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Features at the Sohm Abyssal Plain terminus
Marine Geology -
ElsevierPublishing Company, Amsterdam - Printed in The Netherlands
F E A T U R E S AT T H E SOHM ABYSSAL PLAIN T E R M I N U S B O N N I E A. M c G R E G O R
Graduate School of Oceanography, University of Rhode Island, Kingston, R.L (U.S.A.)
(Received November 9, 1967)
SUMMARY
Bathymetry and magnetic anomalies reflect the complexity of the structure in the area at the southern-most extension of the Sohm Abyssal Plain between 29 -27°N and 55°-52°W in the North Atlantic. The general trend of structures in this area is N20CE with a cross structure trending N70"E. The topography shows ridges and troughs, varying in depth from 4,600 m to 6,100 m. In part this roughness is masked by the sediment pattern 600 m thick in flat sediment fingers, which are the southern extent of the Sohm Abyssal Plain. Deep unfilled valleys emphasize the continuity of the ridges bounding the sediment fingers. A cross structure trending N 7 0 E is a fracture zone, probably a continuation of the Atlantic Fracture Zone. Correlating magnetic anomalies with VINE's (1966) profiles gives an age of Lower Eocene for this area. Dates on two sediment cores near the survey area give an age of Lower Eocene and Eocene. These cores are part of the pelagic blanket, which is found across the area and underlies the stratified plain sediments. The continuity of pelagic sediments beneath the plain sediments indicate that the lat~er are more recent. The topography controls the type of sedimentation whether pelagic or turbidites and also controls the thickness and accumulation of these sediments. This survey area fits in well with the picture of an ocean basin orginating by ocean floor spreading and in time being modified by depositional processes.
INTRODUCTION
A bathymetric and magnetic profile across the western portion of the North Atlantic Basin in 1963 initiated the survey of part of the southern terminus of the Sohm Abyssal Plain (Fig.l). A series of sediment tongues, located at this southern terminus, extend into the abyssal hills and mark the mergence of the abyssal hills and abyssal plains provinces. The topographic features in the area are more massive and generally higher than the abyssal hills on either side (Fig.2B). This physiographic difference suggests a structural difference as well. A survey between 29°-27°N and 55~'-52~W was conducted to obtain bathyM a r i n e Geol.,
6 (1968) 401~414
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metric, magnetic, and sub-bottom data, which would trace the Sohm Abyssal Plain to its southern-most extension and determine the structural control of this terminus, The research vessel "Trident" of the University of Rhode Island was used in this survey. The main part of the survey was conducted during R/V "Trident" cruise ~¢28 and was supplemented by data from cruises ~:,'+'04and 7~29 (Fig.3). Adjusting the ship's tracks where they cross gives tile position of the survey lines to within 2-4 kin. Coordination of the bathymetric, magnetic, and sub-bottom data gives a geologic picture of the processes which have occurred and are occurring, that are important in the formation and general character of this area.
GENERAL
AREA
The Sohm Abyssal Plain is bounded on the east by the Mid-Atlantic Ridge and on the west by the Bermuda Rise (Fig. l). The plain between 3 T and 2 5 N slopes to the south with an overall gradient of 1 : 2,700 and a change in depth from 5,204 m to 5,703 m (corrected with Matthews tables, MATTHEWS,1939) (taken i¥om sounding records of Woods Hole Oceanographic Institute and University of Rhode Island). At the southern end of the Plain, the gradient is 1 : 1,865 with a reverse in slope at the terminus indicating a piling-up of sediment against a barrier. Abyssal plains such as the Sohm and the Nares terminate by sediment feathering out into the abyssa 1 hills in deepening and narrowing valleys (HEEZEN et al., 1954). These authors also suggest that canyons may connect the Sohm with the Nares Abyssal Plain, which at the northeast end has a depth of 6002 m and slopes to the northeast with a gradient of 1 : 3,300. However, this is not the case since a barrier prevents any exchange of Marine Geol.. 6 (1968) 401-414
SOHMABYSSALPLAINTERMINUS
403
sediment. The bedrock beneath the sediments of the Sohm Abyssal Plain is increasingly exposed from north to south due to thinning of sediments with distance from the source. The data at hand definitely support the origin of these Abyssal Plains as burial of original topography by turbidity currents (HEEzEN and LAUGHTON, 1963). EWING et al. (1964) report that the near-surface sediment type in this area is red clay with < 20 ~o CaCO3 with a deposition rate or 1 ram/1000 yrs. The bottom topography beneath these sediments is very rough with maximum sediment accumulations where the depth is greatest, hence, the topography controls the sedimentation pattern in this area.
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BATHYMETRY
The bathymetry was recorded on a PESR (Precision Echo Sounding Recorder, Alpine Geophysical Associates inc.) using an Edo transmitter and transducer (AN/UQN-IC). Fig.2 and 4 show the extension of the Sohm Abyssal Plain as a finger of sediment present only in the northern profiles. This sediment is ponded at two different depths which are separated by a continuous ridge trending north-south through the area. The deeper sediment level is at about 5,615 m and the shallower at about 5,570 m. The sediment surface in both levels slopes downward to the south and is continuous with a similar slope on the main part of the Sohm Abyssal Plain. This is to be expected if the primary movement of sediments is by turbidity currents. The two-level sediment finger divided by a ridge is bounded on the east by a steep scarp trending almost east-west and on the west by an irregular ridge. Deep valleys flank these highs with depths generally greater than 5,800 m. Although these valleys are deeper than the plain, they are not filled with sediment. The sediment finger narrows to the south until, in profile H (Fig.2) the topography becomes irregular and the sediment finger ends. Fig.4 is a bathymetric map with a contour interval of 100 m. The separating ridge between the two sediment levels trends N20°E, and is sufficiently high and continuous to prevent any loss of sediment from the high to the low level. The 5,6t5 m Marine Geol., 6 (1968) 401-414
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sediment level appears io be hounded on the southeast and north by at contitlu,}t~, scarp, while on the south, it is d a m m e d b \ a 5.563 m sill. l he 5.570 m lcxci i, i-:,~undctl on the west by a continuous north ~,outh ridge and on the south b~, t~tcrsecling ridges, as evidenced by the spreading out of sediments in this area to t\)rm ~t i -shapc~ structure. In the castern section el" the survey area a continuous high flanked b 5 d c c p trends N 2 0 E , and in the northeast corner the N 2 0 E and the N 7 0 E sll-tictures hltci" sect at an angle of 5 0 . The highs and lows continue to lhe south, but ii~, cl<,lliii-i:~i~t features are as obvious as they are in the north.
