Seismic profiling survey of the east coast submarine canyons Part 1. Wilmington, Baltimore, Washington and Norfolk Canyons

Deep-SeaResearch, 1968, Vol. 15, pp. 613 to 616. Pergamon Press. Printed in Great Britain.

SHORTER

CONTRIBUTIONS

Seismic profiling survey of the east coast submarine canyons Part 1. Wilmington, Baltimore, Washington and Norfolk Canyons* ELAZ~ UCHUPIt"

(Received 19 July 1968) Abstract--Seismic profiler recordings indicate that the morphology of Wilmington, Baltimore, Washington, and Norfolk Canyons are erosional not structural in origin. No evidence of folding or faulting was observed on any of the profiles. INTRODUCTION OVER the years the submarine canyons off the east coast of the United States have received considerable attention from many geologists. The topography of the canyons on the shelf and continental slope is well known from soundings from the U.S. Coast and Geodetic Survey hydrographic surveys. Results from these surveys have been described and discussed by VEATCHand SMIra (1939) and many other geologists (see SrmPARD and DILL, 1966, p. 143-161 for pertinent references). The seaward extensions of the slope canyons on the continental rise have been studied by HEEZEN and TOLSTOY (HEEZEN, THARP and EWING, 1959, pl. 2), NORTHROP, FROSCHand FR~SSETTO(1962), PRATT (1967), and RONA, SCHNEIDER and I-I~EZEN (1967). More recently, observations made from the Woods Hole Oceanographic Institution submersible Alvin have yielded considerable information on the morphology of the canyons (Ross, 1968; TRUMBULLand HATHAWAY,1968 ; TRUMBULLand M c C ~ n s , 1967). Data obtained using this new technique suggests that slumping has played a significant role in the formation of the canyons. Sediments from the slope canyons and. the channels on the rise have been described by STETSON(1949), ERICSON,EWING, WOLLIN and I-IEEzEN(1961), and STANLEY and KELLING(1968). Dredging, coring, and sampling during dives by Alvin have yielded data on the pre-Pleistocene stratigraphy of the canyons (EPdCSON, EWIN~, WOLLINand I-I~EZEN,1961 ; NORTHaOP and I'IEEzEN, 1951; STETSON,1949; TRUMBULLand I-IATHAWAV,1968; GIBSON, HAZEL and MELLO, 1968 in press). Data on the structure of several of the canyons have been obtained through the use of seismic profilers (EWING, LUSTS, ROBERTS and HaRSm~N, 1960; KNOTT and HOSrdNS, 1968; ROBERSON, 1964). During May, 1966 several seismic profiler lines were recorded on Wilmington, Baltimore, Washington, and Norfolk Canyons (Figs. 1 and 2). The survey was made in order to determine the sedimentary framework of the canyons, with the hope that such information would lead to a better understanding as to their origin and evolution. The results are described below. RESULTS Wilmington, Baltimore, Washington, and Norfolk are four major submarine canyons south of New York (Fig. I). Wilmington Canyon is the largest of the four, and can be traced for over 300 km beyond the base of the continental slope (PaATT, 1967). Baltimore Canyon farther south only extends 60 km onto the rise. Washington Canyon has a seaward extension across the rise of about 200 km. Norfolk Canyon the southernmost one investigated during the present survey dies out rather rapidly on the continental rise merging with Washington Canyon by way of two discontinuous channels (PRATT, 1967). Figure 1 shows the locations of the seismic profiles in relation to the submarine canyons. Profile 184 is located on the upper continental rise, most of 180, all of 182 and 186 are on the continental slope, and the remaining profiles across the segments of the submarine canyons incised into the contin*Contribution No. 2126 of the Woods Hole Oceanographic Institution. "t'Woods Hole Oceanographic Institution, Woods Hole, Mass. 02543, U.S.A. 613

614

Shorter Contributions 75 °

74 °

:/

73 °

39°

2'

BALTIMORE CANYON

t

38 °

/s" /

/ 37 °

/ 0

IOOKM i

50

I

, , CONTOURS

i IN

METERS

Fig. 1. Locations of seismic profiles shown in Fig. 2. The dashed line north eastofWilmington canyon is the reference line to which the seismic profiles were adjusted. ental shelf. The profiles on the shelf display the same type o f structure. Reflectors beneath the shelf arc horizontal, can be traced for a considerable distance, and are deeply entrenched by the suhmaiin¢ canyons (Fig. 2). Horizons o n the shelf do not bend as they approach the canyons indicating that the strata have compressive velocities similar to that of water. The downward bending o f reflectors as they approach canyon walls is known as the velocity effect. Velocities in water generally are less than in the sediment, and consequently as the sediments over the reflector become thinner in the direction of the canyon wall travel time to the reflector increases. Thus, a horizontal interface will show a n apparent dip as the interface approaches the canyon wall. Several prominent terraces nearly 5 k m wide (see 176 and 188) are present o n th~ canyon walls. Possibly several hundred meters of sediment fill (assuming a velocity of 1800 m/see) may be present in the centre of Wilmington (profile 188)and Norfolk (180)canyons. The seismic profiler recordings also suggest that the canyon side-slopes may have more than one origin; for example, the southwest wall of Washington Canyon

Shorter Contributions 200 --

r

t50 1

I

KILOMETERS

615

100

5(3

:

,

0

• i

i

I

I

~m,naton

190

~'~'~

Canyon ~

Norfolk

Washington

.7 caoyon

~=~awo~

Norfolk

=

~,.oL

:~ ~=

-

Norfolk

~anyon

. . . .

