep in less steep sectors near the axes, are ongoing processes: these have cont inued to displace sediment of shallow marine origin towards the canyon axes. The presence of graded beds on the rise indicates that down-axis flowing turbidi ty currents also occur in these two canyons: intense b io turba t ion is particularly effective in mixing these and other deposits in the slope reaches of the canyons.

Observat ions from submersibles in Wilmington Canyon, nor th of the present study area, indicate that the most active zone of down-axis coarse sediment movement (by debris flow) is actually restricted to a very narrow belt little more than 100 m wide 2 ~. Such narrow belts of down- axis t ransport have not, to date, been recognized in the Norfolk and Washington canyons. The potential impor- tance of this phenomenon should not be overlooked, for unless recovered specifically from such active thalwegs, cores would tend to recover primarily material that has (a) slumped or creeped down canyon walls, (b) been emplaced by overbank spill dur ing gravity flows, or (c) settled out of suspension and moved by bot tom currents. Definition of downslope t ranspor t trends in Norfolk and Washington canyon systems requires more precise core recovery, specifically in the narrow, potentially active canyon axis proper, and radiocarbon dating of such cores to calculate sedimentat ion rates.

ACKNOWLEDGEMENTS

This study was funded by the N O A A Atlantic Oceanographic and Meteorological Laboratories and a Smithsonian Research Award. The authors thank Drs. G. H. Keller, Oregon State Univeristy, and R. Bennett,

can vmt.s: E. B. km'de et al.

NOAA, Miami, for reviewing the manuscript , and G. Merrill, NOAA. Miami, for providing lhe detailed bathymetric base used for the study.

REFERENCES

10

I I

12

13

14

15

16

17

18

19

20

21

Veatch, A. C. and Smith, P. A. Atlantic submarine valleys offthe United States and the Congo submarine valley, Geol Soc. Am. Spec. Pap. 7, 1939, 101 pp. Shepard, F. P. and Dill, R. F. Submarine Can)ons and Other Sea Vulleys. Rand McNally, Chicago, 1966, 381 pp. Emery, K. O. and Uchupi, E. Western North Atlantic Ocean: topography, rocks, structures, water, life and sediments, Am. Ass. Petrol. Geol. Memoir 17, 1972, 532 pp. Keller, G. H. Sedimentary processes in submarine canyons off norlhcastern United States. IX Conqr~s International de Si'dimentolo~tie, Nice, 1975, 6, 77 Keller, G. H. and Shepard, 1.. P. Currents and sedimentary processes in submarine canyons off the northeastern United States, in Sedimentation in Submarine Canyons, Fan.s and Trenches, (Eds. Stanley, D. J. and G. Kelling), Dowden, Hutchinson and Ross, Stroudsburg, Pa. 1978, pp. 15 32 Doyle, L. J., Pilkey, O. H. and Woo, C. C. Sedimentation on the eastern United States continental slope, in Geology ~fContinental Slopes, (Eds. Doyle, L. J. and O. H. Pilkeyl, SEPM Spec. Publ. 27, 1979, 119 129 Kelling, G. and Stanley, D. J. Morphology and structure of Wilmington and Baltimore submarine canyons, eastern United States. J. Geol. 1970, 78, 637 Wear. C. M., Stanley, D. J. and Boula, J. [';. Shelf-break physiography between Wilmington and Norfolk canyons, ,~/lur. "Fechnol. Soc..l. 1974. 8, 37 Stanley, D. J. and Wear. C. M. "lhe "mudlme': an erosion- deposition boundary on the upper conteinental slope. Mar. Geol. 1978. 28, M19 M29 l-orde, E. B. Processes in Ihree East Coast submarine canyons EOS, 1978, 59, 303 Milliman. J. D., Pilkey, O. H. and Ross, D. A. Sediments of the continental margin off the eastern United States, Geol. Soc. Am. Bull. 1972, ~3, 1315 Richards, A. F. and Keller, G. H. A plastic barrel sediment corer. Deep-Sea Res. 1961, 8. 306 Nelsen, T. A. An automated rapid sediment analyser. Sedimemol. 1976, 23, 867 Kelling, G.. Sheng, H. and Stanley, D. J. Mineralogic com- position of sand-sized sediment on the outer margin off the Mid- Atlantic State: assessment of the influence of the ancestral Hudson and other fluvial systems, Geol. Soc. Am. Bull. 1975, ~6, 853 Forde, E. B. Evolution of Veatch, Washington and Norfolk submarine canyons: inferences from strata and morphology, Mar. Geol. 1980, in press Cleary, W. J., Pilkey, O. H. and Ayers. M. A. Morphology and sediments of three ocean basin entry points, Hatteras Abyssal Plain, J. Sed. Petrol. 1977, 47, 1157 Stanley, D..I.. Sheng, H and Pedr~.za. C. P. Lower continental rise east of Middle Atlantic States: predominant dispersal perpendicular to isobaths, C-col. Soc. Am. Bull. 1971, I]2, 1831 Stanley, D. J. Sedimentation in slope and base of slope environ- ments. Lecture 8, "Fhe Ne~ Concepts ~[ Continental Margin Sedimentation, Am. Geol. Inst. Short Course, 1969, p. DJS-8-25 Stanley, D. J. Submarine canyon wall sedimentation and lateral infill: some ancient examples, Smithsonian ('m~tr. Marine Sci. 5, 1980, 20 pp. Malahofl'. A., Emblcy, R. W. and Fonari. D. J. Geological observations from the ALVIN of the continental margin from Baltimore Canyon to Norfolk Canyon EOS 1979, 60 (18), 287 Stanley, D. J. Pebbly rand transport in the head of Wilmington Canyon, Mar. Geol. 1974, 16, MI M8

62 Applied Ocean Research, 1981, Vol. 3, No. 2