MAGNETICS
The total magnetic field intensity was recorded in gammas by a shipborne Varian precession magnetometer. The observed magnetic field of the 1963, 1965, and 1966 cruises was adjusted to 1965 using a 60 ,,/year decrease and was found to agree roughly with the 1965 edition of the Total Intensitr Map of the Earth's Ma,em,Hc Forcu (Naval Oceanographic Office, H.O. 1703). An additional 200 7 was added to the magnetic field and the field was tilted down to the southeast 100 7, indicating that in this area the total intensity map is in error by 200 300 7. The contours of the earth's magnetic force are assumed to be parallel for this small area. N o principal magnetic storms were recorded for any of the survey dates and the daily variations were small, classified as quiet days (LINcot.y, 1963, 1966), so that no correction of the data ~as necessary.
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Marlin' Geol., 6 (1968) 401-414
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The magnetic data and the magnetic anomalies are shown in Fig.5 and 6, while magnetic profiles are given in Fig.2. From these data it can be seen that the general trend of magnetic anomalies is N20 E. In general, negative anomalies appear in the regions ol" the bathymetric lows, and positive anomalies are associated with the ridges such as those around the ponded sediment tongue, with the ridge on the west side coinciding with the most continuous positive anomaly probably because of the close proximity of two ridges in this area. The 5,615 m sediment finger is outlined by the 50 ;' contour associated with the scarp on the east, and on the west by the 100 ;, contour associated with the north south separating ridge. Positive anomalies in the 5,615 m sediment finger may be due to buried topographic highs or a change in basement depth; both are shown in the sub-bottom profile. The magnetic lows correspond with the tOFographic lows to the east of the 5,615 m sediment pond (Fig.4 and 6), while the discontinuous ~ 50 ,' contour follows the eastern-most topographic ridge. At the east and west edge of the survey area the topography and magnetics do not correlate.
SUB-BOTTOM PROFILE
A sub-bottom profile was made across the area (Fig.3, track C) with a Bolt Associates P A R (Pneumatic Acoustic Repeater, Model P-300-2). The profiler was towed at 9 km/'h. A compressor s.lpplied 2,500 p.s.i. (43.5 dynes/'cm ~) air pressure to Marine Geol., 6 (1968) 401-414
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the 121 cubic inch (1,984 cm3) chamber which was fired once every 22 sec. A Bolt Associates Model 1010 hydrophone was towed behind the ship, and the sound was recorded unfiltered by a Sanborn Magnetic Tape Recorder at 7.5 inches/sec (19 cm/sec). In the laboratory the magnetic tapes were played back at 60 inches/sec (150 cm/sec) and filtered for 400-1,600 Hz (50-200 Hz at 7.5 inches/sec) by Mr. Gilbert Fain of the Electrical Engineering Department at the University of Rhode Island. The PAR sub-bottom profile was made across the area to investigate the sediment thickness. At an assumed sediment velocity of 1.5 km/sec, the thickness of the sediment is 600 m to the bedrock (Fig.7). Numerous reflectors are apparent in the ponded sediment with a blanket of transparent sediment overlying the rough topography on either side. DISCUSSION
Bathymetric and magnetic trends Two major trends having different origins exist in the bathymetry and the magnetics. The N20°E trend of bathymetry and magnetic anomalies is the same as that of the Mid-Atlantic Ridge and the scarp at the east flank of the Bermuda Rise. To the southwest near the eastern edge of the Nares Abyssal Plain, magnetic data in a topographically similar area also show a trend of N20°E (AVERY et al,, 1966). Therefore, in this part of the North American basin, the general trend in the magnetics and bathymetry appears to be northeast-southwest. The second major trend in the area crosses the dominant trend striking NT0°E Marine Geol., 6 (1968) 401-414
409
SOHM ABYSSAL PLAIN TERMINUS
as a scarp bounding the eastern side of the 5,615 m sediment finger. The magnetic data show this trend as a small positive anomaly outlined by the "- 50 7 contour. This topographic cross trend controls the type of sedimentation and hence, the terminus of the Sohm Abyssal Plain, as defined by the stratified turbidity current sediments. A statistical analysis was made of the trend directions with a grid of 17 ;~ 17 km spacing laid out on the map area. Of the 130 blocks in the grid, random number tables were used to select 65 for analysis and the trend of each bathymetric contour contained in a selected box was measured. The total number of contours measured was 372. The mean trend X is NI9.8°E with a standard deviation of 24 ~' (Fig.8). A chi-square test for randomness showed that the frequency is highly significant above the 99 % confidence interval for 0-30 ~ and the N20CE trend is therefore significant for this area. The N70°E scarp is not significant, but is obvious in Fig.8. The intei'section of the N70°E scarp and the median N20°E ridge created a sill at 5,563 m, damming the sediment