-Ral/~bTore

Conyon~

\

Wi ~gto

¢"

Canyon~

Barn?note__

Wash/hgfon Canyon

Canyon ~

Wilming¢o~ \~-~ Canyon\ "

-

Norfolk anyon

IRq

~

laO 182

co_n~::,,, ~

Washtnglon C a n y a ~

on~;

178

.....

q

Washington /Canyon

Washthgton

/co~o~

_=/Canyon

"

~---

~a,nmore an on ~ ,,l W/lmingfon

Balhmore

canyon

Wilmington Canyon

~__~

Fig. 2. Seismic profiles of Wilmington, Baltimore, Washington and Norfolk Canyons. The profiles were taken using a 10,500 .r sparker as a sound source programmed at a 5 sec repetition rate. Ship's speed during the survey was about 10 kin/hr. Water depth and sediment thicknesses expressed in time may be converted to approximate depths and thicknesses by assuming values of 1500 m/see (for water) and 1800 m/see (for the sediment).

(profile 176) and the northeast side of Wilmington Canyon (188) may have been formed by slumping or deposition and not just simply downcutting. Between the major canyons the profiles also reveal several buried channels having reliefs in excess of 500 m. Profiles on the continental slope and rise display a different structure than the profiles on the shelf. The deeper strata, for example, bend as they approach the canyons suggesting that these sediments have higher velocities than water. In Wilmington Canyon (profile 186) a broad terrace along the contact between the sediments has higher and lower velocities than water. On both the rise and lower continental slope (182 and 184), the internal reflectors are somewhat more irregular than those on the shelf. The lower slope as shown by profile 182 is serrated in outline being cut by a large number of small channels rather than several V-shaped canyons as on the shelf and upper slope. On the upper continental rise the seaward extensions of the slope canyons consist of low relief, broad, flat-bottomed channels (profile 184). These channels are cut into a gently undulating terrain containing several prominent angular unconformities. CONCLUSIONS

Results from the present seismic profiler survey indicate that Wilmington, Baltimore, Washington, and Norfolk Canyons are erosional in origin. The canyons appear to be entrenched into essentially horizontal beds that can be traced for a considerable distance. No suggestion of the type of structural control (faulting) reported by YERKES, GORSLINE, and RUSNAK (1967) from Redondo Canyon in southern California was encountered on any of the profiles. Possibly as much as several hundred meters of sediment fill may be present in the center of Wilmington and Norfolk Canyons.

Acknowledgements--Financial support for this project came from th.e U.S. Geological Survey. The writer wishes to thank K. E. PRADA of Woods Hole Oceanographic Institution and D. W. FOLGER of Lamont Geological Observatory for standing watches during the cruise. Appreciation is also expressed to the captain, officers, and crew of R.V. Gosnold who made this survey possible. REFERENCES

ERICSON D. B., M. EWING, G. WOLUN and B. C. HEEZEN (1961) Atlantic deep-sea sediment cores. Bull. Geol. Sac. Am., 72, 193-286.

616

Shorter Contribututions

EwINo J., B. LusmN, A. ROASTS and J. ~ (1960) Sub-bottom reflection measurements on the continental shelf, Bexmuda Banks, West Indies Arc, and in the West Atlantic Basin..r. geophys. Res., 65, 2849-2859. GmSON T. G., ~'. E. HAZELand J. F. MELLO(1968) Fossiliferous rocks from submarine canyons off the northe~tern United States, U.S. Geol. Surv. Prof. Paper 600 C, in press. KNOTTS. T. and H. H o s m ~ (1968) Evidence of Pleistocene events in the structure of the continenta) shelf off the northeastern United States. Mar. Geol., 6 (1), 5-43. NORTHROP J., R. A. FROSCH and R. FRAgSETrO(1962) Bermuda-New England Seamount Arc. Bull. Geol. Soc. Am., 73, 587-594. NORTHROP J. and B. C. HE~Z~N (1951) An outcrop of Eocene sediment on the continental slope. J. Geol., 59, 396-399. RONAP. A., E. D. Scm,wm~g and B. C. HEEZEN(1967) Bathymetry of the continental rise off Cape Hatteras. Deep-Sea Res., 14, 625--633. Ross D. A. (1968) Pexsonal Communication. SI-IEPARDF. P. and R. D. DILL (1966) Submarine Canyons and Other Sea Valleys. Rand McNally, Chicago, 381 pp. STANLEYD. J. and G. KELLING(1968) Sedi_mentation patterns in the Wilmington Submarine Canyon area. In: Trans. ham. Syrap. Ocean Sci. Engng o f the Atlantic Shelf, Mar. Tech. So(:., 127-142. Sr~SON H. C. (1949) The sediments and stratigraphy of the east coast continental margin; Georges Bank to Norfolk Canyon. Papers Phys. Oceanogr. Met., 11, 1-60. TRUMBULLJ. V. A. and 3. C, HATHAWAY(1968) Personal Communication. TRU~mULLJ. V. A. and M. 3. M c C ~ I s (I967) Geologkal exploration in an east coast submarine canyon from a research submenible. Science, 1 ~ , 370-372. VEATCHA. C. and P. A. SMITH(1939) Atlantic submarine valleys off the United States and the Congo Submarine Valley. Spec. Paper Geol. Soc. Am., 7, 101 pp. Ym~g.es 1L F., D. S. GOasLINE and G. A. RUSNAK(1967) Origin of Redondo Submarine Canyon, southern California. U.S. GeoL Surv. Prof Paper 575-C, C97-C107.