in the 5,615 m sediment finger. The 5,570 m sediment finger appears to be dammed also on the south end by this east west scarp. The sub-bottom record (Fig.7) shows these sediment fingers to be buried valleys. The general topography reflects the intersection of trends such as the truncated deep valleys, a continuous north-south ridge near 53cW which is truncated or offset from the topographic highs to the north and east, and an eastward trending ridge or seamount of 4,700 m depth which is probably part of the structure associated with the scarp. Some magnetic highs are located just to the west of 53c:W in the 5,615 m sediment finger. They may be associated with buried topographic features, but this necessitates the feature be less than 600 m high and have a high magnetic suscepta-
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Marine Geol., 6 (1968) 401-414
410
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bility to account for the 150 )' anomaly. On the sub-bottom record of the track to the south, a general rise in the bedrock does occur, suggestir~g the anomalies arc related to the east-west cross structure. Atlantis Fracture Zone
In many places the Mid-Atlantic Ridge is offset by fracture zones, the St. Paul's, Vema, Azores, Guinea, Chain, Romanche, and Atlantis Fracture Zone (H E~ZE>~ et al., 1964a, 1964b; KRAUSV, 1964, 1965; HEEZEN and TnARP, 1965). TO obtain an estimate of fracture zone length, MENARD (1962) proposed a correlation between the offset distance and length of wrench faults. For offsets less than 600 kin, the length is seven times the offset. On the Atlantis Fracture Zone the offset is 120 km (HEFzEN and Triage, 1965) so the length should be 840 kin. The area presented in this paper is 960 km from the ridge, so that the postulated length of the Atlantis Fracture Zone is sufficient to reach this area. The N70~'E scarp with its i 50 7 anomaly appears to be part of a fracture zone, which the author interprets as part of the Atlantis Fracture Zone. Testing MENARD'S (1955) criteria for fracture zone identification on the N70~E scarp gives the following results: (1) The fracture zone should follow the plan of a great circle, and in this case the scarp has such a plan. (2) The length and width should be extensive. No estimate can be made of the width, but the length is at least 840 km. (3) Longitudinal ridges and troughs are associated with the fracture zones in the Pacific. Discontinuous ridges and troughs are found in this area. The anomalies under the 5,615 m sediment finger may be associated with a parallel structure as may also the discontinuous 4,900 m highs at the northeast side of the survey area. (4) Fracture zones offset older features such as the 120 km offset of the MidAtlantic Ridge at 31 N . In the survey area sparse data suggest that an offset may exist in the north-south features. HEEZEN et al. (1959) suggest that earthquake epicenters clustered at particular places on the Mid-Atlantic Ridge may indicate cross fault activity. Fault plane solutions of epicenters clustered at 31°N on the Ridge satisfy a transform fault (SYKES, 1967). (5) The fault scarp is asymmetrical. The sub-botton profile shows that the N70°E scarp has asymmetry. (6) A regional change in depth is associated with a fracture zone. North of 28~N a change in depth occurs, as the valleys are deeper north of the N7@'E scarp, but this is partly masked by sedimentary fill in the 6,200 m-deep valleys. The argument for the existence of a fracture zone in the survey area is supported by the six criteria. The trend of the Atlantis Fracture Zone indicates that the fracture zone in the survey area is a westward continuation. Also, with a length of 840 km an offset in the MidAtlantic Ridge should be observable, and the only offset reported in the literature for this latitude is the Atlantis Fracture Zone. Marine Geol., 6 (1968) 401-414
411
SOHM ABYSSAL PLAIN TERMINUS Million
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The unusual shape of the 5,615 m sediment finger indicates a possible fracture zone association. The outermost fringes of sediment from the Nares Plain partially fill long linear depressions which are associated with fracture zones, such as the Vema Fracture Zone which offsets the Mid-Atlantic Ridge (EwING et al., 1966b). Similar lineations are seen along the Sohm Abyssal Plain. The shape of the 5,615 m sediment finger is a configuration similar to these fracture zone-associated lineations. A correlation of such features with the clustering of epicenters on the Mid-Atlantic Ridge seems to be a good criterion for tentative identification of fracture zones.
Magnetic correlations Fig.9 shows the magnetic anomalies from the track of "Trident" Cruise ~,¢04 and the anomalies from VrNE'S paper (1966). The "Trident" track correlates reasonably well as a rough approximation between 50 and 60 million years. Some discrepancy in detail occurs, but Vine shows some variation in the detail of the tracks from RAFF (1966) and VACQUIERet al. (1961). The age of the survey area is thus indicated to be 55 million years or Early Eocene according to KULP'S geologic time scale (1961).
Sedimentation The surface sediments in the fiat sediment fingers are very fluid, as indicated by sub-bottom echoes obtained with the PESR to a depth of 50 m. The reflecting layers range from 3-5 m apart, but are not spaced a uniform distance. A uniform, wide distribution of beds indicates a probable turbidity current origin (HFRsEY, 1965). When a turbidity current enters a basin and is dammed by its borders, the current is turned back and circulates within the basin, distributing suspended matter more or less evenly with sorting taking place as the material settles out differentially (HERsEY, 1965). The surfaces most likely to reflect sound, according to Hersey, are Marine Geol., 6 (1968) 401 414
412
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between coarser beds with a h)w water content and liner sediments with a high porosity. The material ill the sediment finger has a high water content as indicated by the 50 m penetration. Because of the Iocat ion. turbidity currents actively contribute material to the sediment fingers and since tile fingers were closed basins, tile type of sorting and settling that Hersey proposes would hold for this area. The sub-bottom profile (Fig.7) shows the deeper sediment structure. Two types of sediment are easily observable, an acoustically transparent layer, and stratified sediments. The transparent sediments, 100 m thick in places, blanket the topography on either side of the sediment fingers and compose the bottom half of the sediment fingers with the stratified sediments filling the other half. The origins of these two types of sediment are different. The stratified zone results from turbidity currents moving from the Sohm Abyssal Plain into this area. The transparent zones represent quiescent deposition of pelagic sediment or deposits of fine particles transported by a nepheloid layer (EwIN(; et al., 1966a). On either side of the sediment fingers the bottom is rough with a blanket of acoustically transparent sediments across the area. Some sediment accumulation in the valleys have a few weak reflectors. This sediment is slump or slide material from the topographic highs on either side, and in some cases visibly slopes away from the highest topography. The top 300 m of sediment in the 5,570 m sediment finger consists of sediments stratified parallel to the water-sediment interface rather than the bedrock surface, and the bottom 300 m of sediment is more acoustically transparent. Some stratification exists at the bottom of the eastern side, but this is at the base of the steeper slope and may be slump material. Some irregularity occurs at the interface between the acoustically transparent and stratified sediment due to slumping material. No blanket of pelagic sediments is obvious on top of the dividing ridge between the sediment ponds, but the bottom return is not sharp, so that the layer may be present or the steep slope may cut down on its thickness. The 5,615 m sediment pond is also asymmetrical with a steep slope on a buried irregular topographic high in the eastern portion. Stratification of the sediments more paralleling the basement is present, but definitely not as sharp as in the other finger. Stratified sediments 200 m thick in this eastern portion overlie 100 m of acoustically transparent sediments. In many places these transparent sediments come to the surface, which may be due to diapiric structures, although no bending of the beds is obvious. As the topography rises, the stratified sediments disappear and only a continuous blanket of transparent sediment is found. The pelagic sedimentation has been going on for some time and at some more recent time turbidity current activity began in this area, as indicated by the continuity of pelagic sediments beneath the plain sediments. This indicates a sill to the north, which permitted passage of sediments into the fingers when the sediments on the plain accumulated deeply enough. The small thickness of sediment with stratification indicates that turbidity current activity here is not as old as on the Sohm a short distance to the north of the survey area where the sediments are much thicker. SMTO et al. (1966) have dated two cores near the survey area as Lower Eocene and Eocene. The cores were located at 30:~N, 51~52'W and 2T52'N, 54°38'W in 4,673 m and 4,680 Marine Geol., 6 (1968) 401-414
SOHM ABYSSAL PLAIN TERMINUS
413
m of water respectively. The depth of water indicates that the material was cored on a topographic high and therefore, was part of the pelagic, transparent layer. The length of the cores is 540 cm and 627 cm, respectively, which may indicate the sediments have been reworked, since the cores are short for Eocene if a uniform deposition rate of 1 ram/l,000 years is assumed. The acoustically transparent sediment accumulation is the same over the whole area, which indicates the age of the scarp and fracture zone is similar to the initial formation of the area, which one would expect for a transform fault. The difference in surface depths in the two sediment fingers, 5,570 m and 5,615 m, is a result of a difference in the depth of the bedrock. The sediment thickness is the same in both fingers so they have a common source to the north. The dividing ridge is either truncated to the north, or more likely is buried by the thicker sediment accumulation on the plain. The stratification is more pronounced in the 5,570 m finger because it is narrower than the 5,615 m finger where the sediments can circulate more evenly before the settling of material begins.
ACKNOWLEDGEMENTS
I would like to thank Dr. Dale C. Krause for his assistance and advice during the research and preparation of this manuscript. Thanks are also due Dr. Louis E. Garrison for critically reviewing the manuscript. Dr. Robert L. McMaster and Dr. Frank T. Dietz critically read earlier drafts, and William P. Dillon reviewed the final illustrations. Funds for this study were provided by Office of Naval Research cont~ act Nonr 396(08).
REFERENCES AVERY, O. E., CARROLL, J. C. and BRACEY, D. R., 1966. Shipboard magnetic survey of an area north of the Lesser Antilles. U.S. Naval Oceanog. Office Informal Rept., H-5-66 : 1-19. EWING, M., EWING, J. and TALWANI, M., 1964. Sediment distribution in the oceans, the MidAtlantic Ridge. Bull. Geol. Soe. Am., 75 : 17-36. EWING, M., LE PICHON, X and EWlNG, J., 1966a. Crustal structure of the midocean ridges, 4. Sediment distribution in the South Atlantic Ocean and the Cenozoic history of the Mid-Atlantic Ridge. J. Geophys. Res., 71 : 1611-1636. EWlNG, M., Worzel, J. L., EWING, J. and WINDISCH,C.~ 1966b. Ages of horizon A and the oldest Atlantic sediments. Science, 154 : 1125-1132. HEEZEN, B. C. and THARP, M., 1965. A symposium on continental drift. Phil. Trans. Roy. Soc. London, Ser. A, 258 : 90-106. HEEZEN, B. C., EWING, M. and ERICSON, D. B., 1951. Submarine topography in the North Atlantic. Bull. Geol. Soc. Am., 62 : 1407-1409. HEEZEN, B. C., EWING, M. and ERICSON, D. B., 1954. Reconnaissance survey of the abyssal plain south of Newfoundland. Deep-Sea Res., 2 : 122-133. HEEZEN, B. C., THARP, M. and EWING, M., 1959. The floors of the oceans, 1. The North Atlantic. Geol. Soc. Am., Spec. Papers, 65 : 1-122. HEEZEN, B. C. and LAUGHTON, A. S., 1963. Abyssal plains. In: N. M. HILL (Editor), The Sea, Ideas and Observations on Progress in the Study of the Sea. Interscience, New York, N.Y., 3 : 312-364.
Marine Geol., 6 (1968) 401-414
HF,EZEN, B. C., BUNCE, L. 1., t]IRSt'~. J. B. alld lt]Al~,P, M., 19{~4a. ('hain and t~om:uwhc I~~:l~iJ~ zones. Deefl-Sea Rc,~., I 1 : I I 33. HEEZEN, B. C., GIRARD, R. D. and TI|AI~.P, M., 19t~ab. The Vellla Fr~cture Zone !i~ !!~c cqua~ ~ieal Atlantic. J. Geop/o,s. Reds., ~9 : 733- 739. H~.RSEY, J. B., 1965. Sediment ponding in the deep sea. Bull. Geol. Soc. Am., 76 : 1251 I259~ KRAUSE, D. C., 1964. Guinea fracture zone in the equatorial Atlantic. Science, 146 : 57 59. KRAUSE, D. C., 1965. East and west Azores fracture zones in the North Atlantic. 17lh £.V~*i[~.('ozsto~e
Research Sot., Col,~ton Paper,s. 17 : I ~3 17~. KULP, J. L., 1961. Geologic tinge scale. Science, t33 " 1105- tl 14. LINCOLN, J. V., 1963. Selected gcomagnetic and solatr data. J. Geophy.s. Res., 68 : 4863. LINCOLN, J. V., 1966. Selected geomagnelic and solar data. J. Geophys. Re~'., 7i :2411-2414: 3793-3798. MA'rrHEWS, D. J., 1939. T~lble.vq/'the l eloeity {4'Sotmd #~ Pure Uq~ter and Sea Water /i~r U.~','in Ec'ho Sounding and Eeho Rang,ing. Admiralty Hydrographic Dept., London, 52 pp. MENAr~D, H. W., 1955. Deformatioll of the northeastern Pacific Basin and the west coast of North America. BMt. Geol. Soe./Ira., ¢~6 : 1149 1198. MI-NARD, H. W., 1962. Correlation between length and offset on very large wrench ihuhs. J. Geophys. Res., 67 : 4096 4098. NAVAL OCEANOGRAPHICOFFICk, 19(~5. T~Jlal Intcn.sity Mayo o[the Earlh's Maenetic torte, RAW, A. D., 1966, Boundaries of an area of very long magnetic anomalies in the northeast Pacific. J. Geophys. Res., 71 : 2631 263f~. SmTO, T., EWlnC;, M. and BtJrcKJl, 1,. H., 1966. lertiary sediments from the Mid-Atlantic Ridge, Science, 151 : 1075 t079. SYKES, L. R., 1967. Mechanism of earthquakes and nature of faulting on the mid-oceanic ridges. J. Geophys. Res., 72:2131 2153. VaCQOIER, V., RAW, A. D. and W a r r E n , R. E., 1961. Horizontal displacements in the lloor of lhe northeastern Pacific Ocean. Bid[. Geol. Soc. Am., 72 : 1 2 5 1 - 1 2 5 8 . VINE, F. J., 1966. Spreading of the ocean floor: new evidence. Science, 154 : 1405-1415.
Marine Geol., 6 (1968) 401---414
ElsevierPublishing Company, Amsterdam - Printed in The Netherlands
F E A T U R E S AT T H E SOHM ABYSSAL PLAIN T E R M I N U S B O N N I E A. M c G R E G O R
Graduate School of Oceanography, University of Rhode Island, Kingston, R.L (U.S.A.)
(Received November 9, 1967)
SUMMARY
Bathymetry and magnetic anomalies reflect the complexity of the structure in the area at the southern-most extension of the Sohm Abyssal Plain between 29 -27°N and 55°-52°W in the North Atlantic. The general trend of structures in this area is N20CE with a cross structure trending N70"E. The topography shows ridges and troughs, varying in depth from 4,600 m to 6,100 m. In part this roughness is masked by the sediment pattern 600 m thick in flat sediment fingers, which are the southern extent of the Sohm Abyssal Plain. Deep unfilled valleys emphasize the continuity of the ridges bounding the sediment fingers. A cross structure trending N 7 0 E is a fracture zone, probably a continuation of the Atlantic Fracture Zone. Correlating magnetic anomalies with VINE's (1966) profiles gives an age of Lower Eocene for this area. Dates on two sediment cores near the survey area give an age of Lower Eocene and Eocene. These cores are part of the pelagic blanket, which is found across the area and underlies the stratified plain sediments. The continuity of pelagic sediments beneath the plain sediments indicate that the lat~er are more recent. The topography controls the type of sedimentation whether pelagic or turbidites and also controls the thickness and accumulation of these sediments. This survey area fits in well with the picture of an ocean basin orginating by ocean floor spreading and in time being modified by depositional processes.
INTRODUCTION
A bathymetric and magnetic profile across the western portion of the North Atlantic Basin in 1963 initiated the survey of part of the southern terminus of the Sohm Abyssal Plain (Fig.l). A series of sediment tongues, located at this southern terminus, extend into the abyssal hills and mark the mergence of the abyssal hills and abyssal plains provinces. The topographic features in the area are more massive and generally higher than the abyssal hills on either side (Fig.2B). This physiographic difference suggests a structural difference as well. A survey between 29°-27°N and 55~'-52~W was conducted to obtain bathyM a r i n e Geol.,
6 (1968) 401~414
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metric, magnetic, and sub-bottom data, which would trace the Sohm Abyssal Plain to its southern-most extension and determine the structural control of this terminus, The research vessel "Trident" of the University of Rhode Island was used in this survey. The main part of the survey was conducted during R/V "Trident" cruise ~¢28 and was supplemented by data from cruises ~:,'+'04and 7~29 (Fig.3). Adjusting the ship's tracks where they cross gives tile position of the survey lines to within 2-4 kin. Coordination of the bathymetric, magnetic, and sub-bottom data gives a geologic picture of the processes which have occurred and are occurring, that are important in the formation and general character of this area.
GENERAL
AREA
The Sohm Abyssal Plain is bounded on the east by the Mid-Atlantic Ridge and on the west by the Bermuda Rise (Fig. l). The plain between 3 T and 2 5 N slopes to the south with an overall gradient of 1 : 2,700 and a change in depth from 5,204 m to 5,703 m (corrected with Matthews tables, MATTHEWS,1939) (taken i¥om sounding records of Woods Hole Oceanographic Institute and University of Rhode Island). At the southern end of the Plain, the gradient is 1 : 1,865 with a reverse in slope at the terminus indicating a piling-up of sediment against a barrier. Abyssal plains such as the Sohm and the Nares terminate by sediment feathering out into the abyssa 1 hills in deepening and narrowing valleys (HEEZEN et al., 1954). These authors also suggest that canyons may connect the Sohm with the Nares Abyssal Plain, which at the northeast end has a depth of 6002 m and slopes to the northeast with a gradient of 1 : 3,300. However, this is not the case since a barrier prevents any exchange of Marine Geol.. 6 (1968) 401-414
SOHMABYSSALPLAINTERMINUS
403
sediment. The bedrock beneath the sediments of the Sohm Abyssal Plain is increasingly exposed from north to south due to thinning of sediments with distance from the source. The data at hand definitely support the origin of these Abyssal Plains as burial of original topography by turbidity currents (HEEzEN and LAUGHTON, 1963). EWING et al. (1964) report that the near-surface sediment type in this area is red clay with < 20 ~o CaCO3 with a deposition rate or 1 ram/1000 yrs. The bottom topography beneath these sediments is very rough with maximum sediment accumulations where the depth is greatest, hence, the topography controls the sedimentation pattern in this area.
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Marine Geol.,
6 (1968)401~-14
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BATHYMETRY
The bathymetry was recorded on a PESR (Precision Echo Sounding Recorder, Alpine Geophysical Associates inc.) using an Edo transmitter and transducer (AN/UQN-IC). Fig.2 and 4 show the extension of the Sohm Abyssal Plain as a finger of sediment present only in the northern profiles. This sediment is ponded at two different depths which are separated by a continuous ridge trending north-south through the area. The deeper sediment level is at about 5,615 m and the shallower at about 5,570 m. The sediment surface in both levels slopes downward to the south and is continuous with a similar slope on the main part of the Sohm Abyssal Plain. This is to be expected if the primary movement of sediments is by turbidity currents. The two-level sediment finger divided by a ridge is bounded on the east by a steep scarp trending almost east-west and on the west by an irregular ridge. Deep valleys flank these highs with depths generally greater than 5,800 m. Although these valleys are deeper than the plain, they are not filled with sediment. The sediment finger narrows to the south until, in profile H (Fig.2) the topography becomes irregular and the sediment finger ends. Fig.4 is a bathymetric map with a contour interval of 100 m. The separating ridge between the two sediment levels trends N20°E, and is sufficiently high and continuous to prevent any loss of sediment from the high to the low level. The 5,6t5 m Marine Geol., 6 (1968) 401-414
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sediment level appears io be hounded on the southeast and north by at contitlu,}t~, scarp, while on the south, it is d a m m e d b \ a 5.563 m sill. l he 5.570 m lcxci i, i-:,~undctl on the west by a continuous north ~,outh ridge and on the south b~, t~tcrsecling ridges, as evidenced by the spreading out of sediments in this area to t\)rm ~t i -shapc~ structure. In the castern section el" the survey area a continuous high flanked b 5 d c c p trends N 2 0 E , and in the northeast corner the N 2 0 E and the N 7 0 E sll-tictures hltci" sect at an angle of 5 0 . The highs and lows continue to lhe south, but ii~, cl<,lliii-i:~i~t features are as obvious as they are in the north.
MAGNETICS
The total magnetic field intensity was recorded in gammas by a shipborne Varian precession magnetometer. The observed magnetic field of the 1963, 1965, and 1966 cruises was adjusted to 1965 using a 60 ,,/year decrease and was found to agree roughly with the 1965 edition of the Total Intensitr Map of the Earth's Ma,em,Hc Forcu (Naval Oceanographic Office, H.O. 1703). An additional 200 7 was added to the magnetic field and the field was tilted down to the southeast 100 7, indicating that in this area the total intensity map is in error by 200 300 7. The contours of the earth's magnetic force are assumed to be parallel for this small area. N o principal magnetic storms were recorded for any of the survey dates and the daily variations were small, classified as quiet days (LINcot.y, 1963, 1966), so that no correction of the data ~as necessary.
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Marlin' Geol., 6 (1968) 401-414
407
SOHM ABYSSAL PLAIN TERMINUS
29 ° N.
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The magnetic data and the magnetic anomalies are shown in Fig.5 and 6, while magnetic profiles are given in Fig.2. From these data it can be seen that the general trend of magnetic anomalies is N20 E. In general, negative anomalies appear in the regions ol" the bathymetric lows, and positive anomalies are associated with the ridges such as those around the ponded sediment tongue, with the ridge on the west side coinciding with the most continuous positive anomaly probably because of the close proximity of two ridges in this area. The 5,615 m sediment finger is outlined by the 50 ;' contour associated with the scarp on the east, and on the west by the 100 ;, contour associated with the north south separating ridge. Positive anomalies in the 5,615 m sediment finger may be due to buried topographic highs or a change in basement depth; both are shown in the sub-bottom profile. The magnetic lows correspond with the tOFographic lows to the east of the 5,615 m sediment pond (Fig.4 and 6), while the discontinuous ~ 50 ,' contour follows the eastern-most topographic ridge. At the east and west edge of the survey area the topography and magnetics do not correlate.
SUB-BOTTOM PROFILE
A sub-bottom profile was made across the area (Fig.3, track C) with a Bolt Associates P A R (Pneumatic Acoustic Repeater, Model P-300-2). The profiler was towed at 9 km/'h. A compressor s.lpplied 2,500 p.s.i. (43.5 dynes/'cm ~) air pressure to Marine Geol., 6 (1968) 401-414
408
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the 121 cubic inch (1,984 cm3) chamber which was fired once every 22 sec. A Bolt Associates Model 1010 hydrophone was towed behind the ship, and the sound was recorded unfiltered by a Sanborn Magnetic Tape Recorder at 7.5 inches/sec (19 cm/sec). In the laboratory the magnetic tapes were played back at 60 inches/sec (150 cm/sec) and filtered for 400-1,600 Hz (50-200 Hz at 7.5 inches/sec) by Mr. Gilbert Fain of the Electrical Engineering Department at the University of Rhode Island. The PAR sub-bottom profile was made across the area to investigate the sediment thickness. At an assumed sediment velocity of 1.5 km/sec, the thickness of the sediment is 600 m to the bedrock (Fig.7). Numerous reflectors are apparent in the ponded sediment with a blanket of transparent sediment overlying the rough topography on either side. DISCUSSION
Bathymetric and magnetic trends Two major trends having different origins exist in the bathymetry and the magnetics. The N20°E trend of bathymetry and magnetic anomalies is the same as that of the Mid-Atlantic Ridge and the scarp at the east flank of the Bermuda Rise. To the southwest near the eastern edge of the Nares Abyssal Plain, magnetic data in a topographically similar area also show a trend of N20°E (AVERY et al,, 1966). Therefore, in this part of the North American basin, the general trend in the magnetics and bathymetry appears to be northeast-southwest. The second major trend in the area crosses the dominant trend striking NT0°E Marine Geol., 6 (1968) 401-414
409
SOHM ABYSSAL PLAIN TERMINUS
as a scarp bounding the eastern side of the 5,615 m sediment finger. The magnetic data show this trend as a small positive anomaly outlined by the "- 50 7 contour. This topographic cross trend controls the type of sedimentation and hence, the terminus of the Sohm Abyssal Plain, as defined by the stratified turbidity current sediments. A statistical analysis was made of the trend directions with a grid of 17 ;~ 17 km spacing laid out on the map area. Of the 130 blocks in the grid, random number tables were used to select 65 for analysis and the trend of each bathymetric contour contained in a selected box was measured. The total number of contours measured was 372. The mean trend X is NI9.8°E with a standard deviation of 24 ~' (Fig.8). A chi-square test for randomness showed that the frequency is highly significant above the 99 % confidence interval for 0-30 ~ and the N20CE trend is therefore significant for this area. The N70°E scarp is not significant, but is obvious in Fig.8. The intei'section of the N70°E scarp and the median N20°E ridge created a sill at 5,563 m, damming the sediment in the 5,615 m sediment finger. The 5,570 m sediment finger appears to be dammed also on the south end by this east west scarp. The sub-bottom record (Fig.7) shows these sediment fingers to be buried valleys. The general topography reflects the intersection of trends such as the truncated deep valleys, a continuous north-south ridge near 53cW which is truncated or offset from the topographic highs to the north and east, and an eastward trending ridge or seamount of 4,700 m depth which is probably part of the structure associated with the scarp. Some magnetic highs are located just to the west of 53c:W in the 5,615 m sediment finger. They may be associated with buried topographic features, but this necessitates the feature be less than 600 m high and have a high magnetic suscepta-
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Marine Geol., 6 (1968) 401-414
410
I~. ~. :~ICGRF(i¢~R
bility to account for the 150 )' anomaly. On the sub-bottom record of the track to the south, a general rise in the bedrock does occur, suggestir~g the anomalies arc related to the east-west cross structure. Atlantis Fracture Zone
In many places the Mid-Atlantic Ridge is offset by fracture zones, the St. Paul's, Vema, Azores, Guinea, Chain, Romanche, and Atlantis Fracture Zone (H E~ZE>~ et al., 1964a, 1964b; KRAUSV, 1964, 1965; HEEZEN and TnARP, 1965). TO obtain an estimate of fracture zone length, MENARD (1962) proposed a correlation between the offset distance and length of wrench faults. For offsets less than 600 kin, the length is seven times the offset. On the Atlantis Fracture Zone the offset is 120 km (HEFzEN and Triage, 1965) so the length should be 840 kin. The area presented in this paper is 960 km from the ridge, so that the postulated length of the Atlantis Fracture Zone is sufficient to reach this area. The N70~'E scarp with its i 50 7 anomaly appears to be part of a fracture zone, which the author interprets as part of the Atlantis Fracture Zone. Testing MENARD'S (1955) criteria for fracture zone identification on the N70~E scarp gives the following results: (1) The fracture zone should follow the plan of a great circle, and in this case the scarp has such a plan. (2) The length and width should be extensive. No estimate can be made of the width, but the length is at least 840 km. (3) Longitudinal ridges and troughs are associated with the fracture zones in the Pacific. Discontinuous ridges and troughs are found in this area. The anomalies under the 5,615 m sediment finger may be associated with a parallel structure as may also the discontinuous 4,900 m highs at the northeast side of the survey area. (4) Fracture zones offset older features such as the 120 km offset of the MidAtlantic Ridge at 31 N . In the survey area sparse data suggest that an offset may exist in the north-south features. HEEZEN et al. (1959) suggest that earthquake epicenters clustered at particular places on the Mid-Atlantic Ridge may indicate cross fault activity. Fault plane solutions of epicenters clustered at 31°N on the Ridge satisfy a transform fault (SYKES, 1967). (5) The fault scarp is asymmetrical. The sub-botton profile shows that the N70°E scarp has asymmetry. (6) A regional change in depth is associated with a fracture zone. North of 28~N a change in depth occurs, as the valleys are deeper north of the N7@'E scarp, but this is partly masked by sedimentary fill in the 6,200 m-deep valleys. The argument for the existence of a fracture zone in the survey area is supported by the six criteria. The trend of the Atlantis Fracture Zone indicates that the fracture zone in the survey area is a westward continuation. Also, with a length of 840 km an offset in the MidAtlantic Ridge should be observable, and the only offset reported in the literature for this latitude is the Atlantis Fracture Zone. Marine Geol., 6 (1968) 401-414
411
SOHM ABYSSAL PLAIN TERMINUS Million
Yeors
60
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Fig.9. Comparison of magnetic anomalies with Vine's anomaly profile.
The unusual shape of the 5,615 m sediment finger indicates a possible fracture zone association. The outermost fringes of sediment from the Nares Plain partially fill long linear depressions which are associated with fracture zones, such as the Vema Fracture Zone which offsets the Mid-Atlantic Ridge (EwING et al., 1966b). Similar lineations are seen along the Sohm Abyssal Plain. The shape of the 5,615 m sediment finger is a configuration similar to these fracture zone-associated lineations. A correlation of such features with the clustering of epicenters on the Mid-Atlantic Ridge seems to be a good criterion for tentative identification of fracture zones.
Magnetic correlations Fig.9 shows the magnetic anomalies from the track of "Trident" Cruise ~,¢04 and the anomalies from VrNE'S paper (1966). The "Trident" track correlates reasonably well as a rough approximation between 50 and 60 million years. Some discrepancy in detail occurs, but Vine shows some variation in the detail of the tracks from RAFF (1966) and VACQUIERet al. (1961). The age of the survey area is thus indicated to be 55 million years or Early Eocene according to KULP'S geologic time scale (1961).
Sedimentation The surface sediments in the fiat sediment fingers are very fluid, as indicated by sub-bottom echoes obtained with the PESR to a depth of 50 m. The reflecting layers range from 3-5 m apart, but are not spaced a uniform distance. A uniform, wide distribution of beds indicates a probable turbidity current origin (HFRsEY, 1965). When a turbidity current enters a basin and is dammed by its borders, the current is turned back and circulates within the basin, distributing suspended matter more or less evenly with sorting taking place as the material settles out differentially (HERsEY, 1965). The surfaces most likely to reflect sound, according to Hersey, are Marine Geol., 6 (1968) 401 414
412
~, ~. MCGREG()R
between coarser beds with a h)w water content and liner sediments with a high porosity. The material ill the sediment finger has a high water content as indicated by the 50 m penetration. Because of the Iocat ion. turbidity currents actively contribute material to the sediment fingers and since tile fingers were closed basins, tile type of sorting and settling that Hersey proposes would hold for this area. The sub-bottom profile (Fig.7) shows the deeper sediment structure. Two types of sediment are easily observable, an acoustically transparent layer, and stratified sediments. The transparent sediments, 100 m thick in places, blanket the topography on either side of the sediment fingers and compose the bottom half of the sediment fingers with the stratified sediments filling the other half. The origins of these two types of sediment are different. The stratified zone results from turbidity currents moving from the Sohm Abyssal Plain into this area. The transparent zones represent quiescent deposition of pelagic sediment or deposits of fine particles transported by a nepheloid layer (EwIN(; et al., 1966a). On either side of the sediment fingers the bottom is rough with a blanket of acoustically transparent sediments across the area. Some sediment accumulation in the valleys have a few weak reflectors. This sediment is slump or slide material from the topographic highs on either side, and in some cases visibly slopes away from the highest topography. The top 300 m of sediment in the 5,570 m sediment finger consists of sediments stratified parallel to the water-sediment interface rather than the bedrock surface, and the bottom 300 m of sediment is more acoustically transparent. Some stratification exists at the bottom of the eastern side, but this is at the base of the steeper slope and may be slump material. Some irregularity occurs at the interface between the acoustically transparent and stratified sediment due to slumping material. No blanket of pelagic sediments is obvious on top of the dividing ridge between the sediment ponds, but the bottom return is not sharp, so that the layer may be present or the steep slope may cut down on its thickness. The 5,615 m sediment pond is also asymmetrical with a steep slope on a buried irregular topographic high in the eastern portion. Stratification of the sediments more paralleling the basement is present, but definitely not as sharp as in the other finger. Stratified sediments 200 m thick in this eastern portion overlie 100 m of acoustically transparent sediments. In many places these transparent sediments come to the surface, which may be due to diapiric structures, although no bending of the beds is obvious. As the topography rises, the stratified sediments disappear and only a continuous blanket of transparent sediment is found. The pelagic sedimentation has been going on for some time and at some more recent time turbidity current activity began in this area, as indicated by the continuity of pelagic sediments beneath the plain sediments. This indicates a sill to the north, which permitted passage of sediments into the fingers when the sediments on the plain accumulated deeply enough. The small thickness of sediment with stratification indicates that turbidity current activity here is not as old as on the Sohm a short distance to the north of the survey area where the sediments are much thicker. SMTO et al. (1966) have dated two cores near the survey area as Lower Eocene and Eocene. The cores were located at 30:~N, 51~52'W and 2T52'N, 54°38'W in 4,673 m and 4,680 Marine Geol., 6 (1968) 401-414
SOHM ABYSSAL PLAIN TERMINUS
413
m of water respectively. The depth of water indicates that the material was cored on a topographic high and therefore, was part of the pelagic, transparent layer. The length of the cores is 540 cm and 627 cm, respectively, which may indicate the sediments have been reworked, since the cores are short for Eocene if a uniform deposition rate of 1 ram/l,000 years is assumed. The acoustically transparent sediment accumulation is the same over the whole area, which indicates the age of the scarp and fracture zone is similar to the initial formation of the area, which one would expect for a transform fault. The difference in surface depths in the two sediment fingers, 5,570 m and 5,615 m, is a result of a difference in the depth of the bedrock. The sediment thickness is the same in both fingers so they have a common source to the north. The dividing ridge is either truncated to the north, or more likely is buried by the thicker sediment accumulation on the plain. The stratification is more pronounced in the 5,570 m finger because it is narrower than the 5,615 m finger where the sediments can circulate more evenly before the settling of material begins.
ACKNOWLEDGEMENTS
I would like to thank Dr. Dale C. Krause for his assistance and advice during the research and preparation of this manuscript. Thanks are also due Dr. Louis E. Garrison for critically reviewing the manuscript. Dr. Robert L. McMaster and Dr. Frank T. Dietz critically read earlier drafts, and William P. Dillon reviewed the final illustrations. Funds for this study were provided by Office of Naval Research cont~ act Nonr 396(08).
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Marine Geol., 6 (1968) 401---414